Now, what the actual fuck happened here.
LiFePO4 (LFP) batteries aren't as flammable as other Lithium battery chemistries, primarily because they're not self-igniting, and secondarily because they aren't self-oxygenating. These things I knew. Biggest issue you can have is that if a battery blows up, the LFP battery will vent. I also knew this. What I didn't know, is that when a LFP battery vents, that vented gas from the LFP battery is itself flammable. This is a pretty fuckin important detail that I did not previously know. Reference below. Do your own research:
So, let's get into what went wrong.
Ever since I had this battery array in the trailer I had one cell that was marginal. Meaning, it didn't have the measured capacity of the others, and in general brought down the whole pack, via BMS protection, because it would get close to overvoltage, or hit undervoltage, before the rest. Not a big deal, that's why the bms is there right. Well, if a LFP battery is left in the undercharged state for too long, it can fail and offgas. Also, if it gets too cold and is attempted to be charged or discharged, it can also offgas. This is important.
Mistake #1. I left the inverter off for a few days / week, and it's been below freezing for the first time this year. Can't do that. I know the pack was in a protection state when I clicked on the inverter, because the inverter told me low battery. So, don't leave them to discharge all the way in the winter, that was dumb.
Mistake #2. I didn't have a fire extinguisher immediately next to the batteries. I didn't think that fire was a concern, but that was, in hindsight, fuckin dumb. Always have a fire extinguisher right next to your electronics, one rated for electronics fires.
Mistake #3. The battery box was right next to the inverter. This was the biggest mistake I think.
Remember when I said that the LFP produces flammable gas when they fail and offgas? Right, well having the battery next to an inverter that has large contactors within it is fuckin stupid. What I think happened is that at some point over the past few days/weeks, the battery failed and started to offgas. There's not a ton of ventilation in that area, so I assume this flammable gas was building up. When I clicked on the inverter, it didn't blow up immediately, but what I would bet happened is that one of the contactors switched on to try to charge the battery, and the tiny arc from that (inside the inverter case) was enough to ignite the flammable gas from the LFP. This flame front traveled up into the battery that had failed and caused it to blow up, blowing apart the plywood battery box.
Mistake #4. The flame at first was so small that it took me 10 seconds or so to realize it was on fire, but between that time and the time it took me to find a fucking extinguisher / tool, the plywood lit on fire. The plywood fire is what caused most of the soot and scorching.
So, fuck. What are we going to do about it, and what should I take away from this?
First of all, don't put your LFP batteries in wooden boxes.
Second, keep your LFP batteries away from ignition sources.
Third, keep electronics rated extinguishers next to your batteries, no matter the chemistry. Lithium rated extinguishers for lithium.
Wanted to share my experience. I'm going to have a rethink of my layout to deal with these new problems. I'm a bit rattled, but thankfully I was there when this happened, instead of my camper burning to the ground for me.
Be safe out there.
