I'm a bit paranoid with lithium ion cells and don't want to charge or discharge them unattended, like when I'm several miles away from them.
What causes cells to combust? Obviously short circuiting and overcharging. If I have a cell not charging or discharging, is "spontaneous" combustion possible. Meaning like an overcharged cell doesn't explode until several hours afterwards, so even if it's not charging or discharging it can explode later on?
I've also seen power tool packs explode on YouTube in the back of someone's truck unattended in a fire safety video. I don't know if storing batteries in an enclosed metal box (a vehicle) can result in combustion because it gets really hot in the summer?
I think storing cells not charging or discharging in a dry cool place s relatively safe thing to do and should not result in combustion?
There are a lot of failure mechanisms that can lead to a cell catching fire. Not all of them occur right after some noticeable abuse. For example, a cell with an internal short circuit can sit right at the point where the exothermic reactions cause just enough heat to keep things “smouldering” internally until a different density area is reached, or a different level of electrolyte is present, and suddenly the reactions flare up and you get full thermal runaway (fire, smoke, possible cell bursting).
Common? No, incredibly rare. But the (literally) billions of cells that don’t catch fire aren’t talked about.
The lowest temp I’ve ever seen for runaway is for LCO chemistry cells (typically “LiPo”) and that’s about 130°C internally. The ”standard“ round cell li-ion chemistries, NMC and NCA won’t go into runaway until wayyyyyy over 200°C. I’ve brought standard chemistry round cells to over 100°C hundreds of times without problems. Pouch “LiPo” cells are different, some are pretty low quality cells...stay under 60°C and assume a fire can happen at any time.
But for most li-ion cells it all gets started at about 75°C where the electrolyte starts to decompose and generate excess gas. Any heat just from those reactions is easily dissipated unless the cell is being brutally discharged and overheating. So this “low” temp is no problem, except for the additional damage/aging this causes.
As the temp gets higher and higher you get more reactions occurring like the plastic separator between positive and negative melting at 105°C-115°C. Once that happens you can get an internal short circuit and the exothermic reactions get going. If the heat from these is not dissipating quickly enough then the temp keeps rising until the thermal runaway (decomposition) temp of the cathode material is reached and the cell catches fire.
Sooooo....a hot box or car shouldn’t cause a cell fire unless there’s internal damage or connected electronics (BMS, whatever) gets fried and causes current to flow and heats up the cell. But we can’t see inside our cells or easily detect impending shorts so the general advice is to avoid higher temps. Keeps the cells from aging faster too.
Overcharging can cause enough damage to, maybe, possibly, cause a fire some time later. Again, verrrrrry uncommon. But not impossible. The vast majority of fires occur during charging, use, or abuse. Not hours later. So always stick around during charging.
Internal defects can lead to a fire with some vibrations/shocks. Incredibly rare with quality cells from one of the big manufacturers. Can definitely happen with crappy “commodity” LiPo’s or round cells from the smaller China factories.
Cool storage (in a non-flammable location) is good as it lowers risks and extends cell life. But it can never be counted on to eliminate the tiny, tiny, TINY chance that the cell could go into runaway sitting there. But ALWAYS assume you’ll be the one-in-a-million to get a fire. The odds are tiny but the consequences are huge.
It’s like seat belts and air bags. We don’t have them because we know we’re going to crash. We have them for the tiny chance we might crash and because the consequences of the crash can be huge without that protection.
