Battery Vehicle Society

mmmmm interesting insurance ? surely if its caught fire then its caught fire ?? must admit I've not noticed if my policy has a "if its a faulty component we're not paying out" feature on it



Sorry my mistake, yep as you say the Lifepo4 cells are supose to be idiot proof which makes me wonder if it was something else ?



Now that is a good idea but you would hope that the manufactures had done some to put out the claims of their indestructablity

ChrisB

Sorry to hear of your car's demise Greg; strange how no-one noticed a burning car.

I was seriously considering a battery swap in my cinquecento to LiFepo4, well, pricing it up anyway. But I might leave it for a while now!

Just gonna pop outside and unplug my charger...

Greg,

OMG, I can't believe nobody heard or saw anything!

I have followed your build and admired your very professional approach. So for me, from the information you have provided, I suspect a component bought in, rather than cells alone or something you have developed. I think I would agree with your logic that the charger appears the likely culprit, but I'm not sure I understand the logic of the charger coming back on if the cell voltage dropped to 3.6V?

LiFePO4 don't just fail. If it was a cell going short circuit, from what I understand of your BMS you would have noticed, the problem develop over time, a cell taking a lot longer to charge than the others, for some days or even weeks before.

In normal auto fires, 99% of the time its wiring catching fire from lose terminals getting hot and the insulation or sound deadening material catching light.

With your delivery of cells, did you get a UN transport certificate? This would have confirmed the puncture, overcharge and short circuit test results. I haven't seen a single LiFePO4 cell fail these tests, but several cells have got up to 150+ degrees C and although they haven't caught fire themselves, anything made of a combustible material touching the cell may well ignite.

I feel your pain, I would be devastated if any of my EV's ended up this way, after the many hours of time devoted to them.

Ian

The problem with the analog version of my BMS and many others available which operate in a simple clamper type mode with a high/low v warning bus and no Master controller, is that if a cell goes short circuit and falls below the minimum operating voltage for the electronics on the bms board it basically stops working, and becomes invisible to the system.

But that does not stop the charge/discharge cycle from trying to force current into the duff cell or reversing it etc possibly leading to a serious problem.

The digital version is diferent in that all the cell boards are polled once a second via a serial link, if they don't respond due to having shut down/broken then the Master would report the fault and act accordingly. So I need to keep working on my Master board and sort out my layout to get rid of the interference issue I had.

I am still running my car as normal with bms in analog mode at present, but I do have a test circuit as my layout was different to Greg. So I can confrim each day by pressing the test button and sending a signal around the pack that each cell board is operating.

So I know at least once a day that my boards are working.

It's all a bit depressing

I concur about bad connections, and I am fastidious about this during the build.

I power filed each terminal on my cells so the were perfectly flat and clean before I started interconnections.

After reading this I was 'googling' the safety on them and came across this
http://www.youtube.com/watch?v=tw4RHdPH ... re=related
If this is what happens to 1 then a whole pack will go up nicely

Hi alexcraiggtv,

The problems Andy had were related to the procedure he used to install his cells and would NOT have caused a cascade effect as you imply. This video bares no relation to Gregs 'incident' and in fact shows how safe LiFePO4 cells are when physically damaged. In the video Andy confirms they only vented, this is not a fire or explosion, just the electrolyte releasing it's oxygen content in a safe manner.

Sorry, I don't wish to come across as defensive of the cells in Andy's video or dismissive or your theory, but want to ensure we don't cloud Greg's investigation with false leads.

Ian

Appologies that I haven't been as active as I would like on the forum, not enough hours in the day! Some good questions have been asked, so hopefully some answers.



The bms boards turned on a 250ma balancing load at 3.6 volts and switched it off at 3.55 volts. Since I didn't have any way to control the charger I simply switched it off when a cell got to 3.75 volts. If I had left it off my pack would not have been balanced since some of the cells would have been at lower voltage. The BMS board continued to load the cell with a 250ma load and when that cell got back down to 3.6 volts the BMS would have allowed the charger to switch back on. This allowed the lower cells to catch up since their BMS boards would not be loading their cells.




My simple analogue setup provided no individual data for the cells. Had planned on moving to the full digital version as that offers individual cell monitoring and also any BMS failures are immediately detected and the system can just shut down. I had been using a Paktrakr that gave me a reading of each "12 volt" battery but I had stopped using it because it powers itself from the first 12 volt battery and was increasing my balancing time. I was also happy that all the cells were behaving normally and had no reason to suspect that a problem was brewing.



That was exactly the cause of this PHEV Prius fire. BTW, its a 2mb pdf. In this case the heat caused from the poor connection caused the cells to overheat and vent, but in a much more violent manner than shown in the youtube video above. In my case their was no high currents to generate large amounts of heat. One possibilty is that the relay controlling my charger simply stuck closed in the night. The charger would have stayed on, overcharging a cell, possibly causeing it to swell. Since they were not restrained against swelling, they could have burst out of their boxes and that could have possibly allowed them to short and start the fire. This is of course all hypothetical, but a plausible explanation, so should be kept in mind for future projects.

One thing that the Prius fire and the you tube video show is that these cells can generate tremendous heat when something goes wrong and that heat can easily start a fire even if the cells themselves don't catch fire. Temperature monitoring is essential and doesn't need to be complex. Even simple temp snap switches that open at say 40 Celsius would be a step in the right direction. And give careful thought about what happens if any one component in your system fails, what is the backup?

I think that's one of the most important messages to come out of all this. It's obvious, but worth stressing, that any cell in an EV power pack contains a lot of energy, and that energy will be released if there is a short. From all the reports I've read and videos I've watched I'm still convinced that lithium phosphate are relatively safe cells. They don't contain any metallic lithium, which is the component that burns so fiercely. But if there is a short, all that energy has to go somewhere, so if there's anything flammable nearby you have a potentially catastrophic fire.

I've seen lots of smaller lithium packs built with (flammable) plastic enclosures and I was considering doing it myself to keep weight down – not any more though. I now plan to enclose my battery packs in a ventilated steel box – probably lined with rockwool or a similar non-flammable thermal insulator.

Peter mentioned a while back that he was installing a smoke alarm above his pack. That sounds like a very good idea. Are there any readily available alarms that can be used to trigger a relay, so it could activate the vehicle horn for example?

One thing I'm not sure about is if or how you could actually get an internal short in a lithium ion cell, since all the reactions are supposed to be ionic – no metal deposits are formed. Are they caused by mechanical movement or damage within the cell? The reason I'm interested is that we can't do anything to prevent internal shorts.

I'm not suggesting that a short was the initial cause of this fire, just that it's brought home to me the fact that if something can go wrong it will go wrong eventually. It's just a shame it was you who had to demonstrate this Greg – that was a nice little car. Hope it hasn't put you off too much.

if these cells vent oxygen when overheated this could account for the very high temperatures of the fiat fire JOHN CRAYTON

Malcolm, no not too much. The Fiat was a great little car. It offered many advantages due to its small size and was proving to be very practical. Because of its small size, the battery pack was small, this meant relatively cheap (for a lithium pack) and could be fully recharged in around 4 hours from a standard household outlet. The small size and weight of the pack meant that it fit completely in the engine bay, so left the passenger compartment fully intact with all seats and storage space fully usable. The car weighed around 30kg over its published kurb weight, so no suspension modifications were needed. The usable range was around 30 miles, even in the winter. Keeping the distance within 30 miles gives a level of "reserve capacity" for unexpected detours. Maximum range was 50 miles and a larger pack could have increased the range further for those that need it.

I have some DIY projects that need finishing at home (moving kitchen ) so they will get priority. The Volvo needs some new batteries so I am pricing a AGM pack and it will be fitted with a temperature safety/warning system.



John, it would be nice to know what gasses vent from these cells if overheated. Is it just oxygen, or a mixture of gasses? Is it flammable or poisonous? What causes the swelling observed in the prismatic cells? Is it just a build up of gasses before they escape, or actual swelling of the plates?

Since we don't know the answers to the above questions or the precise cause of my fire, the best we can do is to protect against as many possible failures as we can think of.