I guess a picture of the bike before it burned might help?
Here is the thread on that bike. https://endless-sphere.com/forums/viewt ... mans+mixte
The bike was built to fit the battery, in the front saddlebags. Nice tight fit, cushioned underneath, It never required any strap to hold it down tight. It did not rattle around in there like is sometimes the case with bags or looser fit boxes.
The battery that was charging when the fire started thread here. https://endless-sphere.com/forums/viewt ... =4&t=71499
This battery was a blue shrink wrap 48v 20 ah. On arrival, it was in good shape, no indications of damage, the box looked intact, no electrolyte stink. The pack was then enclosed in a coroplast box, to protect mostly against chafing in the battery bag/ saddle bag. The battery was never dropped, nor the bike crashed. NO reason to suspect a crushed cell in there, or shorted nickel strips. But a short could have been the cause, nothing is ruled out here. It could have slowly developed a rub between a + strip, and the - can, and rubbed through the cells shrink wrap.
After about a weeks light use, as balanced as I could get it, a discharge test produced around 18 ah, so presumably some cells inside were not up to snuff somehow.
Discharging, the rate was usually about .5c on a pack supposedly rated for 3-4c. I did put it on a 40 amps bike once as a test, and saw it could not do 2c very comfy, so I never put it on a controller stronger than 22 amps after that. With 20" wheel and the slow DD motor on that bike, it was very seldom that you saw more than 800w being pulled, even towing a trailer uphill, it just would not do 1000w continuous.
About half the time, including the discharge that day, the pack was paralleled with a 13 ah allcell pack, which also sagged like grannies tits under 2c. So lots of the discharge was 800w from a 33 ah combined pack. Typically, cycles were about 15 to 20 ah from the combined packs, or 10 ah from the single 20 ah.
The packs were never charged parallel. Always each one charged separately. If in a rush, I had two chargers. The packs were never connected paralell except when both were full, at 54.6v.
That day though,, both packs were run till the bms popped, for my spring capacity test. Each spring I do this, so I can plan a very long summer ride based on current capacity. I think perhaps the bms did allow one cell group to go below spec voltage. Then when it got full charging, a damaged cell went runaway.
The other just as likely cause, the bms or the charger or both allowed an overcharge of a normal cell. No way to tell which happened, there was no trace of the bms left after the fire. The bms was in a plastic case/ box, did this cause the bms to overheat and fail? who knows, the plastic box never got scorched or melted deformed. Never should have gotten that hot, even when in balance mode. But I do wonder if the balancers were still, or even ever, working. I had not checked individual cell group voltages since it was new, you had to open the box to get at the bms plug.
So in summary, other than it was a bargain pack from alibabba, nothing in this batteries history would make you suspect it was going to flame on recharge. There was not a loss of capacity from when it was new beyond normal, it was only 1.5 years old, it was seldom run lower than 75% Dod, it was not dropped, crashed, or rattled around dirt riding, or discharged at high rates. I did not heat this pack up each ride ( So the coroplast box did not cause it to get hotter), did not series connect it.
NO reason to think it would flame. BUT IT WAS VERY STUPID TO CHARGE IT SO CLOSE TO FLAMABLE SHIT.