dnmun wrote:again a lot of misinformation about how lipo fires start. it is not because they were overdischarged as in this case or because they are punctured.
thermal runaway in lithium polymer is initiated when the cell is pushed beyond the 4.2V level where it is already 100% charged. at some point while exposed to this high voltage, the crust that forms on the anode as the lithium is deposited, breaks off in sections which then exposes the underlying anode to the charge and this causes an exothermic reaction to begin which then causes more of the crust on the anode to break off and lead to more heat released as it is charging, and the continuation is thermal runaway. this leads to the fire as the pouch burns because the organic polymer will burn in the presence of air.
It's actually not anything remotely like that. Where do people get this stuff?
You de-Lithiate the cathode during over-charge to the point that it liberates oxygen that reacts with the solvent. If the cell was old and the anode side wasn't built with much capacity overhead, then it sometimes can intercalte all the lithium ions and forms plating, and lithium has strong tendencies to form dendrites, which damage the structure of things, but aren't what drives the fire. The polymer seperator just becomes additional fuel, but isn't driving any reaction, and certainly doesn't burn in the presence of air, lol, I have a dozen various polymer seperators out on my desk right now, which would be a much more exciting desk if they burned in the presence of air. They are exactly as hazardous as a chunk of a plastic bag, meaning, if you light them, they burn, if you blast them with O2 while on-fire, they burn impressively (along with wood or anything else on fire you blast with O2.)
You can learn about what happens here, which is remarkably a decently correct paper.http://www.intechopen.com/source/pdfs/1 ... teries.pdf
there is no reason to fear using lipo, but it should be treated with common sense. methods sells a simple and effective solution and the new zephyr BMS that gary and richard have designed also will protect the battery from allowing any one cell to be overcharged past the 4.2V level.
with this level of surveillance there is no reason to fear that lipo will inevitably lead to fires. literally billions of cellphones and other devices have lipo batteries and the incidence of failure is very low, because they all use a BMS built into the charging electronics in the cellphone.
so with a proper BMS, there is no real reason to be unnecessarily afraid of using them. i don't think the packs have to be removed from the bike and stored in the oven like some think since the risk is during charging so just monitoring the charging along with use of a BMS should essentially eliminate the risk of thermal runaway.
With healthy cells, I agree with all of this. However, it seems not all cells are healthy, some have some included impurity or tear or flaw that causes them to literally randomly have a thermal event. I've not seen it myself, but I trust JohnInCR absolutely, and he had one burn just chilling on his desk. The same desk I have sat and worked at years ago in fact.
dnmun wrote:the risk is not during storage, only during the charging process.
even when john's pack shorted out internally while stored on his shelf and melted the pouch, it did not go into thermal runaway and cause a fire.
We don't know his pack shorted internally. We only know that it was sitting and then released a ton of heat. I agree that it's awesome that it was sitting with other packs and didn't catch any of them on fire or spread. The pack itself actually just looked like an extreme thermal venting rather than a flaming event, which is great, but doesn't mean it couldn't have just as easily been a flaming event, which jumps the temps up drastically.
The problem is not that it's LiPo. The safest cells in the world right now happen to be LiPo (not RC LiPo obviously, but grid storage cells). The problem is that we've seen a few events of defects showing up in RC cells (which have terrible QC) that resulted in random catastrophic failure modes. That's the part that sucks.
For making large packs that get stored in homes etc, we really need to be using Tier1 grade automotive LiPo. It's not as high of C-rate, it's saggy, it sucks to build packs with it, it generally always ends up being heavier and bulkier than you wanted, it has limited form-factors for packaging. And yes, all those things suck about it, but your cycle life is in the 3000's to 10,000's of cycles, they are matched so well your meter won't even read precise enough to try to improve the balance on them, they handle being over-charged to 8v and do nothing, the handle being over-discharged and reverse charged and crushed and over-heated and just sit and do nothing. Unfortunately, none of the Tier1s will even talk business if your purchase volumes aren't in the $10,000,000 range, and your design needs to be thoroughly approved by there teams.