# Lessons learned



## tomofreno (Mar 3, 2009)

Thought it worth posting a "lessons learned" post from Greg Fordyce (guy in UK whose ev burned while charging at night about two years ago) on the Yahoo TS site. His best guess is that the cause was a cell developing high resistance during charge, resulting in higher than designed voltage across his un-fused shunt circuit. 

Lessons learned;

Don't mount slave boards directly on cells. If you do, test one board at
a voltage level above the maximum pack charging voltage and make sure it
won't catch fire. If it does you will have a very nasty fire on your
hands. Lifepo4 burns very nicely, in my car the glass in the doors
melted and the gearbox casing ended up as a very expensive piece of
artwork! Mount slaves in a separate enclosure to the cells.

Fuse all wires to the cells.

Make sure your bms has some sort of "dead mans logic" like the dead man
switch found on trains. If the bms can't confirm all cell voltages are
o.k. or if the bms fails the charger should shut down.

Your charger should be able to safely charge your pack without a bms and
your bms should be able to switch off or reduce charger output.

For added safety, use a mechanical timer at the wall set to cut power to
the charger shortly after the car should have finished charging.

Don't get to hung up about having a "perfectly balanced" pack. It
doesn't exist, you only need it balanced enough that you have all of the
weakest cells capacity available.

Greg


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## BMI/LiFeTech (Aug 12, 2009)

I strongly disagree with the comment that "LiFePO4 burns very nicely".
This may be the case with poor quality LiFePO4 batteries from China which most likely also have all kinds of flammable additives but I can tell you professional grade LiFePO4 batteries absolutely DO NOT BURN. 
For anyone who doubts me feel free to drop me a quick email to [email protected] with "LiFePO4 crash test" in the subject line and I will send you the full powerpoint presentation to show you what actually does happen to a battery pack during crash lab testing.


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## Tesseract (Sep 27, 2008)

BMI/LiFeTech said:


> I strongly disagree with the comment that "LiFePO4 burns very nicely".
> This may be the case with poor quality LiFePO4 batteries from China which most likely also have all kinds of flammable additives but I can tell you professional grade LiFePO4 batteries absolutely DO NOT BURN.


My understanding is that the solvent has to be polar, aprotic and capable of solvating the Lithium in the actual electrolyte, LiFP6. The best solvents by far for this job appear to be carbonate esters (e.g. - Propylene Carbonate, Dimethyl Carbonate, Ethyl Carbonate, etc..) and they are all highly flammable. Just goes with the territory, them being composed of carbon, oxygen and hydrogen atoms, after all.

And the concern here is not so much whether the cells can survive puncturing, crushing or other mechanical injury, rather, it is whether a combination of a BMS failure and overcharging can cause the solvents to boil and then find an ignition source. Ie - overcharging causes the solvents to boil and an already-burning BMS board makes a handy source of ignition.


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## DawidvC (Feb 14, 2010)

I would suspect the most important lesson should be that any failure should result in a "Fail Safe" mode. In this instance, the boards must fail open on all failures. Total failure of any single slave should get the bms to stop the charging process. Excessive heat in any part of the pack should result in the charging process being stopped.

BTW, the whole problem surrounding bms systems, especially slave systems, seems to relate to the failure mode, in as much as they do not seem to be designed t fail safe.

Just my 0.02 worth.
Regards
Dawid


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## tomofreno (Mar 3, 2009)

> And the concern here is not so much whether the cells can survive puncturing, crushing or other mechanical injury, rather, it is whether a combination of a BMS failure and overcharging can cause the solvents to boil and then find an ignition source. Ie - overcharging causes the solvents to boil and an already-burning BMS board makes a handy source of ignition.


 I think that's it in a nutshell. My understanding from the CMU prof and journal articles is that all LiFePO4 cells use some combination of the flammable solvents Tesseract listed. The on-cell bms boards on my cells are sitting right next to the vent, so if a shunt gets hot enough to start the pc board burning and the cell gets hot enough to vent, the jet of gases would likely be ignited. I think Greg is saying if a cell develops high resistance during charging, it then heats more, and the shunt circuit has higher voltage across it resulting in larger shunt current and heating of the shunt resistor. If the increase in cell resistance is large enough, it can result in the cell becoming hot enough to vent solvent, and the bms shunt hot enough to ignite the pc board. Even if a cell develops high resistance during driving you may well not notice it unless you are monitoring cell temperatures or are very well calibrated with how much voltage sag you normally see (I'm not). The little circuit for watching pack balance may show it due to uneven voltage sag in the two pack halves, but you would have to be well calibrated and paying attention to notice it. You would also have to be diligent enough to further investigate any little change.

The minibms boards are fused to prevent the shunt from overheating in the event of a cell failing open or with high resistance. I don't know about others, that is the only one I have experience with.


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## dtbaker (Jan 5, 2008)

hence my hesitation to install anything resembling a shunt resistor near my batteries.


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## EVfun (Mar 14, 2010)

How are the current BMS offerings protected from shunts that get stuck on, cell over voltage, and cell reversal issues?

It seems that the stuck shunt can be made into a benign failure (except for loosing a cell if it is not noticed in a timely fashion) simply by sizing the system so that overheating will not occur, even if the shunt is on for hours. 

If I was designing a cell module I would consider a fuse and zener diode to address excess voltage and reversals. A cell reversal would short through the zener blowing the fuse quickly. Excess voltage on a cell would also cause the zener to conduct and blow the fuse. It is important to size the zener and fuse so that the fuse opens before the zener fails. 

If remote mounting the cell modules I would be very concerned if cell wires where not fused at the cells. Fusing on the board doesn't protect from cell wires getting abraded or cut.


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## DawidvC (Feb 14, 2010)

EVfun said:


> not occur, even if the shunt is on for hours.
> 
> If I was designing a cell module I would consider a fuse and zener diode to address excess voltage and reversals. A cell reversal would short through the zener blowing the fuse quickly. Excess voltage on a cell would also cause the zener to conduct and blow the fuse. It is important to size the zener and fuse so that the fuse opens before the zener fails.
> 
> If remote mounting the cell modules I would be very concerned if cell wires where not fused at the cells. Fusing on the board doesn't protect from cell wires getting abraded or cut.


Good idea. I would also move the shunt resistors off-board and use a type that can be mounted on a heatsink. I know it is overkill, but in the presence of flammable gases I would rather be overly carefull - Belts and braces. 

It would also be a good idea to have thermal monitoring on every cell, in case anything overheats. I would rather stop charging early on a suspected fault, than keep on charging until something breaks.

Regards
Dawid


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## dimitri (May 16, 2008)

EVfun said:


> How are the current BMS offerings protected from shunts that get stuck on, cell over voltage, and cell reversal issues?


I can only speak for my BMS of course  , but I have taken all of these considerations when making miniBMS. In fact, 2 most important features which often are overlooked by BMS-jihad crowd are normally closed signaling and PTC fuse on each module. Virtually any failure is caught by normally closed system and PTC fuse protects from overheating, regardless of the source of heat, whether its caused by shunting or anything else. As for shunting capacity, it must be kept low and resistor must be oversized for the job, which is why I limit shunting to 0.75A with 5Watt resistor, allowing comfortable temperatures up to 7V or so, at which point any Lithium cell would be a toast anyway.

The only comment I disagree with is not to install BMS at the cell. In case of large prismatic cells distributed system has several advantages over centralized system, its safer, easier to install and more robust.

3-4 years ago when we knew much less about LiFePO4 cells early BMS designes were hell bent on maximum shunting capacity, I'm guilty of such myself  , but as we learned more and more it became evident that less shunting is better, although I can still argue that no shunting at all is not ideal long term, but can be managed. If I had a choice of heavy shunting ( over 1A ) or no shunting at all, I would prefer no shunting at all, but given a choice of safe small amount I would go for it.

Greg's experience was very unfortunate, but I applaud him for sharing details in a calm logical manner, as opposed to majority of panic inducing posters with oversized mouths and/or imagination.


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## DawidvC (Feb 14, 2010)

Thanks for sharing that, Dimitry. I didn't want to say anything about the minibms, because I have no experience with it, but I did pick up about the signaling - it was the first thing I actually check for when I first heard about it. 

With current that low through the shunt it should be ok. My comment was for systems shunting amps, not milliamps. 

Damn this english. Everytime I think I know how to speel, they change the rules  

Dawid


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## corbin (Apr 6, 2010)

dimitri said:


> ...normally closed signaling


I totally agree! I have the Elithion BMS, and I explicitly set the "high level output" to be normally closed (and open when it is "high"). That way, the relay is normally open, and *doesn't* allow charging with a Manzantia PFC charger (by allowing pin 1's 5v out to go into pin 2, see: http://lithiumate.elithion.com/php/manzanita_micro.php ). 

Now, if someone wired it the default way, then the "high limit" is usually indicated by the BMS closing the circuit. If the power for the BMS fails (i.e.: 12v dies), the charger could still be left on charging.

Bottom line: there is a right way and a wrong way to install a BMS. For me, it was the most complex piece of the car to get working. (And I don't have it all working right yet!).

corbin


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## tomofreno (Mar 3, 2009)

> I totally agree! I have the Elithion BMS, and I explicitly set the "high level output" to be normally closed (and open when it is "high"). That way, the relay is normally open, and *doesn't* allow charging with a Manzantia PFC charger (by allowing pin 1's 5v out to go into pin 2


 Corbin, I'm not following you here (part I underlined). It sounds to me like you are saying you have a N.O. relay between Regbus pins 1 and 2 so it keeps the charger output off by not connecting pin 1 to pin 2. But the charger output is held low by connecting pin 1 to pin2, i.e. applying 5V to pin 2, hence my confusion. I must be misunderstanding what you mean.


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## DawidvC (Feb 14, 2010)

Crazy design. Most definitely not a very safe design on that charger. I can understand why they are doing it, but if you have a slightly flaky connection in that plug - after plugging it in 1000 times or so - your bms can try to limit the charge, and the flaky connection will ensure that it keeps on charging!

Definitely not a good way of doing it. It is safer to interrupt the signal to pin 2 to stop charging, that way, if something dies, the charging stops. However, that will let the charger charge away happily with *no* connection  I would prefer 2 signals, to ensure that it cannot start without a load, and can be stopped whenever there is a problem. Belts and braces.

Yikes, my industrial background is hanging out 

Regards
Dawid


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## corbin (Apr 6, 2010)

tomofreno said:


> Corbin, I'm not following you here (part I underlined). It sounds to me like you are saying you have a N.O. relay between Regbus pins 1 and 2 so it keeps the charger output off by not connecting pin 1 to pin 2. But the charger output is held low by connecting pin 1 to pin2, i.e. applying 5V to pin 2, hence my confusion. I must be misunderstanding what you mean.


Hey Tom -- I think I worded it poorly. I can draw a circuit to make it more clear, but first I'll try with words.

On the charger, Regbus pin 1 outputs 5V. pin 2, when supplied with 5V, stops charging. If you complete that circuit, it won't charge.

I have a standard automotive relay. The charger Regbus Pin 1 is hooked up to "87a" and pin 2 is hooked up to "30", allowing it to *always* complete the circuit from pin 1 to pin 2, and thus making the charger not charge.

The BMS is hooked up to "85" and "86" on the relay and is normally open. When powered, it turns on the relay (completing 30 to 87, which isn't hooked up to anything), and just opens the circuit from pin 1 to pin 2, allowing the charger to charge.

--corbin


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## corbin (Apr 6, 2010)

DawidvC said:


> Crazy design. Most definitely not a very safe design on that charger. I can understand why they are doing it, but if you have a slightly flaky connection in that plug - after plugging it in 1000 times or so - your bms can try to limit the charge, and the flaky connection will ensure that it keeps on charging!
> 
> Definitely not a good way of doing it. It is safer to interrupt the signal to pin 2 to stop charging, that way, if something dies, the charging stops. However, that will let the charger charge away happily with *no* connection  I would prefer 2 signals, to ensure that it cannot start without a load, and can be stopped whenever there is a problem. Belts and braces.
> 
> ...


Hi Dawid -- Yes, I agree! There is the potential for a problem here, as you noted. It would be better if there was a dipswitch on the charger that flipped the way this logic worked. However, I never unplug that Reg BUS connection to the charger; it is hardwired. 

Even so, I still feel it is quite safe. The reason is that I have the chargers voltage max set to right around my pack voltage. The charger ramps down the charging current right around the end of the charge, and then the BMS turns it off right about when it is done ramping down. 

So, this ensures the charger won't just keep charging forever. 

-corbin


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## tomofreno (Mar 3, 2009)

> When powered, it turns on the relay (completing 30 to 87, which isn't hooked up to anything), and just opens the circuit from pin 1 to pin 2, allowing the charger to charge.


 So I guess if a cell then hits HVC the bms output to the relay goes low, connecting pins 1 & 2 and shutting down the charger output? That's nice that it shuts down the charger if you lose 12V to the bms.



> Even so, I still feel it is quite safe. The reason is that I have the chargers voltage max set to right around my pack voltage. The charger ramps down the charging current right around the end of the charge, and then the BMS turns it off right about when it is done ramping down.


 Keep an eye on it as outside temperature warms up this summer. You may have to re-adjust your limit voltage to keep it behaving the way it currently does. That is the way mine is set up too, but I found last summer that the bottom balanced pack hit the limit voltage at higher SOC in warmer temperatures so the charger would overcharge my lowest capacity cell if not stopped by me or the bms, unless the limit voltage was re-adjusted lower. 

It may not do that with a top balanced pack though. I top balanced mine a couple months ago and there seems to be a lot more latitude in temperature and charging current I can use without over-charging or re-adjusting the voltage limit, due to the much larger voltage change near end of charge as all the cell voltages start going up the exponential part of the charge curve. That makes the Manzanita cut the current way back fairly quickly, so when the timer times out charge current is down to almost nothing for lower charge currents. At higher charge currents the charger times out before the cell voltages even get to the exp part of the curve, since the pack hits the limit voltage at lower SOC than with lower charge currents. I'll see how it behaves this summer, but you may not have to re-adjust at all since you are top balanced, I just thought it prudent to mention there may be an issue.


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## gdirwin (Apr 7, 2009)

I also use Elithion and Manzanita chargers - one "gotcha" I found:
- I set the FLT signal from the BMS to be grounded under non-fault conditions
- this ground signal goes to a SSR AC relay that enables the charger AC input power

The idea is that the BMS HVC should normally turn the charger off via the Regbus - if there is an error (say a cell gets too high of a voltage etc...) then the BMS FLT signal will remove power to the charger.

Here's the problem - if you ever run-down your pack to the point where a cell is below its minimum voltage, you get a FLT, but then you cannot charger anymore! I had to go into the software, and clear the FLT to allow charging... I suppose you could also have put a manual/bms switch on the SSR relay ground...


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