Extenuating circumstances, but EVTV's Jack Rickard found out the hard way that charging a pack with the same cells as the Nissan Leaf that had been dragged down too low without a BMS resulted in a fire that destroyed his workshop.
"This pack was right out of the cargo container and we never even attempted a bottom balance. We were only going to use it for testing chargers and DC-DC converters and the UQM test bench. But as the result of one of our assclowns playing around with the bench while I wasn’t in the shop, it had drained down very slowly overnight to a very low level.
It seemed to charge back up ok. But never quite got to full charge. So I had hooked it up earlier in the afternoon to bring it up some more."
Yes - bad idea to try and charge a pack that is suspicious without any BMS.
"I quickly shut off the charger and cut off the contactors. But it continued to BANG and POP irregularly... Suddenly the pack begins to issue the familiar white smoke – just a bit at first, then more. The pack weighs 450 lbs, and the fork lift is at the other end of the building...."
"By this time the white smoke was coming out pretty good. I don’t know why, but I was curious what the temps were. So I grabbed an infrared gun and shot all the cells. Most were warmish in the 35-40C range but there were two sitting at 95C. Not good."
"Suddenly the pack spewed a spear of sparks and flame about six feet straight out the front – right where I had been a moment before. And then it exploded into a massive fireball shooting flames up to the ceiling with such velocity that they splashed laterally from there."
Lessons to take from this - don't overdischarge, and don't let them get out of balance! If they are out of balance, don't try and charge them without a BMS.