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BMS Ideas

BMS Ideas

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Home grown BMS ideas ! click="needToConfirm = false;" href="javascript:insertAt('editwiki','idea');">idea



acmotor

Posted: 16 January 2009 at 12:48pm



Johny said to start this topic....

It started as a stain on the page in.....



hyjacked topic



Go for it...



Here is my circuit again just to start the topic here.

 


 




NOTE. Transistor used in final version was KSH127



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Posts: 49       Quote jackhyq Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 1:05pm

just circut or can do that by yourself ?

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Posts: 273       Quote zeva Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 1:19pm

Ah what a coincidence, I've started playing with BMS design myself this week..



As many of you have heard, I'll be building a new pack for the MX5 over the coming months (based around K2 26650EV cells) and will be doing a homebaked BMS for it myself. Personally I plan to construct 4S30P modules, and will have a single BMS per module with a microprocessor monitoring the four cell voltages and triggering shunt transistors/resistors through optoisolators. And probably communicating with some kind of master unit (for monitoring voltages, controlling a charger or warning about low cells etc) over I2C serial. Well that's the theory anyway, time will tell if it actually works out! Will post more info as it happens.



I'm certainly interested to hear what others may have done or are planning to do for their BMS too, in case I may want to steal their ideas

Ian Hooper

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"Never doubt that the work of a small group of thoughtful, committed citizens can change the world." - Margaret Mead

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Posts: 1321       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 1:28pm

Jackhyq,







I can't claim all the fame.

The circuit is mine but a friend layed out the PCB. (master of the art)

The aim was to have no adjustments required as the zener are very accurate 1.250V. This has worked.

I've tested 4 out on TS 40Ah and now assembling the others.

Only 216 to go !

My early BMS were pic-axe micro based, but worked out too expensive.

This one should come in under $10 all up.





Hoops is probably on the track by using 1 micro to control / monitor multiple cells.



I've run a module of 4 cells with voltage monitoring via resistor tree (and software subtraction). That works fine but the idea needs to be done over 20 cells.



Red Suzi   Industrial AC while we wait for real emotors.

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Posts: 716       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 1:29pm

OK. I stayed away from prog.zeners because I thought that dual or quad comparator would be cheaper, but I see they are $UA0.12 on Octopart so your system looks pretty inexpensive in parts.



I'll look at a quad comparator and prog.zener and see if I can get the parts count down. A single 14 pin DIP is simpler to load if they are to be DIY built.



A minor variation would also work for lead-acids (my back up plan with the Oz$ going dooowwwnnn). Reason being that I see that "smart" lead-acid chargers uses low current 0.02C to sense if the battery has fully charged then switch to float. My concern is that the shunt zener-resistor system may prevent that switch. We would need an opto input bus as well to switch the eq. off for charging. Do eCrazymans's chargers work OK with the zener-resistor system?



Edit: Added 'a'











Edited by Johny - 16 January 2009 at 1:32pm

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Posts: 1321       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 1:50pm

Johny,



Do go for surface mount as any through hole is potentially a mechanical issue on the underside (cell side) of the PCB near cell terminals.



I started with comps.and zener but the prog zener has the comparator built in.TLV431



Re the SMPS chargers on red suzi SLA and zener/res eq.

See red suzi about the float charge.

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Posts: 49       Quote jackhyq Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 1:53pm

Acmotor, That's great !

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Posts: 1321       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 1:59pm

The best thing about building something and posting it is that others will know what not to do !

... or at least have fun flaming it !

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Posts: 716       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 2:11pm

OK on surface mount - I guess I'm a Luddite. Im a bit used to doing DIY PCBs in single so bias toward DIP - I must change.



I see the float reference but my point was that some chargers wait at 14.2->14.5 Volts for the current to drop to less than 100mA which isn't going to happen with a 200mA drain via the zeners.

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Posts: 645       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 2:19pm

LED + Darlington Clamper would have to be the minimalist approach.



They also mention an alternative: Zener Shunt Regulator



Using LEDs is attractive to me, easy to see which cells are charged :-)



Also could use a similar system to limit under-voltage I guess.



Can you get opto-isolators with a 2.5V cutout to use for a similar low-voltage cut-out circuit?

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Posts: 1321       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 2:20pm

I'm not a fan of the overcharge to 14.5V.

I just give it another 8 hours at 13.6 to 13.8 and it is charged anyway.

Chill out, be patient !



The simple zener/res. bms is not for a 'smart' charger.

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Posts: 1321       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 2:31pm

The problem I found with normal zeners on SLA 12V level was the need to 'select' the voltages out of a batch of zeners. These varied from 6.4 to 6.9 V on '6.8V' zeners.

The prog. zeners are very accurate up front.



There are many minimal systems for lead acid (I use one) however I am not content with minimal on the TS !



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Posts: 645       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 2:42pm

What are the important/unimportant BMS features?



These are the ones I can think of:

0. Safe

1. Overvoltage protection while charging

2. Overvoltage protection while regenerative brakeing

3. Undervoltage protection while driving (buzzer or cut throttle after 20 seconds or what?)

4. cheap (seems silly to spend more on the BMS than the cell)

5. efficient

6. turn off charger when charged

7. optimal charge curve for long cell life

8. identify faulty cell

9. bypass faulty cell / cell group



For 9, if you have 1 reversing contactor per cell block of 48V (60-72V acmotor?), the reversing contactor connects the 48V block either to the 48V bus for charging, or in series with the rest of the blocks to make the 600V or whatever. In case of failed cell in the block, the BMS could take that block out of the series, giving you 200 cells instead of 220 for your VFD, still very drivable.

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Posts: 1321       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 2:58pm

All important.



0 Safe... excludes bipolar /fet junction directly across cell ! and resistor whose power rating is exceeded by full on pwm from micro.



7 optimal charge curve. how simple is Constant voltage current limited ?



8 can be just visual at the cell ?



9 interesting... are you thinking of this while pack powered down only ? Just use a mechanical SPDT switch ? can be AC rated as it will not be operated with live DC.

Change-over 600VDC relays may be expensive.

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Posts: 645       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 3:13pm

Same site has links to about 15 differnet BMSes available, in progress.

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Posts: 716       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 3:18pm

acmotor. What charger(s) are you using for Red Suzi's lead-acid now? I though they were eCrazyman. Aren't his "smart"?



Edited by Johny - 16 January 2009 at 3:19pm

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Posts: 645       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 3:50pm

Originally posted by acmotor



All important.



0 Safe... excludes bipolar /fet junction directly across cell ! and resistor whose power rating is exceeded by full on pwm from micro.





Originally posted by acmotor





8 can be just visual at the cell ?





I think is both simplest and best. (hard to stuff up)



Originally posted by acmotor





9 interesting... are you thinking of this while pack powered down only ? Just use a mechanical SPDT switch ? can be AC rated as it will not be operated with live DC.

Change-over 600VDC relays may be expensive.





I saw a4x4kiwi contactors which split his pack into 12 x 48V packs and a 24V pack for charging. It made sense for me to join them all in parallel to 1 48V pack for use with a big charger.



The way to do this was DPDT contactors:





    48V Bus + -O  O- + 48v block - -O  O- - 48V Bus



                   /                        \



prev 48V block -O  O-(bypass)---O  O-  - next 48 V block







When the contactor was energised (show), the blocks join together to make the 600V chain, when unenergised, they join onto a common 48V bus for charging.



The reversing contactors have 4 terminals on each end. I thought they were double pole double throw, but they aren't are they, a reversing contactor is pretty much quad pole single throw, which you can wire as double pole double throw, I don't think I can do the bypass thing anyway unless I use even more hardware.



I think your manual cutover is a better option.



Edited by woody - 16 January 2009 at 4:08pm

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Posts: 645       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 4:10pm

Originally posted by Johny



Do eCrazymans's chargers work OK with the zener-resistor system?





I think a4x4kiwi is testing that for us at the moment!



I think his first full charge never went from CC to CV, but I can't be sure.



Also one charger let out the magic smoke...

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Posts: 645       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 4:14pm

Same site's take on requirements.

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Posts: 1321       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 4:33pm

Woody, if the DPDT contactors were rated at 600V I would feel at ease with the module parallel switching / bypassing idea. Otherwise I would be taking great care with the operation !

At zero current you may well get away with it.



The chargers I have ended up with are fixed voltage, current limited (actually ecrazyman power supplies). I would need a more effective BMS than simple zener if the charger were taking V up to 14.5V for a faster charge.

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Posts: 114       Quote Mesuge Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 6:07pm

Originally posted by woody



Same site has links to about 15 differnet BMSes available, in progress.





Apart from BMS and his MiniEV build, there are couple of interesting CANbus links as well (lower folder level).

http://carrott.org/cgi-bin/twiki/view/BMS/WebHome

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Posts: 379       Quote a4x4kiwi Quote  Post ReplyReply Direct Link To This Post Posted: 16 January 2009 at 7:00pm

@Woody, All chargers are smart chargers and have worked successfully with the Zener Resistor balancer.



The Zeners and resistors do get quite hot to touch before the charger switches to float. I have an extractor fan on the trey that is powered during charging.

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Posts: 716       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2009 at 7:53am

Are there any specs on the chargers - like current required for float switch? I'll assume not so how hard would it be to gently test a couple of them to determine what the current at switch point is? I suspect this might have to involve a longish time (a few seconds at each current) unless they don't bother to "smooth" the current reading.

My concern is that the zener-resistor system is near the threshold and some switch but some don't - result, unbalanced 600V pack.

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Posts: 49       Quote jackhyq Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2009 at 4:57pm

Originally posted by Johny



Are there any specs on the chargers - like current required for float switch? I'll assume not so how hard would it be to gently test a couple of them to determine what the current at switch point is? I suspect this might have to involve a longish time (a few seconds at each current) unless they don't bother to "smooth" the current reading.

My concern is that the zener-resistor system is near the threshold and some switch but some don't - result, unbalanced 600V pack.







Hi Johny,I have specs of our charger,do you still need ?

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Posts: 379       Quote a4x4kiwi Quote  Post ReplyReply Direct Link To This Post Posted: 17 January 2009 at 5:03pm

FYI here are my charger specs. They are from e-crazyman on ebay.



I was concerned that they would not turn off with the zener / resistor across them, but all have been fine.



input voltage: 98~245V AC 50/60Hz

power efficiency: 90% +/- 2%

size: 17.8*10.4*7.4 CM

indicator method: LED

working temperature: -5~+40 Degree Celsius

cooling method: FAN

output voltage:55.6 +/- 0.4V

output current:DC 2.5A +/- 0.1A

battery fits: 17-20AH

AC wire length: 71.5 CM (i will changing to AU wall plug)

DC wire length: 83 CM

13 units to AU price is 306 usd included shipping

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2009 at 12:34pm

I found an article in EE Times-India that describes an active charge balancing system for Li-ion cells that looks very good. I can't put in a link as you have to log on to read it, but I have it as a .pdf file if anyone wants the full text. It describes "bottom balancing" where a cell lower than the average is given extra charge and "top balancing" where a cell that is at the highest voltage is discharged, the energy going back into all the other cells. The system also allows a voltage scan to monitor all the cell voltages and the way it does this is pretty clever.

It uses a simple multi-winding transformer (20 kHz ferrite). There is a single primary plus a secondary for each cell ie 12 cells need 13 windings. A transistor switch is required for each winding. A microcontroller manages all the switches.

That's all you need for a block of 12 cells, parts cost under $20.

Their prototype is claimed to achieve balancing currents of up to 5 amps with a dissipation in the complete block of only 2 watts.

It would be pretty easy to provide opto-coupled comms to a "master" so you could see every cell voltage in a pack of several hundred! But not absolutely necessary.

I have started work on a prototype for a 4 cell block, just to try it out.

Anyone would like a copy of the .pdf file, send me a PM.

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Posts: 362       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 11:43am

> The best thing about building something and posting it is that others will know what not to do !

> ... or at least have fun flaming it !



Well, perhaps not a flame. Your opto is normally on, which I think is a good idea, so if the cell dies to zero volts, its "I'm OK" signal (conducting) won't be there. So I assume that you will put the outputs in series, so if any are open, the series string will be open.



Will you be putting all 220 outputs in series? If so, that would be a 66v "low" if the Vcesat of the optos is 0.3v; 44v if it's 0.2v.



Perhaps a group of them will be put in series, and combined with a separate board to generate a logic signal?



BTW, thanks a heap for posting this. It's a nice, minimal design with all the essential features. My only criticism is the PNP transistor in series with the programmable zener across the cell with no resistor or fuse. If both fail shorted, as semiconductors often do (when they fail), there will be a great current. Presumably, the "zener" or PCB tracks will vapourise before too much damage is done. I think it would have been nice to put a small resistor in series with the PNP transistor's base.



Will the PCB layout be available? A friend and I are thinking of a high voltage AC system, and our current dreams for a BMS are unlikely to be ready any time soon.



- Coulomb



Edited by coulomb - 22 January 2009 at 11:48am

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Posts: 698       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 12:01pm

Hi coulomb. He describes it's intended operation in "hyjacked topic" - which is linked in the first post in this thread.

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Posts: 634       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 12:06pm

Originally posted by acmotor



All important.



0 Safe... excludes bipolar /fet junction directly across cell ! and resistor whose power rating is exceeded by full on pwm from micro.







Should the zener / resister / zener balancers include a fuse?



Should the LED-darlington include a fuse too?



How should the fuses + resisters be sized?



cheers,

Woody

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Posts: 698       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 12:48pm

It would be far better to have an intrinsically safe design rather than a fuse. A blown fuse would be a right pain. Zener resister balancers can be rated correctly with only a loose connection causing damage. LED/Darlington system should include resisters (IMHO).

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Posts: 362       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 2:08pm

Originally posted by Zeva





I'm certainly interested to hear what others may have done or are planning to do for their BMS too, in case I may want to steal their ideas.





Originally posted by Woody





I saw a4x4kiwi contactors which split his pack into 12 x 48V packs and a 24V pack for charging. It made sense for me to join them all in parallel to 1 48V pack for use with a big charger.



The way to do this was DPDT contactors:





    48V Bus + -O O- + 48v block - -O O- - 48V Bus



                   /                        \



prev 48V block -O O-(bypass)---O O- - next 48 V block







OK, I have this crazy idea, based on this simple circuit per cell or per group of cells:







It's a simple bridge circuit, capable of transmitting DC power from "input" (the cell(s)) to "output" (part of the input to the motor controller), or vice versa. The voltage ratio is set by the duty cycle of the transistors, and the output can be more or less than the input. So this is a buck/boost converter.



My first conversion is likely to be a high voltage AC system, but I'd like to avoid having to wire 180-220 BMS systems in series, if I can avoid it. (How are those 220 boards coming along, acmotor? ). So the idea is to use a smaller number of higher capacity LiFePO4 (LFP) cells, and use the DC-DC converter to translate say 3.2v nominal cells to say 6.4v nominal pseudo-cells. This could also operate on groups of cells, say 16 cells at about 50v nominal.



As per a standard buck/boost converter, Q1 is on for say 2/3 of the time, "charging" the inductor with current flowing left to right (conventional current). Q1 is opened, then Q2 closed; in the dead zone while they are both open, the capacitor continues to supply current to the motor controller, preventing 600-700v from messing things up. Q2 conducts for 1/3 of the cycle, doubling the voltage from the cell(s) to the capacitor. 1:1 duty cycle would just copy the cell(s) voltage to the capacitor (neglecting losses).



Now here is the beauty: we can save the cell(s) during garage charging, motoring, or regenerating (regen). During motoring if the cell voltage gets too low, we just decrease the "turns ratio" of this DC-DC "transformer". So instead of doubling 2.5v to 5v, we multiply by say 1.8, to contribute 4.5v to the pack voltage. This also reduces the current from the cell(s) from 2x the pack current to 1.8x. The pack voltage goes down slightly, which will also reduce the load a tiny amount.



If the cell gets really bad, in the limit, we can just leave Q2 on and Q1 off, bypassing the cell(s) without using contactors.



During charging (garage or regen), if the cell voltage exceeds a preset limit (say 4v), we increase the turns ratio. Instead of doubling 4v to 8v, we multiply it by say 2.2, yielding 8.8v. The current ratio changes from 1/2 to 1/2.2, reducing the current into the cell(s). If this 4v limit is precise, it can be used as the sole balancing method for the cell. If this switching is done at the group level, another balancing method is used for cells within the group.



For garage charging, all that is needed is a source of DC on the DC bus of the motor controller (i.e. across the pack). If the controllers controlling the cells "transformers" are smart enough, they can cope with almost any reasonable voltage level, and use just what current they need by adjusting their "turns ratio". When done, they set the "turns ratio" high enough that the charger doesn't supply any current. The input could conceivably be the unfiltered mains (i.e. a bad boy), and as the waveform rises and falls at 100 cycles per second, the turns ratio could increase and decrease to approximate a sine wave of current, offering good (low) harmonic distortion (often misnamed as good power factor).



When used at the group level, this could be made to do sensible things with a dead cell (bypass the group). It handles over and under voltage, and could handle over temperature as well (at high temperature, just increase the turns ratio if the cell voltage is higher than nominal, and reduce if the cell voltage is lower than nominal).



So, here are the advantages:



1. Relative simplicity, although a lot of the smarts are not shown (driving the gates sensibly).

2. Handles over and under voltage, and overtemperature.

3. A failed cell or group of cells has minimal impact on the pack voltage, so the controller isn't surprised by a huge change in what it assumes is the mains voltage (assuming an industrial controller).

4. Charging is vastly simplified; the charger reduces almost to a rectifier.

5. There is no need for dissipative equalisation, although the FETs will get quite hot.

6. The transistors don't see much voltage in normal operation, so inexpensive 40v devices will suffice.

7. The parts count is quite low, despite needing a microcontroller, gate drivers, isolated supply for Q2's gate driver, etc. The capacitor may take up a bit of room; I haven't calculated its size yet.

8. The microcontroller driving the gates could also act as a monitoring system, sending cell voltages and status on request.



Here are the disadvantages:



1. The transistors handle full cell current, so they need to be capable of handling the current, and on resistance should be very low.

2. The inductor would act as a fuse in case Q1 is turned on hard or fails shorted. That's not ideal.

3. If the two transistors don't conduct for more than a few milliseconds, the capacitor will attempt to charge to full pack voltage, and the transistors would overvoltage.

4. The upper transistor requires high voltage for its gate drive, necessitating transformers or some such to derive a suitable isolated supply.

5. If the circuit isn't very, very efficient, this circuit will dissipate massive power. It may even have to be water cooled. This is the achilles heel of this concept.

6. The losses in all the switchers could add up and affect peak power and range.



No doubt there are other problems I haven't listed. But I think it is a neat concept, and could work out well in some situations, e.g. bicycle or even motor bike sized projects.



So: what does the group think about this idea?

Can it be made to work in a car-sized vehicle?

Are there other problems?

Are there other improvements that could be made?



- Coulomb



Edited by coulomb - 22 January 2009 at 2:10pm

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Posts: 362       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 2:43pm

I should have mentioned this additional advantage:



9. This system is capable of saving the pack in the event that the communications between the BMS and the motor controller (e.g. excess load or regen) is lost. It even saves the pack if the motor controller fails shorted (the groups or cells set their output voltage very low, resulting in low cell currents).



Naturally, I would expect the BMS to ask the controller to wind back the load or regen as needed, as in a standard BMS.



- Coulomb

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Posts: 88       Quote evric Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 2:53pm

Hi Coulomb,

Voltage doubler circuits are inherently lossy. They will only supply very small currents. ...Ric

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Posts: 362       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 5:45pm

Originally posted by evric





Voltage doubler circuits are inherently lossy. They will only supply very small currents. ...Ric





Yes, that seems to be the received wisdom.



Where are the main losses? Ferrite losses in the inductor? FETs are pretty efficient, and the copper losses are pretty easy to keep low.



Perhaps it's junction capacitance (switching) losses?



- Coulomb

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Posts: 1298       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 7:43pm

Coulomb,



Great to see you are on the ball re my simple BMS circuit.



Yes, I had fitted a 200 ohm resistor to the transistor base at the last minute before the PCB was made (sorry, wasn't on posted cct)( I must have known you would spot that shortfall (!).... and a second LED on the PCB to parallel the opto so you can see what is going on.



No fuses, just failsafe design would be my preference. What is the point of a fuse ? extra cost and something else to go wrong.



I see the biggest problem in a series battery pack (particularly higher voltage one) is how to survive an open circuit cell. Danfoss controller will shut down on undervolage but will the offending cell still be there ? A fail to s/c is easier to handle.



Re heat loss / shunting. This is actually needed when you run regen. There is nowhere else to 'shuffle' the power to when cells are full.



Correct on the Vce(on) of the opto and of course its Vce(oc) max.

My battery pack is divided into modules of 22 cells by contactors(70.4V) at present (although I may eventually use half that) so the opto Vce total is less than 10V. This also means that the module can be self contained and pass its status on via another opto to join the other modules, thus maintaining isolated wiring between modules.





Re your BMS thoughts. Interesting.

I guess that the lower the cell count, the fancier the BMS can be. However, after starting with a home made micro BMS I found that the cost per cell and complexity per cell was an issue at 220 off.

Shuffler circuits seem to be around 65% efficient so some energy is saved.



It was also possible to crash the micro on my prototype with the massive electromagnetic fields in the battery wiring when hundreds of amps were flowing. Believe me, I tried it. Sure I could have made it layout less sensitive but it reminded me that simple analogue was fine for bottom line cell protection... use the micro for other nice to know but non critical monitoring.



I am still assembling and have a few layout points I would change on a next run of boards. I will be gettting more boards made once my pack is complete. I will post up results and maybe do another PCB run if you are interested.



Edit:spelling, before Richo sees it



Edited by acmotor - 22 January 2009 at 8:58pm

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Posts: 698       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 8:50pm

On acmotors BMS. How about cascading the optos so that each opto output goes in series with R4 on the next module. No VCC issues and one opto for the output which can be the closest one to signal ground.



You can also lower the opto drive current because the output is running low current. It might stuff up the 2.5V set point though.

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Posts: 1298       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 9:08pm

Just lazy.

One wire daisy chain x 220 is less than 2 wires for cascade. But yes, it would throw out the set point.



The idea is to have the cells as autonomous as possible, even when they are stand alone. edit: did I say that ? You know what I mean though.



I did start on OV and UV separate via 2 optos but cost goes up as does wiring so came back to (Probably Rod Dilkes's) idea of just battery good on one line. After all, you should know if you are charging or discharging !



Edited by acmotor - 22 January 2009 at 9:52pm

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Posts: 700       Quote Richo Quote  Post ReplyReply Direct Link To This Post Posted: 22 January 2009 at 11:44pm

As a side point after seeing thousands of different product boards in for service the tracks on PCB's make quite a good fuse

esp when you are talking about a battery that can supply enough current to vaporise most tracks 300mA v 300A.

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Posts: 1298       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2009 at 11:17am

Agreed on track fuses !



They only get into trouble when the voltages are higher as they tend to take the whole PCB, instrument and room with them !



At only a few volts you can get away with them though.

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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2009 at 6:07pm

From my limited and dated experience, Zener Diodes tend to have a near zero temperature coefficient at about 6.8 V - hence the desirability to aim for there, but Zeners at about 12 V have a far more stable voltage - almost irrespective of current flow (and their temperure coefficient is not all that bad - but if you check the reference voltage in IC chips, then these can be lower voltage along with temperature stability built in - hence the programmable voltage zeners give excellent results as you have pointed out.



After a few weeks thought on Battery Voltage monitoring, I have a mental picture of using firmware approach using a 1 of 16 multiplexer being fed from say 12 10 M ohm resistors (one from from each incremental voltage), and 12 protective diodes to prevent the Multiplexer from being back-biassed, a couple of op-amps (in the Analogue to Digital Converter (ADC)?) to invert and normalise the current referred voltage and then a 16 bit ADC (with its internal voltage reference) to get the digital voltage equivalent, before feeding this into a microprocessor and making a '12 wide, normalised Bar chart display. Comments appreciated...

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Posts: 1298       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2009 at 7:15pm

Just musing here...



I have made a 4 cell ladder voltage measurement to one ADC and subtracted in software to get individual cells voltage to experiment with BMS.



This is fine however voltage numbers degrade as the number of cells increases. Maybe 8 might be a limit. The reason being that there is not much to see on the TS cells I tested once they are equalised. Variations in voltage only occured in the mV range.



At some point I decided I just wanted to know if there was a faulty cell as 220 sets of voltage readings were not very interesting.

I have gone for just reading the overall voltage on modules of 22 or so cells.



Such info as ESR for each cell could be interesting, however the pack is still just limited by the weakest cell. Since you are already measuring energy or Ah in/out of the pack, the individual cell info is fairly dull unless there is something going wrong.



In a pack of 45 TS cells it may be worth reading every cell.

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Posts: 113       Quote Mesuge Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2009 at 8:41pm

Tuarn, I think you are musing in the right direction.

Just look at the AC bigboys, for instance ACP/eBOX frontend seems to indicate they data log only subpacks performance and possibly the highest/lowest VDC cell as an outlier, which should be enough to detect/service any problem with the cells/entire batt. pack. On the other hand detailed BMS and its importance for low voltage system is another story.

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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2009 at 9:12pm

Yes ACMotor, I was thinking about monitoring every cell and then it came to me that if a cell in a battery was faulty, then this would show up in the battery in any case, hence the relax back to 12 voltage test points and not 72 test points - I think we would get the same answer (or information anyway).



The output could be a set of vertical LED arrays forming a horizontal line that shows charge as the height of the line and any leds that are out of line show that the relative battery has a cell that has a problem.



Not that I probably missed it before - how hot do these batteries get, and should this be monitored too??



It almost makes me think that the battery manufacturers could easily install a simple monitoring module in the battery wall that can be plugged into - or have they already done that - or do we have to invent their wheels for them too??

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Posts: 1298       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2009 at 11:21pm

I am glad there is support for battery module rather than cell voltage monitoring, I don't feel so bad now ! (still keeping the cell OV/UV and shunt for eq.)



Built in battery monitor... This has been done by several manufacturers but not on any batteries I can afford ! I think Altair-nano were first some years ago.



On the Rodeo my plan is to discharge at average 0.5C (peak 3C) and charge at 0.1C so I am not expecting temperature to be an issue.

220 x 40Ah TS cells = 704V at 0.5C =14kW average power



Heat at cell (worst case ?) = ESR x (Amps)

2 so on average this would be .004 x (20)
2 = 1.6W        and .004 x (120)
2 = 57.6W for peak.



I can't see this heating things up much !



Now if you pull 5C the game starts to change...

.004 x (200)
2 = 160W but even then it would be in short bursts (and not in my EV).



The TS cell's operating temperature is quite wide (-25 to +75°C) and Matt always tells me they should be 'toasty' for best performance.

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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 7:04am

I am cool with the internal resistance power dissiapation, but what concerns me is the heat generated by the chemical reaction to charge / discharge the cells - and I am thinking that this reaction may generate far more temperature than the internal resistance alone.



On the conceptual side of it; if there is a BMS sitting over the batteries all the time than this will be a constant drain on the batteries - unless the BMS circuitry is made indivual for each battery and in can be remotely activated (else is open circuit) - apart from say a 10 M ohm voltage monitoring probe.



On yet another angle, has anybody done a Finite Element Fourier analysis of the heat distribution of the plates (based on current flow and current density & chemical reaction), so that the temperature can be read by embedding something in the lead plates (or the acid/gel)??



acmotor: If your've got 220 (40 AH) batteries that is a lot of volume (even if they are Lithium) - like about 2 Metres
3 - where do you sit??

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Posts: 634       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 7:55am

Originally posted by EVLearner





acmotor: If your've got 220 (40 AH) batteries that is a lot of volume (even if they are Lithium) - like about 2 Metres
3 - where do you sit??





Er, LFP40's are just over 1 litre each (+ 1.5 kg), so 220 should be 223 Litres (330kg) - quite manageable :-)



cheers,

Woody

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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 8:42am

My bad grammar - I was assuming Lead Acid, not LFP40's! Hey, you don't have to cast out the back seat after all!

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Posts: 1298       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 10:21am

Yep, TS on the tray back of the Rodeo.



FEA on cell plates ? Someone somewhere is probably doing that. Maybe CSIRO with the ultra battery (lead sinker unless they make some real breakthrough though, but it is a good idea. Lead working as well as Lithium would be interesting)



Are you offering to head up battery research in Oz ? That would be great ! It is way beyond me. I have enough trouble just charging and disharging batteries !



Your point is good re heat from chemical reaction.

My thinking is that any heat is energy used in the charge/discharge equation. So if the ESR accounts for the cell losses then the heat(energy) from chemical reaction is already accounted for in the ESR number ? Someone will flame me here.



Maybe we need a careful test of a thermally insulated TS cell in charge / discharge unless someone has the data already ?   





EL,

Correct, minimal battery drain by BMS is part of the design.

My BMS battery drain including LED and opto is <4mA.



40Ah / .004 = 10,000hours to flatten, that is well over a year and probably only just more than self discharge.   

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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 12:20pm

AC,

Regards to the UltraBattery have a look at Firefly.com This is a USA based battery company using a virtually parallel technology to the UltraBattery; ETA about 18 months to the mere mortals.



It looks to me that the Firefly "Carbon Sponge Lead-Acid" technology has a Power/Volume figure that also parallels the Lithium cells (at a large fraction of the cost), and is very low internal Resistance, and is very robust.



My gut feeling about batteery heat is that the chemical reaction is one thing and the I2R losses are another kettle of fish, but these heat sources need to be added.



A BMS runing at <4 mA is commendable (is this for a 12 V battery/pack). Is the LED being run from a little power converter? Does this mean thatyou are simply measuring the voltage for each battery pack or does your BMS do much more - sorry I came into this (EV) life very recently!!





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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 12:34pm

AC,



Yes I finally caught on from going to the start of this thread and saw your little circuit. So the Opto is off for under and over voltage and on if the voltage range is OK - all makes sense there! (Based on nominally 4 V)

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Posts: 1253       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 12:56pm

Yes, there is a lot of battery development in the pipe line. The very development we should have had years ago. AND the development that will make the hybrids look like vintage cars.



I guess to some extent my decision to buy TS was that it was the best value offered so far and it was ACTUALLY available.

Otherwise we all just grow old waiting for the promised EVs and batteries !



I'm glad you follow the circuit.



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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 2:48pm

Hi AC Strange as it may seem, but the circuit that you have is a virtual digital realisation of an expanded analogue voltmeter circuit.



Am I guessing correctly that the separate circuit on the left hand side is an over-voltage shunt to assist in self-balancing the series connections of battery voltges. If this is the case, then I would look to be integrating the two circuits so that the component count is cut by at least 3.



I like the idea of directly mounting the boards on the batteries - that is the general idea - isn't it?

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Posts: 1253       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 5:47pm

Correct.

LHS is voltage shunt.



I did consider combining O/V and shunt but that would make them at the same voltage and that is not quite the plan. Shunt voltage can (needs to) be set independent of O/V.

i.e. shunt is normal operation O/V or U/V is not if you get my drift.



The shunt is set to come in at 4V so eq has a chance to work before any O/V 4.2V) signal (O/V must shut off or at least considerably reduce the charging current (to less than the shunt current))

Also when used with regen the shunt does its job (1-2kW on my pack) without giving an O/V.



The other thought was to keep the voltage monitoring as just that. Separate from the shunt (and more likely to fail) circuit.



I am pleased you are examining the circuit and making suggestions.

Go for it !

I will try to justify my thinking but realise that it will probably have holes in it !



Boards directly mount on TS battery (40Ah) but could be adapted to others in the range.

This idea is quite blatently stollen from others!!!

Some people have a one size fits all PCB. I am not the optimistic yet !



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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 7:18pm

Hi acmotor



Well that answered the question that I was obviously going to ask - what are the voltage thresholds? Does the programmable zener have a sharp knee, and how sensitive is it to temperture (I will check these on a data sheet if I can lay my hands on one!)



I had a thought about arranging a VCO (voltage controlled oscillator) into the Opto, so that the frequency could be proportional to the voltage (over a limited range) - that way the microprocessor could scan and read the cell voltages, as well as know the cell is OK - any thoughts on that?

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Posts: 1253       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2009 at 7:41pm

Accurate 1.24V 1% for the zener, very sharp (mVs) knee. See a TLV431 data sheet (5 pin sm pack) just google for pdf.

O/V 4.2V , U/V 2.5V , shunt 4V although I may move this a bit.



VCO is interesting idea.

My earlier micro version sent serial data via opto but this meant the interconnections were not a simple one wire daisy chain. Also the 4800 baud data rate was too slow to scan 220 cells. 5 bytes x 220 cells = 1100 bytes at onlt 480 bytes/sec and this was before handshake. It was looking like taking 5 seconds for a scan.



As per previous posts, I was reluctant to leave simple analogue as I had experienced micro crash.



Please explore the VCO idea further though as it has merrit.

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Posts: 52       Quote EVLearner Quote  Post ReplyReply Direct Link To This Post Posted: 25 January 2009 at 7:48pm

TLV431 is almost a reference diode and a virtual 741 all in one - nice bit of work there! And at $0.12, resistors are almost more expensive!



With the VCO I was thinking about 131 kHz for 13.1 V, and direct opto scanning into a BCD counter, gated into an 8 or 16 bit bus, on a 10 msec synch per cell or about 2.2 sec for 220 cells - which is still too slow! Better KIS (Keep It Simple).



With the shunt currnt management, has this been tried yet? It concerns me that the TLV431 may leave the switched transistor in an active state - not saturated , and I can't easily see how to impliment a little hysterisis into the circuit so that the transistor 'switches on' and 'switches off' - bit like a schmitt trigger - maybe a 2.2 M from the collector to the TLV input might cause the circuit to switch of the switching transistor does not 'switch' - just a thought!

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Posts: 1253       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 25 January 2009 at 9:16pm

Played with that. Caused oscillations. But I wasn't really bothered since the dissipation from the transistor was in worst case only 2W and it is rated at 20W. It is mounted on plenty of PCB copper by design.

It is all just running linear analogue and seems to work fine.

The transistor saturated at 1A is more than 1V anyway, so it is not really 'saturation' at these low voltages. A FET could be used with low enough Vgs and Vds but the shunt is just a heat loss system so KIS.



After an hour shunting 1.5A at 4V at 25°C ambient the transistor tab is at 45°C and the resistors at 65°C. Measured with IR thermometer.

Real life ambient will be higher but then I see no eveidence from the test bank that eq. will need to run for that much time.



Shunt resistors could be from (edit: forgot to finish the sentence) 1 ohm to some high value so shunt current can be selected between about 3A and not much at all depending on charger rate and regen etc.



The VCO idea still interests me. Keep thinking on it. How does the sync work between BMSs ?





Edited by acmotor - 26 January 2009 at 12:59pm

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 12:38pm

I have used a couple of SMPS in series - 2 x 27V @ 5.4A - quite successfully as a "48V" charger. They are adjustable from a nominal 27V +- 10% and they are current limited at a bit over 5 A. I am charging a 16 x LFP40 pack so I set the two supplies to their max which equates to 4V per cell. They provide CC then CV, admittedly not quite at the TS spec of 4.2v/cell, but I haven't noticed a significant capacity drop using this voltage.

To use them for PbA I would set the o/p to 56.4 or maybe a bit less so as to get CC up to that voltage. A normal PbA charger would drop to the float voltage once the current decreased (at 56.4V) and you won't get that, but if you're fussy you could drop the voltage manually. The advantage of these things is you can buy 2 of them for about $70!

There are a number of SMPS providers that import off-the -shelf stuff from China at pretty good prices. Just watch that the over-current protection is current limiting rather than shutdown/reset.

And you can get higher current ones for a bit more money.

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Posts: 1253       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 1:39pm

Yes, SMPS are quite versatile and the outputs are well isolated from supply or ground.

Nevilleh, what equaliser are you using in the 16 x LFP40 string ?

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 6:09pm

I have to admit that my equaliser is a digital multimeter.

I have been checking the cell voltages whilst charging and just recording them to get a handle on what they do. So far I have found less than .08 volts variation between the 16 cells I am testing. I stick my trusty Dick Smith variable power supply on each one after that to bring it up to 4.2 volts and record all the times etc. The variation is surprisingly small and I wonder if I can get away with just bringing the total pack voltage up to 60 volts. This leaves the cells at voltages from 37.1 to 37.9 and I wonder if this is enough charge - taking them up to 4.2 volts doesn't seem to give appreciable increase in driving range, although I have not measured this precisely as my route varies.

While this is happening I have been working on the design I saw in eetimes-india and just about have a working prototype.

I'm running the cells in an electric scooter and I use the voltmeter to tell me when I am down to about 44volts, then I check the individual cells to see how low each one is and again the variation is quite small. I guess I am accepting a small range/capacity compromise by stopping when the worst cell is at 2.5volts and not worrying about balancing at that point.

My BMS prototype is for only 4 cells but easily extendable to 16 if it works as promised and that will automatically take care of balancing low voltage and high voltage cells and still give me a readout of the lowest and highest voltages.

I have a BMS from TS and it gives the cell temperatures as well, but I'm not sure if this is really needed. Possibly if I ran 5C discharge currents for long times, temperature might be something that I need to monitor too. But charging at 20 amps only increases the cell temperatures by about 7 08 degrees over the couple of hours needed for a full charge. At 5 amps there is not much increase at all.

It's fitted to a big motorcycle and I don't want to pull it to bits just yet! It's interesting that it also shows a voltage range of about .08volts from best to worst and I wonder if it does indeed attempt any balancing at all.

I do need to come up with a good, cheap solution as I plan on fitting 200-odd LFP40 cells to my electric car project and I don't want to destroy them any earlier than necessary.

Just in passing, it strikes me that a couple of 12v 15 amp SMPS's connected (parallel the outputs) across each side of the +- 330volt battery pack would provide the 12volt power needed to run all the other car stuff!

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 6:18pm

Sorry, need to reply to my own post! Duh..

Shift the decimal point one place left in those thirty-something voltages! Also, 7 to 8 degrees.

Must be the couple of glasses of Aussie red.....

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Posts: 650       Quote antiscab Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 11:08pm

a point on charge voltage for TS.



measuring recoverable AH on discharge, the difference between charging to 3.4v and 4.2v was around 0.75AH for a 40AH cell.

as long as current has fallen to around 0.1A at a voltage higher than 3.4v, the cell is charged.



im not sure what other ramifications a lower charge voltage has.



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Posts: 1253       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 11:11pm

I also wonder if there is a downside to LV charging of TS.



I know that with my SLA if you go to 13.6 to 13.8V and wait for current to drop (12-16 hours) the battery is full. Same as 'fast' charge to 14.5V.

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 27 January 2009 at 11:35am

I did read on the Tesla Motors web site that they had dropped the charge voltage by .1v in the interests of battery longevity. Seems like you can do more harm by over-volting the things than anything. Mind you, they are/were using 18650 cells. I'm really starting to wonder if just monitoring the cell voltages is going to be sufficient. I suppose they might get more and more out of step as the number of cycles goes up so you need to re-balance them from time to time to try and keep them together for as long as possible.

Can anyone tell me why Li cells should be so expensive when Li is the 3rd most abundant element? The processing doesn't seem that complicated.

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Posts: 605       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 27 January 2009 at 12:24pm

I did some sums in another thread to see if the drop in minerals market would likely affect battery prices, the raw materials came to 10-20% of our price.

The lithium patents should show you how to do it but legally prevent you :-)

I think the chinese suppliers have ignored the patent laws.

There are photos somewhere online of a TS cell being disected, that should help you if you are going to DIY :-)

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Posts: 140       Quote juk Quote  Post ReplyReply Direct Link To This Post Posted: 27 January 2009 at 4:32pm

Lithium carbonate is selling for around US $8 per kg up from US $0.50/kg a few years ago, which works out to about US $180 per car for our purposes. Remembering that that is for battery grade material, non battery grade material still sells for only 1USD/kg.



Here's a recent diatribe i pulled from my archives:



There’s been chatter here and there about how much recoverable lithium there is in the world, and whether our move toward electric vehicles powered by lithium-ion batteries will create a “peak lithium” scenario.



It all started with William Tahil of U.K.-based Meridian International Research, who back in January 2007 published a paper questioning whether the automotive sector’s expected embrace of lithium-ion technology for next-generation plug-in vehicles was a wise move. Tahil is a fan of the zinc-air battery, largely because “zinc is the only metal which can sustain large battery production in the volumes required by the global automotive industry.” Needless to say, Tahil’s first report whipped up a firestorm of controversy, as you’ll see from some of the comments in a past post here.



Geologist R. Keith Evans published his own report in March 2008 in response to Tahil. Evans’ conclusion: “Concerns regarding lithium availability for hybrid or electric vehicle batteries or other foreseeable applications are unfounded.” Tahil returned fire four months later with a July 2008 report, arguing that Evans failed to make a distinction between practically recoverable lithium carbonate and resources where lithium concentrations are too low to economically exploit. Evans’ document, wrote Tahil, “is not useful for the industrial and strategic planning purposes of the battery and automotive industries. It confounds geological lithium deposits of all grades and types with economically viable reserves that can be realistically exploited and relied upon as a dependable source of sustainable supply by the mass production scale of the automotive industry.”



Evans, keeping the debate alive, issued a quick retort. He argued that it wouldn’t take much of a price spike to economically recover lithium from spodumene deposits, which Tahil had ruled out. He added that other sources of lithium can also be extracted economically as the price of lithium creeps up, which will be necessary to unlock these reserves. “A rise from the current levels is probably necessary but the cost of carbonate in batteries is a very small percentage of the battery cost,” wrote Evans. “Where hectorites, geothermal brines, oil field brines and jadarite stand on the cost ladder remains to be determined.” Evans also called Tahil’s report “alarmist” and “ludicrious.” Ouch.



Of course, Tahil raises other concerns, such as energy security. It doesn’t make much sense, he argues, to move away from oil and all its geopolitical risk and toward lithium, which offers up another batch of geopolitical risk. China, for example, has its own lithium reserves but it’s unlikely to share that with the west. North America gets its lithium mostly from Chile and Argentina, and while Bolivia has huge reserves, that country is beginning to behave like Venezuela. In fact, according to TIME, both Toyota and Mitsubishi have been knocking on Bolivia’s door, hoping to get in on the lithium action, but nobody is answering. Mitsubishi has said that demand of lithium will outstrip supply in less than 10 years unless new sources are found. (Hat tip to Earth2Tech)



Perhaps Tahil’s assessment isn’t so ludicrous, after all. Besides, it’s not an issue of whether the resource exists, it’s a matter of who holds it, how much of it is accessible, at what cost, and at a given time. We saw what that perfect storm of factors did to the price of oil. And unlike oil, lithium batteries will be part of the cars when you buy them; we’re not talking fuel that you pump in later after the vehicle has been purchased. The question must be asked: How would a rapid, steep climb in the price of lithium affect automotive sales? Even if it was a short-term climb, it could have devastating effects.



Toronto-based TRU Group Inc., a leader in lithium resource research, issued a report last week — commissioned by Mitsubishi — which flicked at some of those long-term supply issues but didn’t seem overly concerned. In the short term, TRU said the economic downturn is actually creating a lithium glut. The market, it wrote, “will be pushed into oversupply this year through 2013. Global use of lithium will decline sharply by at least 6 per cent in 2009 and demand is unlikely to bounce back any time soon as consumers put off buying laptops or cell phones containing lithium batteries.”



Notice that there’s no mention of electric or plug-in hybrid vehicles in this period. The impact of those markets won’t begin to be felt until 2013, before which any introduction of the vehicles will be quite limited. Come 2015 the market will regain momentum:



The long range, however, remains bright because new and large uses for lithium will start having a major impact on demand within the five year horizon: Lithium use in electric vehicle batteries and lithium alloys for aircraft. TRU forecasts that demand will be strong and sustained in these two segments over the long term 2020. The industry does need at least one of the announced pipeline production projects to come into production and also could do with another new project as the market tightens around 2015-2017. New lithium producers still will need to be cost competitive with existing salt lake brine based producers in South America and China. Emerging technology may make some of the undeveloped medium sized (brine) lithium resources quite attractive. Certainly the industry through expansion and development of new resources will have no problem meeting demand.



The company said it would post its full report sometime on Tuesday.



So, does all this make you feel more comfortable with the lithium supply-demand situation? China and Chile certainly can’t complain. That said, this isn’t just about forecasting out to 2020. Lithium needs to support decades of growth in both the consumer electronics and automotive sectors, and while recycling of lithium will help, will it help enough?



That said, unlike oil/gas/diesel, the battery is part of the car and can easily be swapped out with different chemistries. By 2020, who knows what chemistries will lead the energy-storage race? To quote GM vice-chair Bob Lutz: “People keep saying we’ve used up the whole periodic table on battery composition and that lithium-ion is about as good as it gets. I don’t believe that.”



Besides, it’s not only new chemistries that could come along, so could technologies that blend different chemistries and energy-storage systems. I’ve got a piece today in MIT Technology Review about a new energy-management system developed by Indy Power that can take two or more different batteries/storage systems and balance them off against each other in a way that optimizes both performance and system life. The system is flexible, allowing multiple combinations with only a software upgrade. It means a car could be designed in the future that blends a little bit of lead-acid, a little bit of lithium-ion, with a touch of ultracapacitor.



If it could be manufactured for less than a 100-per-cent lithium-ion vehicle, if it got better performance, and if the life of each battery system was extended as a result, this could be the way to go…





Edited by juk - 27 January 2009 at 4:34pm

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Posts: 302       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 28 January 2009 at 11:51pm

Originally posted by woody



I saw a4x4kiwi contactors which split his pack into 12 x 48V packs and a 24V pack for charging. It made sense for me to join them all in parallel to 1 48V pack for use with a big charger.



The way to do this was DPDT contactors:





    48V Bus + -O  O- + 48v block - -O  O- - 48V Bus



                   /                        \



prev 48V block -O  O-(bypass)---O  O-  - next 48 V block







When the contactor was energised (show), the blocks join together to make the 600V chain, when unenergised, they join onto a common 48V bus for charging.



The reversing contactors have 4 terminals on each end. I thought they were double pole double throw, but they aren't are they, a reversing contactor is pretty much quad pole single throw, which you can wire as double pole double throw, I don't think I can do the bypass thing anyway unless I use even more hardware.





I think you might be wanting this:







It has the unfortunate effect that each group of cells has three contact sets in series with it (two from one DPDT contactor). But this way, each cell can be active, bypassed, or charging, and you can arrange for the no-power situation to be all paralleled to the bus (charging position).



Edit: Presumably, the coils of all charge/run contactors would be paralleled, so all cells would be in the run (or bypass) position, or the charge position. Also, you could use 1, 5, or as many chargers in parallel as you can afford. The charger bus would have to be thick cables, though, or the last batteries on the bus could be undercharged (though hopefully, the BMS would overcome this).



These contacts obviously have to be of the break before make variety, and DC rated at full pack voltage Edit: and pack current.



Edit: There are ways to reduce the number of contacts per group to 2, but I can't see any safe way that avoids race conditions.



- Coulomb



Edited by coulomb - 29 January 2009 at 12:11am

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Posts: 605       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 31 January 2009 at 10:25pm

Originally posted by coulomb





I think you might be wanting this:







It has the unfortunate effect that each group of cells has three contact sets in series with it (two from one DPDT contactor). But this way, each cell can be active, bypassed, or charging, and you can arrange for the no-power situation to be all paralleled to the bus (charging position).



Edit: Presumably, the coils of all charge/run contactors would be paralleled, so all cells would be in the run (or bypass) position, or the charge position. Also, you could use 1, 5, or as many chargers in parallel as you can afford. The charger bus would have to be thick cables, though, or the last batteries on the bus could be undercharged (though hopefully, the BMS would overcome this).



These contacts obviously have to be of the break before make variety, and DC rated at full pack voltage Edit: and pack current.



Edit: There are ways to reduce the number of contacts per group to 2, but I can't see any safe way that avoids race conditions.





Bypass is hopefully rare enough for it to be a manual operation, but depending on the pack location (e.g. On Ute Tray vs Under Body) it may be desirable to have it remotely switchable.



What did you use to draw the circuit?



cheers,

Woody

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Posts: 302       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2009 at 8:06am

Originally posted by woody



Bypass is hopefully rare enough for it to be a manual operation, but depending on the pack location (e.g. On Ute Tray vs Under Body) it may be desirable to have it remotely switchable.





You might change your mind if you have an older pack with a handful of bad cells.



What did you use to draw the circuit?





MS paint



- Coulomb

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Posts: 1253       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2009 at 10:33am

As a pack ages the monitoring of battery condition becomes more important as Coulomb says. Although the infancy failure needs to be monitored as well, it usually occurs when you are watching things closely anyway.



It seems that many EVers just wait for something to go bang. I have seen some rather scary battery demolition even in lead acid ! Although some failures can be spontaneous, these usually result from pulling too much current !



Fortunately, most batteries weaken evenly so the user gets the message to re-power without individual battery failure.



All this makes BMS important.



The use of contactors to bypass is IMHO not a requirement as batteries are generally reliable and you may be better off stopping altogether and going for the tow truck if there is a problem. This is assuming there is a BMS to say stop now !



One factor to consider is the contact resistance of all the contactors.

This can vary from 10 to 30mohm and will contribute considerable to the effective ESR of the battery pack.



Coulomb, keep thinking about the parallel charging of modules though. Add in some diodes in the charging lines. It would be intersting to see if you come up with something similar to my (yet to be posted (!) circuit. Your ideas will probably be better than mine so I will sit back !

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Posts: 302       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2009 at 3:14pm

Originally posted by acmotor



The use of contactors to bypass is IMHO not a requirement as batteries are generally reliable



Well, it just seems criminal that one bad cell out of perhaps 220 can stop you from getting home. Bypassing a module would allow an almost normal ride home.



One factor to consider is the contact resistance of all the contactors.

This can vary from 10 to 30mohm and will contribute considerable to the effective ESR of the battery pack.



Eek, that is a lot. A contactor per cell is definitely out, then. And I think bypassing might have to be done with an insulated spanner after all.



Coulomb, keep thinking about the parallel charging of modules though. Add in some diodes in the charging lines. It would be interesting to see if you come up with something similar to my (yet to be posted!) circuit.



I'm up for a bit of design, but what's the requirement? Are you concerned that paralleling so many groups of batteries might cause a lot of current flow as they even out?



Perhaps concerned that the last battery on the bus will get less voltage, at least till the others have charged up?



Edit: Can you find a way to use less contactor contacts with diodes used only for charging? That seems worthwhile.



Something else I've overlooked?



- Coulomb



Edited by coulomb - 01 February 2009 at 3:17pm

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Posts: 1253       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2009 at 3:52pm

Yes, cell failure can be an issue but some things we need to have faith in. Computer memory used to be parity checked. Vehicles used to have crank handles and they used to carry real spare tyres. My present dino burner has done 180,000 km without need of the spare (don't tell it though).

Monitor to know there is a problem but don't complicate or expensivate (yes folks, it makes sense as a word !!).

Mind you, module bypass is still an option.



Parallel charging does to some extent need to be controlled/ monitored so that individual modules don't receive overcurrent, although I have not seen this on the 12 modules in red suzi. Once the batteries are equalised once, they share charge quite well.



The idea is to use some blocking diodes rather than all contactors to distribute the charge to modules. This also limits any backfeed between modules in the case of a cell failure during charge. i.e. parallel charging has its downsides.



Your edit is onto the idea.

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Posts: 302       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2009 at 5:23pm

Originally posted by acmotor



Coulomb, keep thinking about the parallel charging of modules though. Add in some diodes in the charging lines. It would be intersting to see if you come up with something similar to my (yet to be posted (!) circuit.





Ah, I think you might mean something like this:





Now there is only one contact per group of cells. The diodes only carry charge current for one group, so they only need to be rated at fully charged pack voltage, and half the total charge current, plus some safety margin. Edit: diode current rating was misleading.



The movistor and fuse is in case the diodes break down and put (partial) pack voltage on the charge bus. Hopefully it would protect the other diodes and cells, and blow the fuse.



Edit: There should be fuses in series with each diode; the one in series with the charger is then redundant.



For extra effect, put a neon bulb (if they are still obtainable) in series with a pair of 100K resistors (for voltage rating) across the fuse, so you can see that there is trouble, and on which half. Unfortunately, if it's the first group or two, the neon may not light.



The vehicle chassis could be used as the common for one side of the charger bus, since the charging current will not be enormous.



A potential problem with this scheme is what happens if a contactor doesn't switch with the others. Kablooey   



Edit: Actually, that's not the case with the current version of the circuit. Please disregard the above sentence.



Also, each contactor has to break before the first contactor makes. So there is certainly room for improvement.



As before, you can have as many chargers as you want, though they have to be paired now. You could have say one pair in the vehicle for opportunity charging, and 6 more at home. (But that means a high current, relatively low voltage connector to connect to the external chargers, as well as a mains connector, which is perhaps not ideal). Maybe you could have all the chargers in the vehicle, but turn some of them off if you only have a 10A mains supply.



Also, this scheme does leave the possibility of a shock from two groups (so 140v DC shock if you manage to get your fingers across the two 70v charge buses). I suppose we could invert the negative batteries and put the whole lot in parallel. No need for paired chargers, either. Hmmm. Next post perhaps.



Your ideas will probably be better than mine so I will sit back !





I doubt that. Maybe I've saved you the trouble of drawing part of your circuit, though.



Edited by coulomb - 01 February 2009 at 6:51pm

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Posts: 302       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2009 at 5:35pm

Originally posted by coulomb



I suppose we could invert the negative batteries and put the whole lot in parallel. No need for paired chargers, either. Hmmm. Next post perhaps.



Oops, the negative side doesn't "invert" quite so easily.



So this arrangement might have to be confined to conversions where the positive and negative pack halves are well separated, either in separate boxes, or one at the front and the other at the back.



Improvement suggestions welcome.

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Posts: 302       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2009 at 6:58pm

If the pack fuses are put in series with the battery connections to ground, then at least any contactor problems like make before break or a flashover will be safe. (In the sense that blowing a multi-hundred amp fuse is ever "safe" )   Of course, that leaves all the batteries in one half of the pack at more than half the pack voltage; one end (across the fuse) is at full pack voltage. So that's probably a nono.

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Posts: 1268       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2009 at 7:46pm

coulomb quotes coulomb in coulomb's post is that a new version of coulomb's law ?



Close.

I place fuses and each diode at the battery module so wiring is safer. I also use 800V PIV diodes to cater for (most) fault conditions.

(both wires to each module are fused at the module)



Never use earth return for traction pack currents. It is not safe and you risk super-imposing voltage on the 12V aux system.

Run a separate wire.



At present I don't use the changeover function of the contactor since the contactors I use are not rated for full pack voltage. I use the multiway charger plug to collect the module wiring together for parallel charging. This does expect the contactors to have openned ! (but both lines to modules are fused 3A)



I don't see an issue with your circuit if any contactor fails to operate. The diodes protect against that. The associated module will simply not get charged.



Can you re-draw with fuses at the diodes ? Your circuit is worth thinking about.



edit:spelling





Edited by acmotor - 01 February 2009 at 7:50pm

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Posts: 326       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 02 February 2009 at 7:01am

Originally posted by acmotor



Never use earth return for traction pack currents. It is not safe and you risk super-imposing voltage on the 12V aux system.

Run a separate wire.





I don't suggest this; only charging current. Traction current will use the ends of the batteries (+350 and -350 wrt chassis).



Can you re-draw with fuses at the diodes ? Your circuit is worth thinking about.



Sure, but right now I'm rushing off to the Ausindustry Green Cars Initiative event. Brisbane is first cab off the rank.



- Coulomb

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Posts: 326       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 03 February 2009 at 9:58pm

Originally posted by acmotor



Can you re-draw with fuses at the diodes?





Edit: 2 right hand side diodes were wrong way around. Thanks, acmotor!



I'm not sure whether to include the contactors inside each battery group, or leave them, as shown, between groups.



I'd also prefer one charge bus rather than two, yet still allow the pack to be earthed in the centre.



Edited by coulomb - 04 February 2009 at 9:40pm

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Posts: 1268       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2009 at 12:38pm

Cut, paste and mirror has got the better of you !

Note diode A/K on positive charge bus.



I would still not use any earth return, even though your were thinking charge current only.

For two reasons....

Fault currents will also flow through earth (chassis). These may be more sizeable currents.

I would only use a single point connection (if used at all) between traction pack and chassis and this may be via a resistor anyway.



This second reason is also necessary to implement any earth leakage system.



Other than that, it is all starting to look very much like my arrangement (other than the changeover contactors as I have noted).

I do have some inline resistors to each module (1 ohm at present) to limit the unbalance of charge current when first starting the charge.



Try thinking one charger and multiple modules without the earth / centre and I think you will see there is a 2 wire bus that does the job.



I have included the contactors in the modules as this makes them a module !



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Posts: 326       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2009 at 9:52pm

Originally posted by acmotor



Cut, paste and mirror has got the better of you !

Note diode A/K on positive charge bus.



Oops!    Corrected now, thanks.



I would still not use any earth return, even though your were thinking charge current only.

For two reasons....

Fault currents will also flow through earth (chassis). These may be more sizeable currents.

I would only use a single point connection (if used at all) between traction pack and chassis and this may be via a resistor anyway.



This second reason is also necessary to implement any earth leakage system.



Yes, I agree. Will change.



Try thinking one charger and multiple modules without the earth / centre and I think you will see there is a 2 wire bus that does the job.



I thought of a solution in the productivity booth (shower) this morning. Replace the contactor earth connections with connections to the most negative part of the pack. When all is switched, this should no longer be -350v anyway. (It might be floating, or earthed, or earthed via a resistor).



So you have a one bus solution for the Rodeo? Yet the pack is earthed (via a resistor) at the centre tap? (When the contactors are live).



I think I'd want all the contactors to be rated at full regen pack voltage.



I have included the contactors in the modules as this makes them a module !



In the sense that you can just daisy chain the hot ends, and parallel the charge bus connections, and there are no other components between the modules?

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Posts: 326       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2009 at 10:20pm

Originally posted by coulomb



So you have a one bus solution for the Rodeo?



Ah, no, I'm guessing this is already done in Red Suzi, with this box:



BTW, are the fuses in there? Or in the modules?



When you say you can charge with up to 12 chargers... do you mean in series? I assume in parallel, for low voltage and safety. But maybe chargers don't parallel nicely.



You have me curious now.



Edit: minor rewording.



Edited by coulomb - 04 February 2009 at 10:23pm

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Posts: 326       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 06 February 2009 at 11:05am

Here is version 3 of my battery paralleling circuit; many thanks to acmotor for his many suggestions:







In this version, there is only one (set of) charger(s), so there is no possibility of a shock of two times the charger voltage. I've also made the contactors (a sort of distributed main contactor) part of the battery modules. Per acmotor's suggestion, there is only one connection to chassis.



The pack is still earthed in the centre, for maximum safety when active. However, the charger(s) have full negative pack voltage on them when active. This may stress their insulation, and a short to chassis or mains wiring will result in loss of on-board charging, along with a (small) fuse blow.



This circuit, more so than earlier earlier versions, assumes that the chargers and their mains inputs are not accessible when the contactors are active. Perhaps it would be better to have another contactor set that isolates the chargers when the contactors are active. Even so, the charge bus and its return are hot when the contactors are active, so these have to be suitably insulated (rated to half the pack voltage).



The 1K resistor near the bottom right corner is to bleed the VFD's bus capacitors without violent currents. It would need full pack voltage rating, and moderate power rating.



I'm still unsure where to put the pack fuses for maximum safety and effectiveness. At present, I'm thinking in series with the negative end of the leftmost battery module; that way any contactor flashovers would blow that fuse. Another one in series with the positive end of the right hand battery module would protect against pack+ shorting to chassis.

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Posts: 87       Quote evric Quote  Post ReplyReply Direct Link To This Post Posted: 06 February 2009 at 11:23am

Do chargers like being paralleled?

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Posts: 326       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 06 February 2009 at 12:28pm

Originally posted by evric



Do chargers like being paralleled?

An excellent question, and I'm not that sure. For bulk charging, they are essentially switch mode power supplies, and each of these has a slight droop with load. I guess that means that some of them might not provide as much current as others. In extreme cases where the voltages are not well matched, only one charger might provide any current at all. This would not be bad for the chargers, it would just increase the time to charge the pack, and waste some investment.



There is also the possibility that one might switch to float mode (assuming lead acid here) before the others, and would be idle until the others switched to float mode.



Acmotor, is that why you use different multipin plugs for different numbers of chargers? To put the chargers in series and avoid current sharing problems? What if one goes to float when the others have not yet; could the output go negative, and would that cause problems? Perhaps you use only the CCCV method even on lead acid, which avoids the issue with one charger going to a lower voltage (float stage)?



I guess with a multipin arrangement as acmotor suggests, you can always split the pack up into suitable pieces (e.g. 3 48v sets if you have 3 48v chargers), and each set would be independent. You would use just single throw contactors, instead of the double throw contactors I have drawn. If you had say 11 modules and 3 chargers, you would split into sets of 4, 4, and 3 modules paralleled. The charger for the set of 3 modules would be idle for something like a quarter of the total charge time, which is not too bad. You can of course avoid this altogether by choosing the pack and module voltages carefully, as well as the number of chargers.



The other thing is that you need the multipin connector (socket only) to be rated at pack voltage, or half pack voltage if you use two (one on each half of the pack). Also, all but one modules are floating in this arrangement without the plug inserted, so some wires could float to plus or minus half the pack or so (with a few hundred picofarads of wiring capacitance behind it). Inserting the plug would presumably ground the modules.



Finally, I initially thought that the multipin arrangement doesn't lend itself towards having some chargers onboard, and some offboard (e.g. in the garage or at a reserved carpark at work). I suppose the onboard chargers could be wired to the multipin connector, and a special plug used to connect the onboard charger to the pack in parallel. The plug could stay with the EV near the socket, perhaps chained to the box with the connectors so it would not get lost. If the garage charger assumed the presence of the onboard charger to charge part of the pack, it could have a small circuit incorporated that sounded an alarm if the onboard charger wasn't outputting voltage (e.g. its mains connector not plugged in).



Then again, with the multipin connector and single pole contactors that just separate modules, there would be no need for the diodes. So what are you using the diodes for, acmotor?



So yet again, acmotor's solution is looking good. It allows great flexibility, without changing the wiring of the vehicle. If I get time, I'll do a fourth version with the multipin connector.

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Posts: 1268       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 06 February 2009 at 2:19pm

I am running out of secrets !!!!

coulomb is a cleaver cookie.



Connectors are 300VDC 6A IP67 18pin

Carefull choice of pin / voltage layout was made. (the fuses and diodes back this up as well)

There is one connector for +ve bank and one for -ve wrt centre(ground)



All good thinking there coulomb.



VFD has internal bleed down resistors and 'fast discharge' option in software already so don't worry about 1k resistor.



If you think of the principle of any control / charge wire leaving the module needing to be fused then you need to add a fuse to the -ve line (charge bus return) at the changeover contactor, as you have already commented on.

That is then all very close to what I am running other than I use a multiway connector to collect the -ve lines together.

A contactor could be added to your circuit (also as you note) to isolate the charging system -ve side from the charger(s).



Re the question re my charging box on red suzi. There are no fuses in there. Fuses are all are at the battery modules.



There are 'dummy' plugs that I put in to the charge box when not charging to retain the IP rating and also these contain the links that are part of the charge interlock / 'ign' Estop wiring of the EV.



Now all this is not perfect and a little fiddly to enter the charge mode but for a first off 600V EV it is probably quite a safe arrangement.



It does need to be a little more automated for Jo average to use.



BTW

The diodes perform an essential task (IMHO) as they stop any backfeed from the battery module to charger caps and also stop any fault condition of one cell / battery from draining any other modules that may be in parallel.



edit:spelling !



Edited by acmotor - 08 February 2009 at 10:03pm

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Posts: 326       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 08 February 2009 at 9:12pm

Originally posted by acmotor



The diodes perform an essential task (IMHO) as they stop any backfeed from the battery module to charger caps ans also stop any fault condition of one cell / battery from draining any other modules that may be in parallel.



Ok, good points. But I still think it may be better not to parallel all the diodes' anodes before the multi-pin connector, so you can split the pack into separate pieces for charging.



Ironically, this eliminates the "charge bus" which is where this circuit started.



Example: 12 modules (6 each side of chassis), 3 chargers. You'd wire the multi-pin connector to split the pack into 3 sets of 4 paralleled modules. The negatives of each module could still be connected to the vehicle chassis (at one point only) for maximum safety when the contactors are inactive.



Then if you get a fourth charger, use a new set of plugs that splits the pack into 4 sets of 3 modules. If one charger dies, use your old cable that fed 3 chargers.



You can still parallel the whole pack with a third cable, for single charger use (on-board, solar, etc).



This way, no chargers are paralleled, so you never get any "lazy chargers", yet modules are paralleled to allow charging from a reasonable number of chargers. There is still at most one module's voltage potential at any point from the chassis (with the contactor inactive, obviously).



I think that will be version 4 of the circuit. Perhaps you don't have the anodes paralleled, acmotor? With 12 chargers and 12 modules, that would make sense, unless you lost say one charger. Then you might want to parallel all the modules and the 11 chargers to charge in 12/11th of the time, instead of twice the time (one charger driving two modules, and 10 chargers driving 1 module each).

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Posts: 105       Quote Tritium_James Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2009 at 10:57am

I realise I'm jumping in late to this topic, but one thing that scares me a bit with some of the circuits in this thread is failure modes.



For a BMS (and especially a lithium BMS) you need to think about what happens if something in your circuit breaks or ages. I think this is probably the biggest problem with the basic comparator switching a FET/resistor type designs - what happens when it's not sensing voltage correctly? This exact thing happened in the AC Propulsion balance nodes in the Porsche, they (after 6-8 years) started failing with the sense resistors and/or voltage reference getting out of whack. This means they were 'balancing' 12V PbA batteries down to 5 or 6 volts, which kills them pretty quickly!   This would potentially cause a fire in a lithium pack.



I think any serious BMS should have a micro on it (with watchdog, checksums, etc) and use two voltage sense circuits, which can be compared against each other. Any failure means that the unit defaults to not balancing at all, and shutting down the charger etc. Much better than over-discharging a lithium cell!



Just my opinion, of course! Please tell me if I'm wrong or paranoid!

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Posts: 1268       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2009 at 11:55am

You are most welcome to question and even be hypercritical of us cave men. If we can't handle that them we limit our ability to learn !

BTW, sorry if I have been first to respond to some of your posts this morning, I just happen to be at the computer and am quickly typing before others get to their keyboards !



Quite correct. Failure modes need to be considered.

Please humour me while I make some observations and excuses.



In your battery pack, was there other module or pack checks on voltage that would pick up the fault you describe ? As you know, reading things two ways can often pick up faults. BTW what BMS was being used ?



A TS lithium cell (LiFePO4) will not catch fire if discharged to zero. (it may be killed and have subsequent problems)



In the BMS circuit in this thread, the difference between micro drive and prog zener drive to the shunt transistor base is your point ?

Either system would require some shunt components (or charge shuffling components) so the question is, would a micro be better than the prog zener since the power side remains the same ?

The rest of the circuit is designed in the failsafe mode (on = good)

If there is a failure to 'on', the shunt probably still works as it is a separate circuit.

Regen (to me) means that the BMS at cell level needs to have a realistic shunt capacity. In the order of amps not mA.



Part of this overall BMS is monitoring module voltages (20 cells) and pack voltage (Yes, with a micro) to see it all adds up OK. This checks for wiring / terminal /contactor and fuse faults. It also checks for those inevitable BMS board failures !



A bipolar is used (normal fail to s/c in this type of circuit) along with parallel power resistors (normally fail open).

A bipolar failure will result (eventually) in a U/V alarm. resistor failure will result (eventually) in O/V alarm.



A micro is not immune to faults, so using a micro is not a gaurantee of function. ( I work with C8051F060,120 and 350 along with atmel mega and various PICs, mostly in C but also Bascom )

Yes, a lot of software goodies (and WDT) and the lack of (some) human assembly makes micros far more reliable than 'dumb' electronics.



Another factor is the cost. Once the BMS cost as much as or even 1/4 the price of the cell, there is the economic problem.



I have built a PIC based BMS. Reproduction cost was an issue and in reality there were probably more features than are actually required for Lithium monitoring. All you need is O/V U/V detection and some eq. shunt. I did also find that the environment on to of the cell was not friendly. Massive EMF, potentially high temps (particularly if there was a terminal contact problem). Once again, as you say, failure modes need to be considered.



No you are not paranoid. But lithium cells are not as evil in the BMS department as was early thought.



After all that, I do agree that the built in micro smarts are the future. I am probably just fighting the inevitable



Please, you are welcome to flame !

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Posts: 326       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2009 at 11:50am

The battery paralleling circuit, which is getting a bit off topic, is continued at this thread.



Just to save people time searching (as I just did).

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 7:16am

I've been playing around with some TS LFP40s and trying various cell balancing methods and chargers. My design based on the EE Times article I saw a while ago is not yet perfected, but looking promising. I'll post details when I get it working OK.

In the meantime, I'm using a 64V, 6A current limited power supply to charge a series string of 16 cells with the voltage clamp circuit as shown to prevent over charging. I set the turn on point to 4.1V and the clamp is in full conduction at 4.13V. It carries the full power supply current (6A) easily with a 50 x 30 x 30 Al, L-shaped heatsink (3 mm thick) although I find that the clamp doesn't turn on for any cell until it is nearly fully charged and the current is down to an amp or so. At that level the heatsink gets barely warm.

For the low voltage end, I am just using a led voltmeter that comes on at 44V which is about 2.7V per cell and that has been adequate so far. The battery pack has now done 16 cycles using this setup and I can't detect any change - and I should hope not to!

It does rely on the charger being current limited, of course, but so do the batteries.

I built it (16 of "it") on a small pcb designed to fit onto the LFP40 terminals with the heatsink on top, so it just screws on, one to each cell.

I'll build a bigger one to fit LFP90 cells next - they are the cells I will likely use in my car, although I'd really like '160s, but the cost is a bit of a deterrent.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 87       Quote evric Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 8:11am

Neville, Don't you think the clamp turn on point of 4.1V is a bit high? Ohters are using 3.65V etc.

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Posts: 1268       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 9:14am

Good point Nevilleh.



One way to limit overvoltage is to have shunt Amp capacity > charger Amps. (provided you do not have regen (!)



The fuse rating may be an issue. T03(P) TIP142 is Ic max 10A (pulse to 15 but that's not fuse blowing stuff) so a fast acting fuse of 6A may be more suitable given the charger current ? Mind you, it has all gone wrong by then. Just limiting the damage when you use only a semiconductor as the heater !



Check the heatsink capacity °C/W and follow the power de-rating curve for TIP142. Probably no issue given your observations so far.



Shunt voltage is still personal choice as a definitive number is still out there somewhere ! TS say 4.2-4.35 but....



Are you going to include over and under voltage monitoring ?



Have fun. Keep us posted !

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 9:37am

Originally posted by evric



Neville, Don't you think the clamp turn on point of 4.1V is a bit high? Ohters are using 3.65V etc.



TS quote 4.2v as the fully charged maximum. This circuit turns fully on at 4.13v with the pot setting I use - the voltage gain is enormous with two darlingtons - so the voltage can't go any higher than that. Others are using a lower voltage for maybe two reasons: their circuit doesn't turn on so quickly, or, they are just being more conservative on the upper voltage limit. If you look at the TS charge graphs, the voltage is limited to 4.2v and held there for 70-odd minutes as the current falls to zero. At the time point where 4.2v is reached and the current starts to drop, the % charge is only 70.

The main reason I dropped it from 4.2v to 4.13v was that I read Tesla Motors have dropped their max charge voltage for 18650 cells from 4.2v to 4.15v "in the interests of increased battery life".

I have noted that if I charge the cells at 3.7V until the current drops to zero, then increasing the voltage to 4.2v causes a current flow (at the 6A limit) for only another 10 minutes or so, so they must be close to fully charged at that voltage.

As far as my car is concerned, I also wonder if I use 4.2v per cell as the max charge voltage, the whole pack of say 46 cells now produces 193.2v instead of the nominal 3.2x46 = 147.2. Sure it's only for a few minutes, but can my 156v rated controller handle that?

I'd be interested to other opinions.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 668       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 9:45am

Hi Neville. Why not remove the PNP darlington? The LM431 could drive the base of Q2 from it's Anode quite comfortably. Reduce semiconductor count by 1?

Reference R2 to Anode and place R1 from Anode to -ve (via fuse).

Like this but use your TIP142.





Just a thought... so many less pins to solder.

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 9:59am

Originally posted by acmotor



Good point Nevilleh.





The fuse rating may be an issue. T03(P) TIP142 is Ic max 10A (pulse to 15 but that's not fuse blowing stuff) so a fast acting fuse of 6A may be more suitable given the charger current ? Mind you, it has all gone wrong by then. Just limiting the damage when you use only a semiconductor as the heater !



Check the heatsink capacity °C/W and follow the power de-rating curve for TIP142. Probably no issue given your observations so far.



Shunt voltage is still personal choice as a definitive number is still out there somewhere ! TS say 4.2-4.35 but....



Are you going to include over and under voltage monitoring ?



Have fun. Keep us posted !





I designed the heatsink to dissipate 40 watts (Ic at 10A or so) and keep the junction temp below 150 deg, so its got a bit in hand. The fuse is that size because I had one and its main function as you surmise is to stop molten silicon spewing everywhere if I get something wrong!

I am working on a micro to monitor volts - since I know a bit about Atmel's AVR ones, I'm using an ATTiny25 Its only a couple of dollars and has 4 x 10 bit ADC in an 8 pin package and a serial comms capability that will do I2C. I figure to use one for each 4 cells to send volts to a master which may yet be my laptop, but might just be another Atmel chip driving a biggish LCD as I am much better at assembler and C than VB.

Problems yet to solve are bus addressing - I need to have the master assign addresses - and to decide if the accuracy is high enough at about 10mv resolution. I think it is and should be OK, especially if I set my upper and lower voltage levels a bit conservative.

The lower end particularly. Looking at the TS graphs again, the capacity drop from 3v to 2.5v is b--- all, so you might as well stop at 3v and not risk damaging the cells.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 10:03am

Originally posted by Johny



Hi Neville. Why not remove the PNP darlington? The LM431 could drive the base of Q2 from it's Anode quite comfortably. Reduce semiconductor count by 1?

Reference R2 to Anode and place R1 from Anode to -ve (via fuse).

Like this but use your TIP142.





Just a thought... so many less pins to solder.



Not a bad thought. I originally used the pnp darlington to get heaps of gain and so a really fast turn on. I added the npn led driver later so I could "see the light". Might try your suggestion on my Veroboard one and see what the difference is.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 10:18am

Well, I tried it and it isn't very good. Starts to turn on at 3.6v and is not in full conduction until 5.2v (without altering the pot setting from the original). I think the extra transistor is well worth it for the "snap-on" effect.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 668       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 10:34am

I should have looked harder. The TIP142 is not quite the conventional darlington (well not as I perceive them).



The culprit is the 0.12 Ohm from base to ground on the second transistor. It gives it a sloppy turn-on characteristic. Done for speed me thinks.

Thanks for trying it.

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Posts: 94       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 10:56am

Yes, it's designed as a fast switch. I only used it because I had some from an SMPS project. If I was buying any, I'd probably look at something else as it is getting a bit old - the 140 would do OK as in this circuit it doesn't need the voltage rating of the 142.

Anyway, the circuit as is doesn't cost much and it works a treat!

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 668       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 11:01am

I totally understand. I have various projects using TL494 PWM controllers which I use just because I have them!

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Posts: 650       Quote antiscab Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 5:56pm

Originally posted by Nevilleh





TS quote 4.2v as the fully charged maximum. This circuit turns fully on at 4.13v with the pot setting I use - the voltage gain is enormous with two darlingtons - so the voltage can't go any higher than that. Others are using a lower voltage for maybe two reasons: their circuit doesn't turn on so quickly, or, they are just being more conservative on the upper voltage limit. If you look at the TS charge graphs, the voltage is limited to 4.2v and held there for 70-odd minutes as the current falls to zero. At the time point where 4.2v is reached and the current starts to drop, the % charge is only 70.

The main reason I dropped it from 4.2v to 4.13v was that I read Tesla Motors have dropped their max charge voltage for 18650 cells from 4.2v to 4.15v "in the interests of increased battery life".

I have noted that if I charge the cells at 3.7V until the current drops to zero, then increasing the voltage to 4.2v causes a current flow (at the 6A limit) for only another 10 minutes or so, so they must be close to fully charged at that voltage.





the difference in recoverable AH, between charging to 3.4v and 4.2v is 2.5%, based on the bench testing i did on a single cell (TS 40AH) a while ago.



from a BMS design perspective, charging only to 3.6vpc with a 4.2v limit reduces complexity somewhat (dont need to shunt full charge current).



you can also use more cells on the same peak voltage rated controller



it also increases charge efficiency, as you wont *have* to balance on every single charge.



as far as the effect on cycle life, im still figuring that one out (ive been charging to 3.6v for more than a year now).



Originally posted by Nevilleh





As far as my car is concerned, I also wonder if I use 4.2v per cell as the max charge voltage, the whole pack of say 46 cells now produces 193.2v instead of the nominal 3.2x46 = 147.2. Sure it's only for a few minutes, but can my 156v rated controller handle that?

I'd be interested to other opinions.





if the logisystem guys are still using 200v parts, then starting at 193.2v is indeed a little dangerous. a little overshoot on the powerstage and BANG.

that takes about 1 sec.



if you let your bms pull the pack voltage down before starting, you should be fine.

Id ask the logisystem guys what the max operating voltage of their controller is.



Matt

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Posts: 1252       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2009 at 8:55pm

Still pondering this recharge voltage myself.



This post raises lithium top voltage vs cycle life issue.

Although generic lithium, I guess even Tesla have tugged the top back a bit. http://www.aeva.asn.au/forums/forum_posts.asp?TID=983



BTW Nevilleh, the circuit at the start of this post turns full on in .05V

The 431 is op-amp output stage.

Electric Vehicles, our future, now.

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Posts: 93       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 26 March 2009 at 2:38pm

acmotor's clamp turns on in 50 mV, mine turns on in 3 mV. I also tried it with an MJE3055 instead of the TIP142 ( I don't have enough '142s) and it works nearly as well, switching full on in 5 mV.

The question is, how fast does it need to switch? Is the reduced parts count worth the slower turn on?

I tend to think of it as the automatic safety device to prevent the cells from over charging, but the charge voltage seems to be OK anywhere from about 3.6v up to 4.2v, so if you have a clamp that turns on on 50 mV, that's probably quite sufficient and if you have to make hundreds of the things, a few less bits is very desirable.

But acmotor's transistor is only half an amp Ic and I think one should allow for a lot more than that, up to the charger current limit. I am inclined to think that the LM431 driving a pnp Darlington with an Ic of at least 6A would be the best solution. That's what I will try next!

After all, I want to stick it on an LFP160 - eventually.



Edited by Nevilleh - 26 March 2009 at 2:43pm

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Posts: 637       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 26 March 2009 at 2:51pm

IMO your PNP darlington (or NPN on the anode side) would be fine. Just don't use a super fast one that has a built-in low value resistor like the TIP14? family.



BTW I know I am coming from a 2 way Radio perspective here but I notice that no-one is including RFI suppression. The CBer or Taxi/Police car/Fire truck that pulls up next to you and transmits might have horrible effects on your BMS. Even the odd cell phone might be a disaster.

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Posts: 1252       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 26 March 2009 at 6:09pm

Johny, true about RFI but at least all my battery boxes are metal so it can only get in via wiring. You could start adding some ceramic caps but take care, it could cause more problems than it fixes. You are welcome to make suggestions !



The hundreds of amps flowing in the battery wiring is more likely to be a problem ( I found it was with pic BMS ).



Nevilleh, I was a bit concerned about too sharp a turn on as it may result in oscillations since the BMS shares the same terminals as the actual battery working load.



Spotted as well....

I ended using KSH127 a surface mount darlington as the shunt transistor.data sheet for 2 reasons. First, the Ic, second the price at digikey. I'll update the circuit on the post.

Electric Vehicles, our future, now.

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Posts: 637       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 26 March 2009 at 6:29pm

Yes it's all a nasty environment. Standard electronics in cars is not usually a problem but we have destroyed after-market mosfet car-stereo amplifiers with 25W of RF at 160MHz into properly matched antennas. rural Ambulance is an interesting proving ground.

Since the wiring would be the main coupling point I would suggest a 1nF straight across the battery terminals on each BMS. As you say this isn't a big issue but better safe...

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Posts: 1252       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 26 March 2009 at 6:39pm

Johny, Good point you raise. It should be included in home grown EV conversion design / testing.

OK, your job is to specify the test proceedure !

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Posts: 196       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 31 March 2009 at 10:56pm

Originally posted by acmotor



Here is my circuit again just to start the topic here.



I can't help thinking that instead of only a single bit of information, we could have an analog quantity represented on that single optocoupled daisy chain, so the motor controller or charger (including regenerating VF drive) could have some warning that things are getting bad and maybe even use the feedback in a linear control loop.



The idea would be to use linear optos and have the opto with the lowest output current calling the tune on the daisy-chain. When every cell is at say 3.3 volts then the daisy-chain current might be at a maximum value, say 20 mA. This might drop by 2 mA for every 0.1 V deviation either above or below 3.3 V. So if, during discharge the lowest cell was at 2.5 V the daisy-chain current would go down to 4 mA. The same would occur if, during charge or regen, the highest cell went to 4.1 V.



Temperature measurement could be included in this. 25^C might be considered optimal and a 30 kelvin temperature change considered to be just as bad as a 1 volt voltage change. Whichever was badder, temperature or voltage, would determine how low the daisy-chain current went.

Working with Coulomb on converting an MX-5

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Posts: 1252       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 10:40am

OK so the most worserer of the badders sets the shutdown. I get it

Keep thinking on it. There is potential.

Final cost must also be kept in mind.

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Posts: 637       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 10:53am

The problem is that linear opto-couplers are fickle devices. In order to get good linearity and defined gain stability they usually require a "servo-loop" back to the driving LED. This would be a pain when trying to keep things simple.

Maybe there is something better than what I have used in the past but they left me wishing I had "done it" another way.

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Posts: 300       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 11:03am

As a point of interest, are the FET output opto isolators any more linear? They would presumably also have less voltage drop (when conducting fully), important when a large number of optos will be in series.



Also, I'm not sure that linearity is necessary, just monotonicity. In other words, when things go from badder to badderest, it's important that the resistance increases (assuming low resistance = least badness), not that 37% extra badness will result in 37% +/- 5% increased resistance. (Weber: no, you won't ever live that one down    )



The overall feedback loop should still settle down to keep all the batteries fairly happy (small badness), while the vehicle has maximum performance.

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Posts: 300       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 11:14am

Originally posted by acmotor



Final cost must also be kept in mind.



Yea, verily.



I'm thinking that diodes rather than low voltage comparators or op-amps could achieve the required "analogue logic". And still be cheap. Perhaps the analogularity (TM) could extend to the red LED: a dull glow from the LED means that this cell is the one causing the cutback of regen or output, but is only a little way from its happy zone (either temperature, over, or under voltage). A bright red LED indicates "I'm really bad, help me!".



It would be good to have a fair range of voltage where there was zero badness reported, e.g. 2.7 to 4.0 volts should be zero badness, with moderately steep ramps at both ends. Same for temperature (probably single ended, so extreme cold is not considered bad). Otherwise, as soon as you drive off, you will start tapering off performance. I believe that the programmable zeners used in acmotor's design can be easily configured to do this.



Edit: The point of this is of course to prevent the controller or charger from continually "hunting" about the badness threshold. Suppose one cell is getting hot; let's say it's at 140°C and we want to keep the batteries under 150°C at all times. With a digital output, you have to choose a safe threshold, say 140°C. The controller doesn't know how much to cut back, so we cut back 10%. The cell rises to 145°C, and the output is still bad. So the controller cuts back 20%, eventually the cell falls to 140°C but by then the controller has cut back 50%. Now the controller goes back to 100% output, and the cycle repeats. In this case, it's a slow cycle, because the time constant for temperature is long. For regen, you might get a bumpy ride.



In the analogue case, we have a ramp that goes from minimum to maximum badness over the 140°C to 150°C range, say. As soon as the cell gets just a but hot, the controller backs off a bit, say 5%. It might settle there, at 142°C and 5% less power. This is a lot better than cutting in and out with wild overshoots.



I guess I'm not being fair to the digital case here; if in the digital case we cut back in 5% increments, the same outcome might be achieved, though it still has to hunt. But what if a cell is rocketing up in temperature? In the digital case, the best you can do is cut back 5%, wait 10 seconds, and cut back another 5% if the badness remains. Meanwhile, the cell could have burned. To prevent that, you have to cut back quicker, at the expense of possible wild overshooting.



In the analogue case, if the temperature shoots up, the output goes quickly to maximum badness, and the controller can shut down quickly. So I'm saying that the analogue output could provide much smoother operation, and/or protect the battery better.



Edited by coulomb - 01 April 2009 at 11:31am

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Posts: 1252       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 12:03pm

We do need to keep in mind what we are actually trying to achieve with the BMS, otherwise we will all have to buy BMI from LBB ! and leave your phone on so it can send you an SMS !



You must stop or (at least reduce) charge or discharge when you get O/V or U/V. You must provide some charge eq.

After that, you are just playing with data and if temperature for instance is an issue then you are most likely pushing the cells way beyond their design rating. IMHO



Understood about digital vs analogue but you know the old saying, there is no such thing as a little bit ........ !



But don't let me put you off. If you can collect more data and still keep it simple, low cost and reliable then got for it.

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Posts: 196       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 12:16pm

Ah'm back and ah'm bad(-er).



Originally posted by coulomb



As a point of interest, are the FET output opto isolators any more linear? They would presumably also have less voltage drop (when conducting fully), important when a large number of optos will be in series.



FET output is resistance output, which merely adds when wired in series, rather than giving the required MIN or MAX function. Bipolar transistor outputs and photodiode outputs are current outputs and when wired in series these give an (approximate) MIN function. But the two have to be sensed in different ways.



The bipolar transistors require you to put about n * 0.3 V across the string and measure the current. So for 200 cells that's 60 V. If one opto wants to pull the current down to zero it has to take the full 60 volts across itself. That's not a problem.



The photodiodes (essentially tiny photovoltaic cells) don't need an external applied voltage, they produce their own approx 0.3 volts each. But they produce a lower current than the transistors, and if one of them wants to reduce the current to zero, it has to take the full 60 volts generated by all the others, across itself _in_reverse_. This aint gonna happen. They can typically only take 5 V reverse. So can't wire more than about 16 of them in series. But that might be OK. 16 LiFePO4's is a 48 V module.



Also, I'm not sure that linearity is necessary, just monotonicity.



I agree, but as Johny mentioned, what is important is that they are all about the same and that they don't change with temperature etc. Typical transistor gains, LED efficiencies, PV efficiencies taken together, vary +-100%. A standard "linear opto" deals with this by having two identical photodiodes. One is your output. The other is your feedback, which you then need an op-amp to deal with.



And you're stuck with a photodiode output in that case, because if you amplify its current with a transistor you then have the highly variable transistor gain unaccounted for.



What did you have in mind re diodes?

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Posts: 196       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 12:43pm

Originally posted by acmotor



If you can collect more data and still keep it simple, low cost and reliable then got for it.



Your circuit is a masterpiece of minimalism. The best I can do to reduce it further is save maybe 2 resistors. One by taking the bypass and overvoltage sensing off a single 3-resistor divider. The other (maybe) by putting the LED where the transistor's base resistor should be.



In theory you could also get rid of the two power resistors and heatsink the transistor instead, e.g. heatsinking it to the battery terminals with fat copper strips as in Rod Dilkes BMS. But I'm sure the resistors are cheaper and simpler and they can run at a higher temperature and so radiate more. It's a shame you can't buy 25 or 50 watt 4 volt lamps. They would be even better than resistors and you wouldn't need the LED. With a clear cover on the batteries and a black painted underside of boot or bonnet above that, they would get the heat right out of the battery box without needing a fan.



I just want to see if we can make some kind of minimal changes such as replacing some precision zeners with a quad op-amp and using a different kind of opto, to get a little more info out on that single wire, to avoid the overshoots by knowing in advance when the badness is coming.



Your bypass circuit could stay as it is now, however if temperature sensing was included it might also be used to temperature-compensate the bypass voltage. Fixed-bypass-voltage boards might overvoltage the cells if the vehicle was sold and moved to a hotter climate. And even in one place, summer winter variation may be significant.



The apparently high voltages specified by Thunder Sky might be due to low ambient temperatures in their part of China.



Edited by weber - 01 April 2009 at 12:46pm

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Posts: 637       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 12:50pm

If anyone wants to get a lot more complex this might be of interest.

A single chip that monitors up to 12 Lithium cells. US$9.95 in 1000 lots.

Can also be used to drive FETs for current sink.

LTC6802 Info

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Posts: 1252       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 1:08pm

Nice but not KISS.



It starts off OK but look at page 36.



It is definitely a sign of good things to come though !



edit: added link to data sheet.

LTC6802 data sheet



Edited by acmotor - 01 April 2009 at 1:51pm

Electric Vehicles, our future, now.

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Posts: 637       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 1:14pm

I did say a LOT more complex.

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Posts: 1252       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 1:49pm

Maybe the new UWA Lotus EV could go this 'lots a data' direction ?

Electric Vehicles, our future, now.

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Posts: 196       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 01 April 2009 at 2:30pm

Here's another idea, in the spirit of brainstorming.



Instead of analog current we use PWM current. We'd need optos going in the opposite direction (two optos per cell) to synchronise. But we'd still use only the single-wire loop.



The LEDs of the input optos could be in parallel with the collector-emitter of the output optos.



Start with all output optos off (o/c) and n * 0.3 V across the loop. Then the master unit puts a big enough voltage pulse (but very narrow) across the loop, to pulse all the input LEDS. This would take something like 1.5 V per cell, so maybe 300 V, but current and energy limited. Done using an inductor flyback circuit. After the n * 1.5 V pulse the master drops the voltage back to n * 0.3 V and watches the current.



All the cell boards respond to this by immediately turning on their outputs (s/c). Then they wait a time proportional to their goodness before turning off again.



goodness = 1 - badness

goodness and badness have values between 0 and 1 (or 0% and 100%)



So the baddest one determines when the whole loop goes o/c. The master waits a fixed time between n*1.5 V pulses. If all cells are 100% good (0% bad) then their output optos all go o/c just before the master sends the big pulse again. If the loop hasn't gone o/c by that time, the master does not send the pulse but registers a fault. Or it might be smart enough to wait just a bit longer and alter its cycle time accordingly.

Working with Coulomb on converting an MX-5

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Posts: 93       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 8:33am

Lots of interesting ideas here!

But I am going with a simple clamp to stop any cell going too high and using fixed resistors in my original circuit sets the activation volyage at 3.93.

In addition, I use an Atmel ATTiny25 to monitor the voltage of each cell. The Tiny 25 has a 4 channel, 10 bit ADC, so one micro does 4 cells. It also has a built in temperature sensor and I plan to use that as the pcb will be attached to the top of one cell in each block of 4.

The Tiny25 is about $2.50 - 3.00 depending on where you get it from.

Using it's SPI capability lets me run a single line, multi-drop comms setup, at the moment using optocouplers ($1.00 or so each) but I might try just capacitive coupling instead - save a few dollars.

The system uses a single byte for addresses. Each micro is programmed as $FF initially. The first one to be turned on talks to the master which allocates it an address in sequence starting at $01 ($00 is reserved for the master). Thus they all receive their own unique address simply by turning them on one at a time and putting a jumper on the pcb is the easiest way to do that.

The master will then just poll them all in turn and receive the 4 cell voltages associated with that particular micro, plus it's temperature.

The comms is async and around 9600 baud is more than adequate. That is about a millisecond per byte (10 bits with start and stop bits) and each data packet will contain an address, 4 cell voltages in 5 bytes, temperature and a CRC byte ie 8 bytes, so 8 msecs per device or 125 per second. I think that is fast enough!

Being a polled system, there are no data collisions and the CRC will provide enough protection against external corruption, although 9600 bps is fairly incorruptible.

Anyone else who wants to try this, drop me a note and I'll give you the software - when its finished!

The master software will collect and store the cell voltages and raise an alarm if any one (or more) goes above or below whatever limits I put in. This is actually the biggest part of the job as I want to use my laptop as the master and my VB skills are very rusty. Any experts out there want to help out?

We could make it an open source thing if enough people are interested.



I tried to upload the circuit diagram, but the image is too big. I'll have to find another way.



Edited by Nevilleh - 03 April 2009 at 8:42am

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 300       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 10:19am

Originally posted by weber



What did you have in mind re diodes?



I've already discussed this face to face with Weber, so this is for others.



The idea is that internally you have a voltage representing temperature badness, say 1-3 V, and another representing voltage badness (also 1-3 V). Maybe two voltage badnesses, one for under, one for over.



Then you just have 2 or 3 diodes that have common cathodes, anodes connected to the internal badness generators. Whichever voltage is the highest gets its diode to conduct (the others will be reverse biased). So the common cathodes have the highest badness, less one diode drop. This is used to generate the badness current.



In practice, we want 0 badness to be max current (so a break in the daisy chain is an error, not "all OK"), so we would invert all this. Each badness signal would be 3-1 V (3V = no badness, 1V = max badness), and common the anodes. Now the signal with the most badness has the lowest voltage, and will conduct its diode, so the output will be the same as the baddest signal, plus one diode drop. A resistor turns this voltage into a current into the opto's LED.

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Posts: 100       Quote Tritium_James Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 11:05am

I can almost guarantee you a capacitive coupling scheme for the comms won't work - I've tried it in the BMS for the Porsche and it runs fine until you have either the battery charger running or the motor running. In the charger case it moves the whole pack around at 1/2 the mains voltage and 100Hz (this will be ACP system specific) and when it's motoring there's fast transients from the PWM switching. The capacitive comms stopped working fairly rapidly. We were doing it properly too, with manchester encoded comms to keep a constant DC value on the caps, idle time filled with 50% transitions, shielded twisted pair cable, all that sort of thing.

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Posts: 637       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 11:09am

In addition you would have to provide ground-supply diode clamping which could well introduce RFI issues (diodes acting as detectors). Reasonable speed optos are very cheap when bought in quantity - even 100 or so.

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Posts: 300       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 11:15am

Originally posted by weber



Here's another idea, in the spirit of brainstorming.



Instead of analog current we use PWM current. We'd need optos going in the opposite direction (two optos per cell) to synchronise. But we'd still use only the single-wire loop.



The LEDs of the input optos could be in parallel with the collector-emitter of the output optos.



I like this idea much better than attempting to use optos in linear mode. The characteristics of optos vary so much. For example, current transfer might be minimum 20% (so 10mA diode -> 2mA transistor), but is typically 70%, and there is no maximum value. Similarly, forward voltage drop is typically 1.15 (for a 4N25), but has a max of 1.5v. It is also temperature dependent, which could be used to advantage, but I suspect it will vary too much from device to device. I think I'd really prefer not to have to adjust 200+ BMS boards.



So the PWM idea is good; it uses optos in digital mode (where they work best), yet we get analogue information.



Another consideration: we'll likely need a quad CMOS op-amp device to generate the various "ramps" we require. A typical device (e.g. TS914) might require a minimum rail voltage of 2.7v, yet we expect the circuit not to go nuts with the cell at 2.5v or (hopefully rarely) less. There are low voltage op amps (e.g. 1.8v), but these may be expensive and possibly hard to find (I haven't looked).



A single programmable zener device could suffice; the 1.4 V reference could be used at one input of all 4 op-amps. The four op-amps would presumably handle low volts, high volts, high temperature, and bypass. That's assuming we don't need an op-amp to handle the timing. An LMC555 works down to 1.5 V, so that's a likely contender.



Edited by coulomb - 03 April 2009 at 11:16am

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Posts: 616       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 11:28am

OK. I just reread weber's PWM pulse idea and I now understand it. Low voltage comparators are cheap, not sure about Op amps.



BUT by the time you have added a timing component (say 555) and a quad comparator, maybe the Tiny25 that Nevilleh is using would be simpler.

Since it could do 4 cells, it sounds way cheaper and 4 times simpler (or more) to make (since there is only 1 for every 4 cells AND it has less components).

Once you get there, why not a serial bus using opto couplers (as per Nevilleh) - and away we go again...sorry

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Posts: 190       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 11:38am

Originally posted by coulomb



An LMC555 works down to 1.5 V, so that's a likely contender.



Yes! That's what's been missing from this thing. Everyone knows every DIY circuit must have at least one 555 timer in it, and preferably a 556 dual. And it must use those timers in the most obscure manner possible.



Actually, I had this web page up and was brushing up when your message arrived. http://home.cogeco.ca/~rpaisley4/LM555.html



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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 06 April 2009 at 12:04pm

Originally posted by Tritium_James



I can almost guarantee you a capacitive coupling scheme for the comms won't work -





Oh well, it was just thought anyway. You've saved me the trouble of even fiddling with it. And, as you say, optos are pretty cheap.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 190       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 10 April 2009 at 11:02pm

Originally posted by Nevilleh



Lots of interesting ideas here!

But I am going with a simple clamp to stop any cell going too high and using fixed resistors in my original circuit sets the activation volyage at 3.93.

In addition, I use an Atmel ATTiny25 to monitor the voltage of each cell. The Tiny 25 has a 4 channel, 10 bit ADC, so one micro does 4 cells. It also has a built in temperature sensor and I plan to use that as the pcb will be attached to the top of one cell in each block of 4.

The Tiny25 is about $2.50 - 3.00 depending on where you get it from.

Using it's SPI capability lets me run a single line, multi-drop comms setup, at the moment using optocouplers ($1.00 or so each)...





Hi Nevilleh,



I'm getting interested in a microcontroller-per-cell arrangement. I thought the ATtiny SPI was only for programming. I didn't think you could use it under software control. Do you mean the USI?



My thought was to avoid having to give them IDs at all, by daisy chaining them in a ring with the master. So the master prompts the first cell which sends its voltage and temp data to the second cell which passes on the first cell's data and adds its own on the end, until you get to the end where the master gets all the data. Each cell is effectively ID'ed by its position in the daisy-chain. The master might have several UARTs and run several rings to avoid any one ring being too long.

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Posts: 310       Quote Electrocycle Quote  Post ReplyReply Direct Link To This Post Posted: 11 April 2009 at 8:13am

yeah you'd have to watch out for the chinese whispers effect there :-)

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Posts: 282       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 11 April 2009 at 8:24am

Originally posted by Nevilleh



I tried to upload the circuit diagram, but the image is too big. I'll have to find another way.



You can zip it and upload it as a file; find the File Manager in the Member Control Panel. Then just let us know it's there, and/or post a link to the uploaded file.



Edit: files have a limit of 1 MB; images a limit of 100 kB. So you could upload a 1.5 MB image if it zip compressed to 1 MB.



Edited by coulomb - 11 April 2009 at 6:57pm

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Posts: 596       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 15 April 2009 at 12:40pm

Something which came up on the ThunderSky list: if you can join multiple cells monitors onto the same heatsink, then you get a better deal, since mostly one cell will be in bypass at a time, when more than a few cells come onto bypass, the charger should be ramping down anyway.



e.g. Tuarn's BMS board for 20 cells should have 1 or 2 big heatsinks, not 20.



And maybe a small fan.



Another possibility I thought of was a clamp/bypass for the whole cell group, which maybe higher capacity - suitable for high regen - sort of a per module braking resistor perhaps.



discuss/flame/etc. :-)



cheers,

Woody

Planned EV: '63 Cortina using Industrial AC and LiFePO4 Battery Pack

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Posts: 1247       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 15 April 2009 at 11:17pm

In a way, that is the plan.

The VFD's 750V braking resistor shunt point starts(lots of kW) at 750/220 = 3.4 V/cell up to 850/220 =3.86V/cell so the single large braking resistor takes over once the average cell voltage is 3.4V (90% charged) and depending on the brake resistor chosen and amount of regen, can go up to 3.86V/cell average (98% charge) before controller backs off regen.

The individual cells are still bypassed by 1.5A to 3.5A (depending on resistor) for eq. along the way. This goes into heat. Remember my BMS is designed for regen.



There are no heatsinks on my BMS boards, just ceramic resistors.



I think there would be a lot more wiring for the module based resistor/heatsink ?

If I went that way I would have one big PCB with two wires running to each cell (quite doable). But then those wires should be fused at the battery terminals.

Electric Vehicles, our future, now.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:11am

Originally posted by weber





Hi Nevilleh,



I'm getting interested in a microcontroller-per-cell arrangement. I thought the ATtiny SPI was only for programming. I didn't think you could use it under software control. Do you mean the USI?



My thought was to avoid having to give them IDs at all, by daisy chaining them in a ring with the master. So the master prompts the first cell which sends its voltage and temp data to the second cell which passes on the first cell's data and adds its own on the end, until you get to the end where the master gets all the data. Each cell is effectively ID'ed by its position in the daisy-chain. The master might have several UARTs and run several rings to avoid any one ring being too long.





Yes, it's proper name is USI (Universal Serial Interface)and it is configured as an SPI (Serial Peripheral Interface) for programming. What's in a name, I know what I mean! (Sorry if that caused confusion).

Atmel have an App Note on configuring it for asynch comms which is fairly straightforward. It has an 8-bit data register so receiving is OK as the start bit just drops off the end. Tx'ing requires it to shift out 4 bits, then reload the register with the rest of the byte so that it can send the start bit (9 bits in all).

Your "ring" idea dates back to the 'sixties and it is simple and easy, but it gets hard if the master wants to send something to a particular device. Also, the message gets pretty long if you have lots of nodes!

I think each micro having a unique address is more flexible and easier to implement. My solution requires no extra hardware no matter how many micros are on the line. Since I use only a 1-byte address, that is limited to 256 nodes, but you could easily use 2 bytes and have 64K nodes!

I've just finished designing the pcb which mounts on top of a LFP40 and I'm about to try making one using a technique I haven't seen before. That is printing the artwork on a bit of transparency film using a laser printer. Then you "iron" the toner pattern onto the copper using your ordinary household iron. Put a sheet of paper between the iron and the film so it wont melt. The artwork is thus transferred to the copper and you just peel the film off. Dunk it in etch solution and voila! a completed pcb.

Apparently the laser toner is made of tiny plastic particles which provide an effective etch resist. Ink jet ink doesn't work because it is just ink.

Anyway, I hope to have my first one up and running shortly.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:19am

Originally posted by coulomb





You can zip it and upload it as a file; find the File Manager in the Member Control Panel. Then just let us know it's there, and/or post a link to the uploaded file.



Edit: files have a limit of 1 MB; images a limit of 100 kB. So you could upload a 1.5 MB image if it zip compressed to 1 MB.





Thanks for the tip.

I don't know if anyone is interested, but I'm happy to upload the schematics for my clamp and micro battery monitor and the pcb layouts as well.

They are quite small files done with ExpressSCH and ExpressPCB.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 282       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:54am

Weber and I would be quite interested, as I'm sure others would be too.



It will be interesting to see how your ironing technique goes; I assume this is just for prototyping.



Also, I'm interested in how micros (especially the serial ports) behave in an electrically hostile environment. Do you have supply rail bypass capacitors on the board, or just rely on proximity to the battery?

Working with Weber on converting an MX-5

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 9:06am

Originally posted by coulomb



Weber and I would be quite interested, as I'm sure others would be too.



It will be interesting to see how your ironing technique goes; I assume this is just for prototyping.



Also, I'm interested in how micros (especially the serial ports) behave in an electrically hostile environment. Do you have supply rail bypass capacitors on the board, or just rely on proximity to the battery?



OK, I'll put the files into a single .zip and see if I can manage to do an upload!

Yes, this pcb technique is just for prototyping and/or small quantities.

I have actually made a board since my last post, that's how quick and easy it is.

I was told to use quite a lot of pressure when ironing, but I think I used too much as the toner on my board blank spread a bit. Also, there are a lot of pinholes, but it worked OK and I regard it as successful for my first try. It'll do to verify everything. I checked all the traces for shorts and opens and it is fine. (see below for finished board but not drilled.)

The circuit uses a 5v regulator with a .33 uF bypass on the input and a .1 uF on the output. This provides pretty good isolation from "crap" on the supply lines. The comms, being async at a fairly low data rate are quite robust and opto-coupling makes them just about bullet-proof. The CRC byte provides sufficient protection against corruption.

I designed a system for connecting poker machines together back in '97 - 2000 and used similar techniques there, although opto-coupled RS485.

The system has been installed in most of the Clubs and Pubs in NSW ("NetCash") and it is very stable in what is really quite a harsh environment. This used the same addressing technique that I described earlier, although the micros were Atmel 8051 derivatives rather than their new(er) RISC chips such as the Tiny25. But running the internal "watch-dog" keeps everything under control.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 282       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 5:47pm

Originally posted by Nevilleh



OK, I'll put the files into a single .zip and see if I can manage to do an upload!



Thanks.



The circuit uses a 5v regulator with ...



Err, 5v? From a 3.2V cell? I assume you mean an LM337 or similar adjustable regulator.



The board looks nice, and simple too. Uncluttered. Is it single sided?

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 10:38am

Originally posted by coulomb



Originally posted by Nevilleh



OK, I'll put the files into a single .zip and see if I can manage to do an upload!



Thanks.



The circuit uses a 5v regulator with ...



Err, 5v? From a 3.2V cell? I assume you mean an LM337 or similar adjustable regulator.



The board looks nice, and simple too. Uncluttered. Is it single sided?





No, remember that this thing monitors four cells in series. It derives its supply from the total ie 4 x 3.2v. Although it is bolted to one cell there is a 6 way connector which supplies wire connections to the other 3 cells, plus 2 wires for the comms (1 unused pin).

The board is single-sided but has 2 wire links.

Here's a picture of the assembled pcb:







I've been thinking about the accuracy of the voltage readings. A 10 bit ADC implies that the 2.5v range has a resolution of (+- 1 bit) 2.44 mv, so there's no point in trying for better accuracy. I suppose +- 10mv overall would be quite good enough, especially as I am limiting the charge voltage to about 3.8v per cell and the low end to about 2.7v.

Here's a .jpg of the circuit.







Edited by Nevilleh - 17 April 2009 at 10:47am

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 11:06am

Well, I zipped the Express schematic and pcb files and uploaded the result as BMS.rar - but I don't know where it went or how to find it!

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Posts: 596       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 11:26am

Originally posted by Nevilleh



Well, I zipped the Express schematic and pcb files and uploaded the result as BMS.rar - but I don't know where it went or how to find it!



Thanks for the upload!



It went here :-)



generally: http://www.aeva.asn.au/forums/uploads//



You can find all your uploads in your Member Control Panel (link at top of each page, 2 lines under the logo), in the files section :-)

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 1:29pm

I've done some calculations on the potential errors in my BMS circuit using the nearest preferred value 1% resistors and the worst case is 101mv on cell 3. A bit much I feel. This comes about because the voltage divider resistor of 22K should actually be 22.6K. Probably I should trim these values by making them up from 2 in series ie 22.6K would be 22K + 600 ohms in series.

Just a thought - I'll explore it further, but if anyone wants to make one of these that is something to be careful of.

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Posts: 190       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 2:43pm

How about changing the 10k to a 12k, and the 22k to a 27k.



Here's a sorted list of all the ratios (with factors of 10 ignored) available with the E12 series plus "16". You get much better resolution from the E12 series by adding the single value "16" from the E24 series.



So for example you want a 2.26 ratio. You look down the left hand column of each pair below and find the closest, which is 2.25 (4th from the top in the middle pair of columns) and you see, immediately to the right, that it is obtained by 27 and 12. You then adjust by whatever factors of 10 you need.



I wish I knew how to make columns line up, with this web interface.



Resistor ratios

1.067     16/15          2.200     22/10          4.556     82/18

1.125     18/16          2.200     33/15          4.615     180/39

1.182     39/33          2.206     150/68          4.667     56/12

1.191     56/47          2.250     27/12          4.681     220/47

1.200     12/10          2.353     160/68          4.700     47/10

1.200     18/15          2.438     39/16          4.756     390/82

1.205     47/39          2.485     82/33          4.821     270/56

1.206     82/68          2.519     68/27          4.848     160/33

1.214     68/56          2.545     56/22          4.853     330/68

1.220     100/82          2.553     120/47          5.125     82/16

1.222     22/18          2.564     100/39          5.455     120/22

1.222     33/27          2.600     39/15          5.455     180/33

1.227     27/22          2.611     47/18          5.467     82/15

1.250     15/12          2.647     180/68          5.556     100/18

1.333     16/12          2.679     150/56          5.556     150/27

1.375     22/16          2.683     220/82          5.600     56/10

1.424     47/33          2.700     27/10          5.641     220/39

1.436     56/39          2.750     33/12          5.667     68/12

1.444     39/27          2.857     160/56          5.732     470/82

1.447     68/47          2.938     47/16          5.735     390/68

1.463     120/82          3.030     100/33          5.745     270/47

1.464     82/56          3.037     82/27          5.893     330/56

1.467     22/15          3.077     120/39          5.926     160/27

1.471     100/68          3.091     68/22          6.250     100/16

1.500     15/10          3.111     56/18          6.667     100/15

1.500     18/12          3.133     47/15          6.667     120/18

1.500     27/18          3.191     150/47          6.667     180/27

1.500     33/22          3.214     180/56          6.667     220/33

1.600     16/10          3.235     220/68          6.800     68/10

1.688     27/16          3.250     39/12          6.818     150/22

1.697     56/33          3.293     270/82          6.829     560/82

1.741     47/27          3.300     33/10          6.833     82/12

1.744     68/39          3.404     160/47          6.912     470/68

1.745     82/47          3.500     56/16          6.923     270/39

1.765     120/68          3.636     120/33          6.964     390/56

1.773     39/22          3.704     100/27          7.021     330/47

1.786     100/56          3.727     82/22          7.273     160/22

1.800     18/10          3.733     56/15          7.500     120/16

1.800     27/15          3.778     68/18          8.000     120/15

1.829     150/82          3.830     180/47          8.148     220/27

1.833     22/12          3.846     150/39          8.182     180/22

1.833     33/18          3.900     39/10          8.182     270/33

1.951     160/82          3.917     47/12          8.200     82/10

2.061     68/33          3.929     220/56          8.235     560/68

2.063     33/16          3.971     270/68          8.293     680/82

2.074     56/27          4.024     330/82          8.298     390/47

2.103     82/39          4.103     160/39          8.333     100/12

2.128     100/47          4.250     68/16          8.333     150/18

2.136     47/22          4.444     120/27          8.393     470/56

2.143     120/56          4.533     68/15          8.462     330/39

2.167     39/18          4.545     100/22          8.889     160/18

2.195     180/82          4.545     150/33          9.375     150/16

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Posts: 99       Quote Tritium_James Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 2:51pm

Ah, why not just pick from a higher series count, eg MRS25 0.6W 1% from Farnell, then you can just choose your 22.6K directly...



And even with the 'wrong' value in there, can't you just calibrate it out in software, it's a known error after all.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 5:15am

Thanks for that guys, those ratios are a great help. Be even more useful to do voltage divider ratios!

And of course the s/w can compensate for known errors.

We are a bit limited here in having only what Jaycar stock. Farnell are OK for mail orders, but it goes against my Scrooge mentality to fork out $20 for postage for $5 worth of resistors!



Edited by Nevilleh - 18 April 2009 at 5:25am

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Posts: 99       Quote Tritium_James Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 6:58am

OK then, Jaycar it is! I was wondering why you weren't using surface mount bits, it makes life a lot easier with homebuilt boards, as you don't need to drill all those holes.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 7:50am

Originally posted by Tritium_James



OK then, Jaycar it is! I was wondering why you weren't using surface mount bits, it makes life a lot easier with homebuilt boards, as you don't need to drill all those holes.





You are quite right and I wish I could get surface mount bits more readily. But I have a small, high speed drill press so the holes aren't too bad for small boards.

I plugged all the resistor values from the E24 range into a spreadsheet and re-worked the voltage dividers to get the values shown below.

The worst error is now only 2.6 mv which is near enough the 10-bit ADC resolution, so that will do just fine.



Using 1% resistors gives an potential error of +- 40-odd mv, so maybe there's a case for going to .2% if you can get them.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 190       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 9:37am

To convert a voltage-divider ratio to a resistor ratio you merely subtract one.



A one-decade table of voltage-divider ratios would be a multi-page table, because there are a large number of possible ratios slightly greater than 1, using resistors from widely differing decades.



In contrast, the one-decade resistor ratio table (for E12 plus "16") is compact enough to stick to the wall above your workbench or resistor storage.



For example, let's say you want a voltage divider ratio of 1.63. We subtract 1 to find we need a resistor ratio of 0.63. We scale that by tens until we get a value between 1 and 9.999... So that's 6.3 which we look up to find that 6.25 is 100/16. Therefore 0.625 is 10/16 and so a 10k above a 16k will give us a 1.625 voltage divider, close enough to our desired 1.63.



The full one-decade E24 resistor ratio table is of course 4 times as big as the E12, but you can probably still fit it on an A4 sheet. You can leave off all the pairs of identical resistors that give a ratio of 1, as I have done above. You might also leave off resistor pairs involving E24-only resistors when an E12 pair give a ratio that is the same (or within say 0.5%).



The idea of just adding one E24 value to the E12 series was in regard to what values to stock for prototyping. All those drawers take up a lot of space. So I spreadsheeted all the ratios available from E12 alone and looked at where the big gaps were, and which E24's were most useful for filling in those gaps. It turns out 16 is the most useful. Adding the entire E24 series does not reduce the size of the largest gap any further than does adding this single value, although the average gap size is of course reduced.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 12:48pm

Originally posted by weber



To convert a voltage-divider ratio to a resistor ratio you merely subtract one.



lot of space. So I spreadsheeted all the ratios available from E12 alone and looked at where the big gaps were, and which E24's were most useful for filling in those gaps. It turns out 16 is the most useful. Adding the





That is really quite interesting and useful, I'll revise my own parts stock accordingly, thanks.



I have a problem with your 'merely subtract one" though.



My voltage divider ratio will always be less than one as it is a divider!



Take R2 as the top leg and R1 as the bottom leg, then the division ratio, Dr = R1/(R1+R2). A bit of algebra and re-arranging gives me R1/R2 = Dr/(1-Dr). So if my Dr is .63 say, then the resistor ratio is .63/(1-.63)= 1.702. from your table, the nearest ratio is 1.697 or 56/33. Putting R2=33 and R1=56 gives an actual ratio of .6292 which is pretty close.



So what have I missed here?



Perhaps "division factor" would be more exact than "division ratio".



Edited by Nevilleh - 18 April 2009 at 12:52pm

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Posts: 1247       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 2:04pm

I used a circuit very much like this a while back and one thing I noticed was that the voltage drop across the battery straps / terminals (particularly by the fourth cell) needs to be corrected for when the cells are under high load (3 to 5C).



The error was typically in the 10's of mV so not gross.

This is not too much problem at the BMS master as it can measure the current and apply a correction (perhaps along with any minor corrections of the voltage dividers)

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Posts: 616       Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 11:28am

OK. I just reread weber's PWM pulse idea and I now understand it. Low voltage comparators are cheap, not sure about Op amps.



BUT by the time you have added a timing component (say 555) and a quad comparator, maybe the Tiny25 that Nevilleh is using would be simpler.

Since it could do 4 cells, it sounds way cheaper and 4 times simpler (or more) to make (since there is only 1 for every 4 cells AND it has less components).

Once you get there, why not a serial bus using opto couplers (as per Nevilleh) - and away we go again...sorry

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Posts: 190       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 03 April 2009 at 11:38am

Originally posted by coulomb



An LMC555 works down to 1.5 V, so that's a likely contender.



Yes! That's what's been missing from this thing. Everyone knows every DIY circuit must have at least one 555 timer in it, and preferably a 556 dual. And it must use those timers in the most obscure manner possible.



Actually, I had this web page up and was brushing up when your message arrived. http://home.cogeco.ca/~rpaisley4/LM555.html



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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 06 April 2009 at 12:04pm

Originally posted by Tritium_James



I can almost guarantee you a capacitive coupling scheme for the comms won't work -





Oh well, it was just thought anyway. You've saved me the trouble of even fiddling with it. And, as you say, optos are pretty cheap.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 190       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 10 April 2009 at 11:02pm

Originally posted by Nevilleh



Lots of interesting ideas here!

But I am going with a simple clamp to stop any cell going too high and using fixed resistors in my original circuit sets the activation volyage at 3.93.

In addition, I use an Atmel ATTiny25 to monitor the voltage of each cell. The Tiny 25 has a 4 channel, 10 bit ADC, so one micro does 4 cells. It also has a built in temperature sensor and I plan to use that as the pcb will be attached to the top of one cell in each block of 4.

The Tiny25 is about $2.50 - 3.00 depending on where you get it from.

Using it's SPI capability lets me run a single line, multi-drop comms setup, at the moment using optocouplers ($1.00 or so each)...





Hi Nevilleh,



I'm getting interested in a microcontroller-per-cell arrangement. I thought the ATtiny SPI was only for programming. I didn't think you could use it under software control. Do you mean the USI?



My thought was to avoid having to give them IDs at all, by daisy chaining them in a ring with the master. So the master prompts the first cell which sends its voltage and temp data to the second cell which passes on the first cell's data and adds its own on the end, until you get to the end where the master gets all the data. Each cell is effectively ID'ed by its position in the daisy-chain. The master might have several UARTs and run several rings to avoid any one ring being too long.

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Posts: 310       Quote Electrocycle Quote  Post ReplyReply Direct Link To This Post Posted: 11 April 2009 at 8:13am

yeah you'd have to watch out for the chinese whispers effect there :-)

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Posts: 282       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 11 April 2009 at 8:24am

Originally posted by Nevilleh



I tried to upload the circuit diagram, but the image is too big. I'll have to find another way.



You can zip it and upload it as a file; find the File Manager in the Member Control Panel. Then just let us know it's there, and/or post a link to the uploaded file.



Edit: files have a limit of 1 MB; images a limit of 100 kB. So you could upload a 1.5 MB image if it zip compressed to 1 MB.



Edited by coulomb - 11 April 2009 at 6:57pm

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Posts: 596       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 15 April 2009 at 12:40pm

Something which came up on the ThunderSky list: if you can join multiple cells monitors onto the same heatsink, then you get a better deal, since mostly one cell will be in bypass at a time, when more than a few cells come onto bypass, the charger should be ramping down anyway.



e.g. Tuarn's BMS board for 20 cells should have 1 or 2 big heatsinks, not 20.



And maybe a small fan.



Another possibility I thought of was a clamp/bypass for the whole cell group, which maybe higher capacity - suitable for high regen - sort of a per module braking resistor perhaps.



discuss/flame/etc. :-)



cheers,

Woody

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Posts: 1247       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 15 April 2009 at 11:17pm

In a way, that is the plan.

The VFD's 750V braking resistor shunt point starts(lots of kW) at 750/220 = 3.4 V/cell up to 850/220 =3.86V/cell so the single large braking resistor takes over once the average cell voltage is 3.4V (90% charged) and depending on the brake resistor chosen and amount of regen, can go up to 3.86V/cell average (98% charge) before controller backs off regen.

The individual cells are still bypassed by 1.5A to 3.5A (depending on resistor) for eq. along the way. This goes into heat. Remember my BMS is designed for regen.



There are no heatsinks on my BMS boards, just ceramic resistors.



I think there would be a lot more wiring for the module based resistor/heatsink ?

If I went that way I would have one big PCB with two wires running to each cell (quite doable). But then those wires should be fused at the battery terminals.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:11am

Originally posted by weber





Hi Nevilleh,



I'm getting interested in a microcontroller-per-cell arrangement. I thought the ATtiny SPI was only for programming. I didn't think you could use it under software control. Do you mean the USI?



My thought was to avoid having to give them IDs at all, by daisy chaining them in a ring with the master. So the master prompts the first cell which sends its voltage and temp data to the second cell which passes on the first cell's data and adds its own on the end, until you get to the end where the master gets all the data. Each cell is effectively ID'ed by its position in the daisy-chain. The master might have several UARTs and run several rings to avoid any one ring being too long.





Yes, it's proper name is USI (Universal Serial Interface)and it is configured as an SPI (Serial Peripheral Interface) for programming. What's in a name, I know what I mean! (Sorry if that caused confusion).

Atmel have an App Note on configuring it for asynch comms which is fairly straightforward. It has an 8-bit data register so receiving is OK as the start bit just drops off the end. Tx'ing requires it to shift out 4 bits, then reload the register with the rest of the byte so that it can send the start bit (9 bits in all).

Your "ring" idea dates back to the 'sixties and it is simple and easy, but it gets hard if the master wants to send something to a particular device. Also, the message gets pretty long if you have lots of nodes!

I think each micro having a unique address is more flexible and easier to implement. My solution requires no extra hardware no matter how many micros are on the line. Since I use only a 1-byte address, that is limited to 256 nodes, but you could easily use 2 bytes and have 64K nodes!

I've just finished designing the pcb which mounts on top of a LFP40 and I'm about to try making one using a technique I haven't seen before. That is printing the artwork on a bit of transparency film using a laser printer. Then you "iron" the toner pattern onto the copper using your ordinary household iron. Put a sheet of paper between the iron and the film so it wont melt. The artwork is thus transferred to the copper and you just peel the film off. Dunk it in etch solution and voila! a completed pcb.

Apparently the laser toner is made of tiny plastic particles which provide an effective etch resist. Ink jet ink doesn't work because it is just ink.

Anyway, I hope to have my first one up and running shortly.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:19am

Originally posted by coulomb





You can zip it and upload it as a file; find the File Manager in the Member Control Panel. Then just let us know it's there, and/or post a link to the uploaded file.



Edit: files have a limit of 1 MB; images a limit of 100 kB. So you could upload a 1.5 MB image if it zip compressed to 1 MB.





Thanks for the tip.

I don't know if anyone is interested, but I'm happy to upload the schematics for my clamp and micro battery monitor and the pcb layouts as well.

They are quite small files done with ExpressSCH and ExpressPCB.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 282       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:54am

Weber and I would be quite interested, as I'm sure others would be too.



It will be interesting to see how your ironing technique goes; I assume this is just for prototyping.



Also, I'm interested in how micros (especially the serial ports) behave in an electrically hostile environment. Do you have supply rail bypass capacitors on the board, or just rely on proximity to the battery?

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 9:06am

Originally posted by coulomb



Weber and I would be quite interested, as I'm sure others would be too.



It will be interesting to see how your ironing technique goes; I assume this is just for prototyping.



Also, I'm interested in how micros (especially the serial ports) behave in an electrically hostile environment. Do you have supply rail bypass capacitors on the board, or just rely on proximity to the battery?



OK, I'll put the files into a single .zip and see if I can manage to do an upload!

Yes, this pcb technique is just for prototyping and/or small quantities.

I have actually made a board since my last post, that's how quick and easy it is.

I was told to use quite a lot of pressure when ironing, but I think I used too much as the toner on my board blank spread a bit. Also, there are a lot of pinholes, but it worked OK and I regard it as successful for my first try. It'll do to verify everything. I checked all the traces for shorts and opens and it is fine. (see below for finished board but not drilled.)

The circuit uses a 5v regulator with a .33 uF bypass on the input and a .1 uF on the output. This provides pretty good isolation from "crap" on the supply lines. The comms, being async at a fairly low data rate are quite robust and opto-coupling makes them just about bullet-proof. The CRC byte provides sufficient protection against corruption.

I designed a system for connecting poker machines together back in '97 - 2000 and used similar techniques there, although opto-coupled RS485.

The system has been installed in most of the Clubs and Pubs in NSW ("NetCash") and it is very stable in what is really quite a harsh environment. This used the same addressing technique that I described earlier, although the micros were Atmel 8051 derivatives rather than their new(er) RISC chips such as the Tiny25. But running the internal "watch-dog" keeps everything under control.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 282       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 5:47pm

Originally posted by Nevilleh



OK, I'll put the files into a single .zip and see if I can manage to do an upload!



Thanks.



The circuit uses a 5v regulator with ...



Err, 5v? From a 3.2V cell? I assume you mean an LM337 or similar adjustable regulator.



The board looks nice, and simple too. Uncluttered. Is it single sided?

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 10:38am

Originally posted by coulomb



Originally posted by Nevilleh



OK, I'll put the files into a single .zip and see if I can manage to do an upload!



Thanks.



The circuit uses a 5v regulator with ...



Err, 5v? From a 3.2V cell? I assume you mean an LM337 or similar adjustable regulator.



The board looks nice, and simple too. Uncluttered. Is it single sided?





No, remember that this thing monitors four cells in series. It derives its supply from the total ie 4 x 3.2v. Although it is bolted to one cell there is a 6 way connector which supplies wire connections to the other 3 cells, plus 2 wires for the comms (1 unused pin).

The board is single-sided but has 2 wire links.

Here's a picture of the assembled pcb:







I've been thinking about the accuracy of the voltage readings. A 10 bit ADC implies that the 2.5v range has a resolution of (+- 1 bit) 2.44 mv, so there's no point in trying for better accuracy. I suppose +- 10mv overall would be quite good enough, especially as I am limiting the charge voltage to about 3.8v per cell and the low end to about 2.7v.

Here's a .jpg of the circuit.







Edited by Nevilleh - 17 April 2009 at 10:47am

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 11:06am

Well, I zipped the Express schematic and pcb files and uploaded the result as BMS.rar - but I don't know where it went or how to find it!

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 596       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 11:26am

Originally posted by Nevilleh



Well, I zipped the Express schematic and pcb files and uploaded the result as BMS.rar - but I don't know where it went or how to find it!



Thanks for the upload!



It went here :-)



generally: http://www.aeva.asn.au/forums/uploads//



You can find all your uploads in your Member Control Panel (link at top of each page, 2 lines under the logo), in the files section :-)

Planned EV: '63 Cortina using Industrial AC and LiFePO4 Battery Pack

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 1:29pm

I've done some calculations on the potential errors in my BMS circuit using the nearest preferred value 1% resistors and the worst case is 101mv on cell 3. A bit much I feel. This comes about because the voltage divider resistor of 22K should actually be 22.6K. Probably I should trim these values by making them up from 2 in series ie 22.6K would be 22K + 600 ohms in series.

Just a thought - I'll explore it further, but if anyone wants to make one of these that is something to be careful of.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 190       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 2:43pm

How about changing the 10k to a 12k, and the 22k to a 27k.



Here's a sorted list of all the ratios (with factors of 10 ignored) available with the E12 series plus "16". You get much better resolution from the E12 series by adding the single value "16" from the E24 series.



So for example you want a 2.26 ratio. You look down the left hand column of each pair below and find the closest, which is 2.25 (4th from the top in the middle pair of columns) and you see, immediately to the right, that it is obtained by 27 and 12. You then adjust by whatever factors of 10 you need.



I wish I knew how to make columns line up, with this web interface.



Resistor ratios

1.067     16/15          2.200     22/10          4.556     82/18

1.125     18/16          2.200     33/15          4.615     180/39

1.182     39/33          2.206     150/68          4.667     56/12

1.191     56/47          2.250     27/12          4.681     220/47

1.200     12/10          2.353     160/68          4.700     47/10

1.200     18/15          2.438     39/16          4.756     390/82

1.205     47/39          2.485     82/33          4.821     270/56

1.206     82/68          2.519     68/27          4.848     160/33

1.214     68/56          2.545     56/22          4.853     330/68

1.220     100/82          2.553     120/47          5.125     82/16

1.222     22/18          2.564     100/39          5.455     120/22

1.222     33/27          2.600     39/15          5.455     180/33

1.227     27/22          2.611     47/18          5.467     82/15

1.250     15/12          2.647     180/68          5.556     100/18

1.333     16/12          2.679     150/56          5.556     150/27

1.375     22/16          2.683     220/82          5.600     56/10

1.424     47/33          2.700     27/10          5.641     220/39

1.436     56/39          2.750     33/12          5.667     68/12

1.444     39/27          2.857     160/56          5.732     470/82

1.447     68/47          2.938     47/16          5.735     390/68

1.463     120/82          3.030     100/33          5.745     270/47

1.464     82/56          3.037     82/27          5.893     330/56

1.467     22/15          3.077     120/39          5.926     160/27

1.471     100/68          3.091     68/22          6.250     100/16

1.500     15/10          3.111     56/18          6.667     100/15

1.500     18/12          3.133     47/15          6.667     120/18

1.500     27/18          3.191     150/47          6.667     180/27

1.500     33/22          3.214     180/56          6.667     220/33

1.600     16/10          3.235     220/68          6.800     68/10

1.688     27/16          3.250     39/12          6.818     150/22

1.697     56/33          3.293     270/82          6.829     560/82

1.741     47/27          3.300     33/10          6.833     82/12

1.744     68/39          3.404     160/47          6.912     470/68

1.745     82/47          3.500     56/16          6.923     270/39

1.765     120/68          3.636     120/33          6.964     390/56

1.773     39/22          3.704     100/27          7.021     330/47

1.786     100/56          3.727     82/22          7.273     160/22

1.800     18/10          3.733     56/15          7.500     120/16

1.800     27/15          3.778     68/18          8.000     120/15

1.829     150/82          3.830     180/47          8.148     220/27

1.833     22/12          3.846     150/39          8.182     180/22

1.833     33/18          3.900     39/10          8.182     270/33

1.951     160/82          3.917     47/12          8.200     82/10

2.061     68/33          3.929     220/56          8.235     560/68

2.063     33/16          3.971     270/68          8.293     680/82

2.074     56/27          4.024     330/82          8.298     390/47

2.103     82/39          4.103     160/39          8.333     100/12

2.128     100/47          4.250     68/16          8.333     150/18

2.136     47/22          4.444     120/27          8.393     470/56

2.143     120/56          4.533     68/15          8.462     330/39

2.167     39/18          4.545     100/22          8.889     160/18

2.195     180/82          4.545     150/33          9.375     150/16

Working with Coulomb on converting an MX-5

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Posts: 99       Quote Tritium_James Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 2:51pm

Ah, why not just pick from a higher series count, eg MRS25 0.6W 1% from Farnell, then you can just choose your 22.6K directly...



And even with the 'wrong' value in there, can't you just calibrate it out in software, it's a known error after all.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 5:15am

Thanks for that guys, those ratios are a great help. Be even more useful to do voltage divider ratios!

And of course the s/w can compensate for known errors.

We are a bit limited here in having only what Jaycar stock. Farnell are OK for mail orders, but it goes against my Scrooge mentality to fork out $20 for postage for $5 worth of resistors!



Edited by Nevilleh - 18 April 2009 at 5:25am

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Posts: 99       Quote Tritium_James Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 6:58am

OK then, Jaycar it is! I was wondering why you weren't using surface mount bits, it makes life a lot easier with homebuilt boards, as you don't need to drill all those holes.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 7:50am

Originally posted by Tritium_James



OK then, Jaycar it is! I was wondering why you weren't using surface mount bits, it makes life a lot easier with homebuilt boards, as you don't need to drill all those holes.





You are quite right and I wish I could get surface mount bits more readily. But I have a small, high speed drill press so the holes aren't too bad for small boards.

I plugged all the resistor values from the E24 range into a spreadsheet and re-worked the voltage dividers to get the values shown below.

The worst error is now only 2.6 mv which is near enough the 10-bit ADC resolution, so that will do just fine.



Using 1% resistors gives an potential error of +- 40-odd mv, so maybe there's a case for going to .2% if you can get them.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 190       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 9:37am

To convert a voltage-divider ratio to a resistor ratio you merely subtract one.



A one-decade table of voltage-divider ratios would be a multi-page table, because there are a large number of possible ratios slightly greater than 1, using resistors from widely differing decades.



In contrast, the one-decade resistor ratio table (for E12 plus "16") is compact enough to stick to the wall above your workbench or resistor storage.



For example, let's say you want a voltage divider ratio of 1.63. We subtract 1 to find we need a resistor ratio of 0.63. We scale that by tens until we get a value between 1 and 9.999... So that's 6.3 which we look up to find that 6.25 is 100/16. Therefore 0.625 is 10/16 and so a 10k above a 16k will give us a 1.625 voltage divider, close enough to our desired 1.63.



The full one-decade E24 resistor ratio table is of course 4 times as big as the E12, but you can probably still fit it on an A4 sheet. You can leave off all the pairs of identical resistors that give a ratio of 1, as I have done above. You might also leave off resistor pairs involving E24-only resistors when an E12 pair give a ratio that is the same (or within say 0.5%).



The idea of just adding one E24 value to the E12 series was in regard to what values to stock for prototyping. All those drawers take up a lot of space. So I spreadsheeted all the ratios available from E12 alone and looked at where the big gaps were, and which E24's were most useful for filling in those gaps. It turns out 16 is the most useful. Adding the entire E24 series does not reduce the size of the largest gap any further than does adding this single value, although the average gap size is of course reduced.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 12:48pm

Originally posted by weber



To convert a voltage-divider ratio to a resistor ratio you merely subtract one.



lot of space. So I spreadsheeted all the ratios available from E12 alone and looked at where the big gaps were, and which E24's were most useful for filling in those gaps. It turns out 16 is the most useful. Adding the





That is really quite interesting and useful, I'll revise my own parts stock accordingly, thanks.



I have a problem with your 'merely subtract one" though.



My voltage divider ratio will always be less than one as it is a divider!



Take R2 as the top leg and R1 as the bottom leg, then the division ratio, Dr = R1/(R1+R2). A bit of algebra and re-arranging gives me R1/R2 = Dr/(1-Dr). So if my Dr is .63 say, then the resistor ratio is .63/(1-.63)= 1.702. from your table, the nearest ratio is 1.697 or 56/33. Putting R2=33 and R1=56 gives an actual ratio of .6292 which is pretty close.



So what have I missed here?



Perhaps "division factor" would be more exact than "division ratio".



Edited by Nevilleh - 18 April 2009 at 12:52pm

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Posts: 1247       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 2:04pm

I used a circuit very much like this a while back and one thing I noticed was that the voltage drop across the battery straps / terminals (particularly by the fourth cell) needs to be corrected for when the cells are under high load (3 to 5C).



The error was typically in the 10's of mV so not gross.

This is not too much problem at the BMS master as it can measure the current and apply a correction (perhaps along with any minor corrections of the voltage dividers)

Electric Vehicles, our future, now.

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Posts: 267       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Topic: Home grown BMS ideas !

    Posted: 22 April 2009 at 11:03am

In response to hemonster's query about my shift register idea, here are some thoughts:



   * Three opto-isolated daisy chains instead of 1. This costs more in optos (cheap) and in wire and/or connectors, but hopefully not much more in effort (maybe crimp multiple wires at once from ribbon cable).

   * Extra two opto-isolated connectors are clock and data for a CMOS shift register. I'm thinking two bits of data, so a 2-bit shift register per BMS board, giving 4 states: 0=normal, 1=read voltage, 2=read temperature, 3=lamp test. Might find a better use for code 3; I thought it might be handy to be able to get a particular cell's BMS to say "here I am" and test its LED(s) at the same time.

  • The data is daisy chained, so in effect with 200 cells there is one 400-bit shift register.
  • In normal mode, the cell puts "badness" on the bus, as usual.
  • In read voltage mode, the addressed BMS reports its voltage in PWM, with a range of say 2.5 to 4.0 volts. This would obviously be fairly low precision, and relies on all the other cells being in zero badness mode. Or mode 3 could be "hands off the bus".
   * In read temp mode, the addressed cell would output a PWM width proportional to temperature, say 0-100C for min .. max PWM width.

   * Only one BMS would be addressed in non-zero mode at any time. Suppose you have 200 BMSs, and want to read the voltage of cell 102 (with 0 as the first one). You would send 204 (2x102) clock pulses, and data of 010000....0000, so the "01" gets shifted to the 103rd cell. It measures its voltage, puts it on the analogue output opto line, and the master BMS displays the voltage. To go back to normal, you output 400-204 clock pulses with all zeroes data. Or just 400 clock pulses to be sure.

   * This is relying on some 555-like device to translate voltage into pulse width. It would be rather non-linear, and the capacitors and resistors may drift with temperature and time. The non-linearity can be corrected for somewhat in software. I'd expect only about 10-20 "buckets" of voltage could be reliably distinguished; that could give the voltage to within about 0.1 volts, and temperature to within 5-10 degrees centigrade. It's not good enough to detect slight imbalances starting to happen, but should be able to tell you something like "cell 102 needs checking" and "this group of cells seems to get hotter than that group".

   * There could be fun with electrical spikes and clock skew, but if it's done slowly enough with proper shielding and capacitors everywhere and so on it should be able to be make to work reliably. At 9600 bps, 400 clock pulses are only 42 ms. Even at 1000 bps and 244 cells, it is still less than half a second. So speed should be fine to do "random access". Reading the voltage of all cells in sequence would be easy, as would be the reading all temperatures.



Comments welcome, as always.



Edit: 2 bits *per BMS*.



Edited by coulomb - 22 April 2009 at 11:04am

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Posts: 583       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 19 April 2009 at 8:45pm

From the Thundersky list:



Good technical article on Lithium battery management:



http://www.edn.com/contents/images/6648791.pdf



Managing high-voltage lithium-ion batteries in HEVs



SKYROCKETING ENERGY PRICES AND THE GROWING CONCERN OVER CARBON EMISSIONS HAVE FOCUSED ATTENTION ON ELECTRIC AND HYBRID-ELECTRIC

VEHICLES. New lithium -battery designs will be key technologies for efficient EVs and HEVs.

Planned EV: '63 Cortina using Industrial AC and LiFePO4 Battery Pack

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 19 April 2009 at 8:29am

Originally posted by weber



Originally posted by Nevilleh



My voltage divider ratio will always be less than one as it is a divider!

...

Perhaps "division factor" would be more exact than "division ratio".



The thing you subtract one from is the answer to the question, "If it's a divider, what's it dividing by?". Division factor is a fine name for it. Clearly your "division ratio" is just the reciprocal of this division factor. Edit: a number you're dividing by is also called a divisor



Take R2 as the top leg and R1 as the bottom leg, then the division ratio, Dr = R1/(R1+R2). A bit of algebra and re-arranging gives me R1/R2 = Dr/(1-Dr). So if my Dr is .63 say, then the resistor ratio is .63/(1-.63)= 1.702. from your table, the nearest ratio is 1.697 or 56/33. Putting R2=33 and R1=56 gives an actual ratio of .6292 which is pretty close.



So what have I missed here?



No need for anything as complicated as that. When designing, you typically start with two voltages. So instead of calculating Vout/Vin (small/big), you calculate Vin/Vout (big/small). Then you can "merely subtract one" to get the resistor ratio to look up in the table.





Yes, I see what you mean. But I have always thought of a voltage divider as something that reduces the voltage by multiplying it by a number <1 and my mental image is that of the bottom bit over the whole, if you like, as a factor. You obviously think of it as the whole divided by the bottom bit which is perfectly logical and gives a divisor rather than a factor.

The result is the same. Taking the .63 figure I used earlier gives your divisor as 1.587, subtract 1 to get .587, find 5.87 in your table (5.89 is nearest) to get resistor values of 330 and 56, now remember that you had to multiply by 10 and so one of the resistors is 10 times too big, so divide it by 10 to get 33.

I like my own way of thinking of it better! Understandable as I've been doing it for lots of years.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 19 April 2009 at 8:04am

Originally posted by coulomb



Originally posted by Nevilleh



I was thinking of using Al for the battery straps too.



Why is that, Neville?





I had thought to use the Al heatsinks for the clamp as connectors. ie the +ve of one cell is connected to the heatsink via the pass transistor collector and it would be simple to extend the heatsink to reach the -ve terminal of the next cell.





Yes, actual SOC is difficult! It's easy enough to see how many AHrs have been used as we have a current measurement, but turning that into available capacity depends on estimating the load current.

I worked out the Peukert exponent for the LFP40 to be 1.0328 and I suppose one could take the average current used at any particular point and say that is likely to be the same for the rest of the cell capacity, put that value into the Peukert equation and hence derive a reasonably accurate figure for available capacity.



Nothing more embarrassing than running out of energy in an EV!



I have found with PbA in my electric scooter that even when the controller shuts down due to low batt volts, if you switch it off and wait a few minutes, it will come to life an struggle another 3 or 4 kms. Running the same machine on LFP40s is a different story. If you let them go down to the shutoff point, there is NO recovery, you have gone as far as you can go. I have 16 cells in series and I find that 44V is the minimum - if you aren't home by then you might manage another 3 or 4 kms if you drop down to 20 kph!



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Posts: 267       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 19 April 2009 at 7:05am

Originally posted by Nevilleh



I was thinking of using Al for the battery straps too.



Why is that, Neville? Are the copper straps rather thin? Remember that only silver conducts better than copper (for the same cross sectional area), so you'd want to use thick aluminium straps.



I also wonder if it is of much concern to measure the cell voltage very accurately when they are under a heavy load? ... I think the no load voltage gives the state of charge and provided it is above 2.5v then the cell voltage can fall below this due to the load current drop and that should not harm the battery as long as the no load voltage can still come back above 2.5v.



Interesting point. I think I read somewhere that a real SOC measurement (taking into account the Peukert effect, less than for lead acid but still present) is better than using the terminal voltage. But how do you "debug" your algorithm without losing a lot of cells?



The other thing is that open circuit voltage may not spring back to a decent "state of charge" value immediately. Lead acid takes at least an hour to settle down, but the chemistries are different enough that this may not apply here.



The limits, either too high or too low a voltage, are very likely to happen at maximum charge or maximum discharge respectively, so I think it's reasonable to assume this and take typical voltage drops into consideration. Internal resistance varies with many factors, so I wouldn't take it too far; best to be conservative with cell protection.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 19 April 2009 at 5:44am

Originally posted by acmotor



I used a circuit very much like this a while back and one thing I noticed was that the voltage drop across the battery straps / terminals (particularly by the fourth cell) needs to be corrected for when the cells are under high load (3 to 5C).









Yes, one tends to regard copper straps as zero ohms, but they do drop a little voltage and the 4th cell gets the drop across 3 straps. I was thinking of using Al for the battery straps too.

I also wonder if it is of much concern to measure the cell voltage very accurately when they are under a heavy load? The point of the BMS is to ensure that no cell gets too high a voltage across it when charging and no cell reaches too low a voltage when discharging. So if your max discharge current is say 5C and you know the cell's internal resistance you should be able to come up with a Vmin figure that allows a bit of a margin. I note from the discharge curves that most of the cell capacity has been used when the terminal voltage drops to 3v (at no load) so if you are sucking 500A and the esr is 1 milliohm then 2.5v is the "alarm" point. If you drop the load, the cell voltage should come back to 3v and so the cell has not been discharged too much. I think the no load voltage gives the state of charge and provided it is above 2.5v then the cell voltage can fall below this due to the load current drop and that should not harm the battery as long as the no load vo;ltage can still come back above 2.5v.

This is still speculation on my part, of course!

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Posts: 181       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 2:29pm

Originally posted by Nevilleh



My voltage divider ratio will always be less than one as it is a divider!

...

Perhaps "division factor" would be more exact than "division ratio".



The thing you subtract one from is the answer to the question, "If it's a divider, what's it dividing by?". Division factor is a fine name for it. Clearly your "division ratio" is just the reciprocal of this division factor. Edit: a number you're dividing by is also called a divisor



Take R2 as the top leg and R1 as the bottom leg, then the division ratio, Dr = R1/(R1+R2). A bit of algebra and re-arranging gives me R1/R2 = Dr/(1-Dr). So if my Dr is .63 say, then the resistor ratio is .63/(1-.63)= 1.702. from your table, the nearest ratio is 1.697 or 56/33. Putting R2=33 and R1=56 gives an actual ratio of .6292 which is pretty close.



So what have I missed here?



No need for anything as complicated as that. When designing, you typically start with two voltages. So instead of calculating Vout/Vin (small/big), you calculate Vin/Vout (big/small). Then you can "merely subtract one" to get the resistor ratio to look up in the table.



Edited by weber - 18 April 2009 at 2:34pm

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Posts: 1247       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 2:04pm

I used a circuit very much like this a while back and one thing I noticed was that the voltage drop across the battery straps / terminals (particularly by the fourth cell) needs to be corrected for when the cells are under high load (3 to 5C).



The error was typically in the 10's of mV so not gross.

This is not too much problem at the BMS master as it can measure the current and apply a correction (perhaps along with any minor corrections of the voltage dividers)

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 12:48pm

Originally posted by weber



To convert a voltage-divider ratio to a resistor ratio you merely subtract one.



lot of space. So I spreadsheeted all the ratios available from E12 alone and looked at where the big gaps were, and which E24's were most useful for filling in those gaps. It turns out 16 is the most useful. Adding the





That is really quite interesting and useful, I'll revise my own parts stock accordingly, thanks.



I have a problem with your 'merely subtract one" though.



My voltage divider ratio will always be less than one as it is a divider!



Take R2 as the top leg and R1 as the bottom leg, then the division ratio, Dr = R1/(R1+R2). A bit of algebra and re-arranging gives me R1/R2 = Dr/(1-Dr). So if my Dr is .63 say, then the resistor ratio is .63/(1-.63)= 1.702. from your table, the nearest ratio is 1.697 or 56/33. Putting R2=33 and R1=56 gives an actual ratio of .6292 which is pretty close.



So what have I missed here?



Perhaps "division factor" would be more exact than "division ratio".



Edited by Nevilleh - 18 April 2009 at 12:52pm

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Posts: 181       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 9:37am

To convert a voltage-divider ratio to a resistor ratio you merely subtract one.



A one-decade table of voltage-divider ratios would be a multi-page table, because there are a large number of possible ratios slightly greater than 1, using resistors from widely differing decades.



In contrast, the one-decade resistor ratio table (for E12 plus "16") is compact enough to stick to the wall above your workbench or resistor storage.



For example, let's say you want a voltage divider ratio of 1.63. We subtract 1 to find we need a resistor ratio of 0.63. We scale that by tens until we get a value between 1 and 9.999... So that's 6.3 which we look up to find that 6.25 is 100/16. Therefore 0.625 is 10/16 and so a 10k above a 16k will give us a 1.625 voltage divider, close enough to our desired 1.63.



The full one-decade E24 resistor ratio table is of course 4 times as big as the E12, but you can probably still fit it on an A4 sheet. You can leave off all the pairs of identical resistors that give a ratio of 1, as I have done above. You might also leave off resistor pairs involving E24-only resistors when an E12 pair give a ratio that is the same (or within say 0.5%).



The idea of just adding one E24 value to the E12 series was in regard to what values to stock for prototyping. All those drawers take up a lot of space. So I spreadsheeted all the ratios available from E12 alone and looked at where the big gaps were, and which E24's were most useful for filling in those gaps. It turns out 16 is the most useful. Adding the entire E24 series does not reduce the size of the largest gap any further than does adding this single value, although the average gap size is of course reduced.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 7:50am

Originally posted by Tritium_James



OK then, Jaycar it is! I was wondering why you weren't using surface mount bits, it makes life a lot easier with homebuilt boards, as you don't need to drill all those holes.





You are quite right and I wish I could get surface mount bits more readily. But I have a small, high speed drill press so the holes aren't too bad for small boards.

I plugged all the resistor values from the E24 range into a spreadsheet and re-worked the voltage dividers to get the values shown below.

The worst error is now only 2.6 mv which is near enough the 10-bit ADC resolution, so that will do just fine.



Using 1% resistors gives an potential error of +- 40-odd mv, so maybe there's a case for going to .2% if you can get them.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 98       Quote Tritium_James Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 6:58am

OK then, Jaycar it is! I was wondering why you weren't using surface mount bits, it makes life a lot easier with homebuilt boards, as you don't need to drill all those holes.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 18 April 2009 at 5:15am

Thanks for that guys, those ratios are a great help. Be even more useful to do voltage divider ratios!

And of course the s/w can compensate for known errors.

We are a bit limited here in having only what Jaycar stock. Farnell are OK for mail orders, but it goes against my Scrooge mentality to fork out $20 for postage for $5 worth of resistors!



Edited by Nevilleh - 18 April 2009 at 5:25am

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Posts: 98       Quote Tritium_James Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 2:51pm

Ah, why not just pick from a higher series count, eg MRS25 0.6W 1% from Farnell, then you can just choose your 22.6K directly...



And even with the 'wrong' value in there, can't you just calibrate it out in software, it's a known error after all.

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Posts: 181       Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 2:43pm

How about changing the 10k to a 12k, and the 22k to a 27k.



Here's a sorted list of all the ratios (with factors of 10 ignored) available with the E12 series plus "16". You get much better resolution from the E12 series by adding the single value "16" from the E24 series.



So for example you want a 2.26 ratio. You look down the left hand column of each pair below and find the closest, which is 2.25 (4th from the top in the middle pair of columns) and you see, immediately to the right, that it is obtained by 27 and 12. You then adjust by whatever factors of 10 you need.



I wish I knew how to make columns line up, with this web interface.



Resistor ratios

1.067     16/15          2.200     22/10          4.556     82/18

1.125     18/16          2.200     33/15          4.615     180/39

1.182     39/33          2.206     150/68          4.667     56/12

1.191     56/47          2.250     27/12          4.681     220/47

1.200     12/10          2.353     160/68          4.700     47/10

1.200     18/15          2.438     39/16          4.756     390/82

1.205     47/39          2.485     82/33          4.821     270/56

1.206     82/68          2.519     68/27          4.848     160/33

1.214     68/56          2.545     56/22          4.853     330/68

1.220     100/82          2.553     120/47          5.125     82/16

1.222     22/18          2.564     100/39          5.455     120/22

1.222     33/27          2.600     39/15          5.455     180/33

1.227     27/22          2.611     47/18          5.467     82/15

1.250     15/12          2.647     180/68          5.556     100/18

1.333     16/12          2.679     150/56          5.556     150/27

1.375     22/16          2.683     220/82          5.600     56/10

1.424     47/33          2.700     27/10          5.641     220/39

1.436     56/39          2.750     33/12          5.667     68/12

1.444     39/27          2.857     160/56          5.732     470/82

1.447     68/47          2.938     47/16          5.735     390/68

1.463     120/82          3.030     100/33          5.745     270/47

1.464     82/56          3.037     82/27          5.893     330/56

1.467     22/15          3.077     120/39          5.926     160/27

1.471     100/68          3.091     68/22          6.250     100/16

1.500     15/10          3.111     56/18          6.667     100/15

1.500     18/12          3.133     47/15          6.667     120/18

1.500     27/18          3.191     150/47          6.667     180/27

1.500     33/22          3.214     180/56          6.667     220/33

1.600     16/10          3.235     220/68          6.800     68/10

1.688     27/16          3.250     39/12          6.818     150/22

1.697     56/33          3.293     270/82          6.829     560/82

1.741     47/27          3.300     33/10          6.833     82/12

1.744     68/39          3.404     160/47          6.912     470/68

1.745     82/47          3.500     56/16          6.923     270/39

1.765     120/68          3.636     120/33          6.964     390/56

1.773     39/22          3.704     100/27          7.021     330/47

1.786     100/56          3.727     82/22          7.273     160/22

1.800     18/10          3.733     56/15          7.500     120/16

1.800     27/15          3.778     68/18          8.000     120/15

1.829     150/82          3.830     180/47          8.148     220/27

1.833     22/12          3.846     150/39          8.182     180/22

1.833     33/18          3.900     39/10          8.182     270/33

1.951     160/82          3.917     47/12          8.200     82/10

2.061     68/33          3.929     220/56          8.235     560/68

2.063     33/16          3.971     270/68          8.293     680/82

2.074     56/27          4.024     330/82          8.298     390/47

2.103     82/39          4.103     160/39          8.333     100/12

2.128     100/47          4.250     68/16          8.333     150/18

2.136     47/22          4.444     120/27          8.393     470/56

2.143     120/56          4.533     68/15          8.462     330/39

2.167     39/18          4.545     100/22          8.889     160/18

2.195     180/82          4.545     150/33          9.375     150/16

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 1:29pm

I've done some calculations on the potential errors in my BMS circuit using the nearest preferred value 1% resistors and the worst case is 101mv on cell 3. A bit much I feel. This comes about because the voltage divider resistor of 22K should actually be 22.6K. Probably I should trim these values by making them up from 2 in series ie 22.6K would be 22K + 600 ohms in series.

Just a thought - I'll explore it further, but if anyone wants to make one of these that is something to be careful of.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 583       Quote woody Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 11:26am

Originally posted by Nevilleh



Well, I zipped the Express schematic and pcb files and uploaded the result as BMS.rar - but I don't know where it went or how to find it!



Thanks for the upload!



It went here :-)



generally: http://www.aeva.asn.au/forums/uploads//



You can find all your uploads in your Member Control Panel (link at top of each page, 2 lines under the logo), in the files section :-)

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 11:06am

Well, I zipped the Express schematic and pcb files and uploaded the result as BMS.rar - but I don't know where it went or how to find it!

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 17 April 2009 at 10:38am

Originally posted by coulomb



Originally posted by Nevilleh



OK, I'll put the files into a single .zip and see if I can manage to do an upload!



Thanks.



The circuit uses a 5v regulator with ...



Err, 5v? From a 3.2V cell? I assume you mean an LM337 or similar adjustable regulator.



The board looks nice, and simple too. Uncluttered. Is it single sided?





No, remember that this thing monitors four cells in series. It derives its supply from the total ie 4 x 3.2v. Although it is bolted to one cell there is a 6 way connector which supplies wire connections to the other 3 cells, plus 2 wires for the comms (1 unused pin).

The board is single-sided but has 2 wire links.

Here's a picture of the assembled pcb:







I've been thinking about the accuracy of the voltage readings. A 10 bit ADC implies that the 2.5v range has a resolution of (+- 1 bit) 2.44 mv, so there's no point in trying for better accuracy. I suppose +- 10mv overall would be quite good enough, especially as I am limiting the charge voltage to about 3.8v per cell and the low end to about 2.7v.

Here's a .jpg of the circuit.







Edited by Nevilleh - 17 April 2009 at 10:47am

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Posts: 267       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 5:47pm

Originally posted by Nevilleh



OK, I'll put the files into a single .zip and see if I can manage to do an upload!



Thanks.



The circuit uses a 5v regulator with ...



Err, 5v? From a 3.2V cell? I assume you mean an LM337 or similar adjustable regulator.



The board looks nice, and simple too. Uncluttered. Is it single sided?

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 9:06am

Originally posted by coulomb



Weber and I would be quite interested, as I'm sure others would be too.



It will be interesting to see how your ironing technique goes; I assume this is just for prototyping.



Also, I'm interested in how micros (especially the serial ports) behave in an electrically hostile environment. Do you have supply rail bypass capacitors on the board, or just rely on proximity to the battery?



OK, I'll put the files into a single .zip and see if I can manage to do an upload!

Yes, this pcb technique is just for prototyping and/or small quantities.

I have actually made a board since my last post, that's how quick and easy it is.

I was told to use quite a lot of pressure when ironing, but I think I used too much as the toner on my board blank spread a bit. Also, there are a lot of pinholes, but it worked OK and I regard it as successful for my first try. It'll do to verify everything. I checked all the traces for shorts and opens and it is fine. (see below for finished board but not drilled.)

The circuit uses a 5v regulator with a .33 uF bypass on the input and a .1 uF on the output. This provides pretty good isolation from "crap" on the supply lines. The comms, being async at a fairly low data rate are quite robust and opto-coupling makes them just about bullet-proof. The CRC byte provides sufficient protection against corruption.

I designed a system for connecting poker machines together back in '97 - 2000 and used similar techniques there, although opto-coupled RS485.

The system has been installed in most of the Clubs and Pubs in NSW ("NetCash") and it is very stable in what is really quite a harsh environment. This used the same addressing technique that I described earlier, although the micros were Atmel 8051 derivatives rather than their new(er) RISC chips such as the Tiny25. But running the internal "watch-dog" keeps everything under control.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 267       Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:54am

Weber and I would be quite interested, as I'm sure others would be too.



It will be interesting to see how your ironing technique goes; I assume this is just for prototyping.



Also, I'm interested in how micros (especially the serial ports) behave in an electrically hostile environment. Do you have supply rail bypass capacitors on the board, or just rely on proximity to the battery?

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:19am

Originally posted by coulomb





You can zip it and upload it as a file; find the File Manager in the Member Control Panel. Then just let us know it's there, and/or post a link to the uploaded file.



Edit: files have a limit of 1 MB; images a limit of 100 kB. So you could upload a 1.5 MB image if it zip compressed to 1 MB.





Thanks for the tip.

I don't know if anyone is interested, but I'm happy to upload the schematics for my clamp and micro battery monitor and the pcb layouts as well.

They are quite small files done with ExpressSCH and ExpressPCB.



The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 88       Quote Nevilleh Quote  Post ReplyReply Direct Link To This Post Posted: 16 April 2009 at 6:11am

Originally posted by weber





Hi Nevilleh,



I'm getting interested in a microcontroller-per-cell arrangement. I thought the ATtiny SPI was only for programming. I didn't think you could use it under software control. Do you mean the USI?



My thought was to avoid having to give them IDs at all, by daisy chaining them in a ring with the master. So the master prompts the first cell which sends its voltage and temp data to the second cell which passes on the first cell's data and adds its own on the end, until you get to the end where the master gets all the data. Each cell is effectively ID'ed by its position in the daisy-chain. The master might have several UARTs and run several rings to avoid any one ring being too long.





Yes, it's proper name is USI (Universal Serial Interface)and it is configured as an SPI (Serial Peripheral Interface) for programming. What's in a name, I know what I mean! (Sorry if that caused confusion).

Atmel have an App Note on configuring it for asynch comms which is fairly straightforward. It has an 8-bit data register so receiving is OK as the start bit just drops off the end. Tx'ing requires it to shift out 4 bits, then reload the register with the rest of the byte so that it can send the start bit (9 bits in all).

Your "ring" idea dates back to the 'sixties and it is simple and easy, but it gets hard if the master wants to send something to a particular device. Also, the message gets pretty long if you have lots of nodes!

I think each micro having a unique address is more flexible and easier to implement. My solution requires no extra hardware no matter how many micros are on the line. Since I use only a 1-byte address, that is limited to 256 nodes, but you could easily use 2 bytes and have 64K nodes!

I've just finished designing the pcb which mounts on top of a LFP40 and I'm about to try making one using a technique I haven't seen before. That is printing the artwork on a bit of transparency film using a laser printer. Then you "iron" the toner pattern onto the copper using your ordinary household iron. Put a sheet of paper between the iron and the film so it wont melt. The artwork is thus transferred to the copper and you just peel the film off. Dunk it in etch solution and voila! a completed pcb.

Apparently the laser toner is made of tiny plastic particles which provide an effective etch resist. Ink jet ink doesn't work because it is just ink.

Anyway, I hope to have my first one up and running shortly.

The beauty of these things is that you don't really know what you've got until you put it all together.

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Posts: 1247       Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 15 April 2009 at 11:17pm

In a way, that is the plan.

The VFD's 750V braking resistor shunt point starts(lots of kW) at 750/220 = 3.4 V/cell up to 850/220 =3.86V/cell so the single large braking resistor takes over once the average cell voltage is 3.4V (90% charged) and depending on the brake resistor chosen and amount of regen, can go up to 3.86V/cell average (98% charge) before controller backs off regen.

The individual cells are still bypassed by 1.5A to 3.5A (depending on resistor) for eq. along the way. This goes into heat. Remember my BMS is designed for regen.



There are no heatsinks on my BMS boards, just ceramic resistors.



I think there would be a lot more wiring for the module based resistor/heatsink ?

If I went that way I would have one big PCB with two wires running to each cell (quite doable). But then those wires should be fused at the battery terminals.

Electric Vehicles, our future, now.

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