# LiFeP04 Fire - Car destroyed



## brainzel (Jun 15, 2009)

Hmm ... I've never seen, read about a LiFePo4 car fire *without* BMS or "controll boards" etc.

Would be cool to see a listing of all fires, incl. data to battery, bms, while charging/discharging etc.

PS: 12.500 ml without BMS or issues and counting 
!! This should *not* be a new flame thread BMS or not !!


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## TigerNut (Dec 18, 2009)

That thread has been running for nearly four years... it's probably safe to say that BMS technology has come a long way in that time. The initial speculation by the guy whose car was destroyed was BMS failure, possibly exacerbated by a design detail in that particular BMS where a cell-open failure would not be properly detected.

Without pointing ANY fingers at anyone in the BMS game, and without having done an appropriate amount of research into the various BMS vendors and their systems' strengths and weaknesses, I think that it is imperative that BMS systems be designed in the same vein as intrinsically safe equipment for industrial applications. The IS design standards require (for the most hazardous applications) that the amount of energy buildup is kept below a threshold that could cause ignition of a flammable atmosphere, even if the circuit experiences a double fault of any sort. That fault could be a component failure or a mechanical one. Additionally, the standards mandate that no component be stressed beyond about 80% of its specified power handling capability even under fault conditions.

In a BMS system the aim would be to prevent the overcharging or overheating of any cell in the battery pack - even if a sensing component failed or a wire came loose. Without that kind of protection, it's better to just do 'monitoring' as opposed to 'management' on every charge cycle.


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## cts_casemod (Aug 23, 2012)

brainzel said:


> Hmm ... I've never seen, read about a LiFePo4 car fire *without* BMS or "controll boards" etc.
> 
> Would be cool to see a listing of all fires, incl. data to battery, bms, while charging/discharging etc.
> 
> ...


As long as you have a way to check individual battery voltages is fine . I've done the test and know what happens to depply discharged cells.

I have seen a few topics on e-bikes (dead cells) and fires both with and without BMS. Althought I believe most come from poor conections. The vibration is quite bad. 

My personal opinion on the issue is balancing. Many efforts are done on ballancing, but keeping the cells in a high voltage may cause stress on them. I dont balance my bike pack (the charger cuts as soon as a cell reaches HVC) and I log the Amps in/out.


On the car I plan to stop charging as soon as the current drops to 1/5 of capacity unless I need the range. This should provide 80% capacity and improved regen capabilities.


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## tomofreno (Mar 3, 2009)

Old news. The owner, Greg, of Greg's Garage, discussed possible causes of failure, with a cell developing high resistance his guess at most likely cause. I think most bms protect themselves against this. For example, the minibms has a fusible link that will open in this case to prevent high current through the shunt path. The bms in this car was a project between Greg and others. It was re-designed after this to protect against such failures.


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## Siwastaja (Aug 1, 2012)

BMS indeed needs to be carefully designed, otherwise it may work opposite to its purpose of being there. IIRC, this was some quick DIY design (correct me, if I remember wrong).

It is not very difficult, though, to do so, but someone with some experience needs to check the design. But I guess there are many people going with their own very simple designs but are forgetting a few important design points.

A BMS should rely on passive security instead of (or in addition to) active error checks. The main point is to recognize all possible error scenarios (there are not that many) and verify that the charger shuts down in every of them, and does so "naturally", or passively, without overly complex electronics or processors actively initiating the shutdown.

Practically, this can mean a normally-open chain that shuts down the charger when any cell board stops working, loses power or blows up for any reason. It is relatively easy to test this. Verify that the chain opens even in a large overvoltage situation, with negative voltage, with very high current (bypass any fuses for the test and let it really blow up)... In all of these cases, the charger should stop. When pointing the cell module with a hairdryer, the charger should stop too.

This also prevents charging when any cell is drained near 0V, and hence adds true security over "no BMS". Not saying that charging totally drained LiFePO4 would set them on fire, but it happens with some other Li chemistries, and is best avoided just to be sure.

In addition, a fuse and a thermal fuse are a must. I would try to avoid large shunting currents, as initially properly balanced LiFePO4 pack shouldn't need much balancing. Then, the BMS's thermal fuse would be rated to relatively low temperatures and would be there for monitoring the _cell_ temperature, so that the charger would be stopped well below the thermal runaway temperature, in the rare case of the cell having some problem.

When properly designed, a BMS can increase security, but badly designed, it really can decrease it. 

With LiFePO4 being a very simple and robust battery chemistry, a BMS does not have so much to do. It is crucial to understand what it needs to do and what it doesn't, and do only the necessary parts, and do them well. Think about every possibility.

LiFePO4 is more forgiving than other Li, but the same principles still apply, at least for full cell lifetime:

1) Never let the cell voltage drop below lower limit
2) Never let the cell voltage rise over higher limit
3) Don't start charging of a cell with voltage below lower limit
4) Monitor temperature and stop charging/draining if too high.

IMO, MiniBMS looks like a very good reference design that does these things just right, but as a disclaimer, this is just my hunch without proper analysis.


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## Sunking (Aug 10, 2009)

Is it possible this is from a poor connection on a terminal somewhere?


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## ruckus (Apr 15, 2009)

brainzel said:


> Hmm ... I've never seen, read about a LiFePo4 car fire *without* BMS or "controll boards" etc.


Evidently you don't watch EVTV and don't consider an incinerated work bench a 'car' fire.

No bms = high likelihood of user error and fire.

There are always risks that are difficult to quantify. The bms vs no bms risk will always draw a crowd. But in the end, who do you trust? a computer, or a human? While the computer will certainly have failures, I would trust it far more than the average college kid or grandmother or old man.

This being said, I am heavily considering going 'bms-free' on my next build. Just for giggles. Experience will school me. It's only $12,000 in batts...

Cheers.


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