About batteries.
Unlike other battery types, Lithium batteries require electronic monitoring for both charging and discharging. Usually each cell voltages are monitored in order to achieve maximum battery performance and avoid damages and early battery death because of slight difference in battery cells.
If certain limits are reached then battery monitoring system stops or limits battery currents, activates battery cooling or warming. So it is a tricky business with a Li battery in high current applications like in electric cars. Battery usage envelope must be maintained at any cost. If battery monitoring system fails, it should switch off the battery itself or limit battery usage considerably.
If Li batteries are over charged they will start to overheat andat certain point they are not able to contain their own charge and self discharge adds to overheating with rapidly rising temperatures starting to boil highly flammable organic electrolyte which usually burst battery container and creates flames which again usually overheat neighbour cell causing same reaction. if not cooled down fast enough and boiled off organic electrolyte safely vented, the whole battery burns down in a chain reaction emitting huge amount of heat if oxygen is allowed to contact overheated battery parts.
How these problems are solved in aircrafts I do not know. But it seems like huge fire was avoided, as closed Li cells can violently explode and burn. It could be that some of battery cells broke down while being operated in specified envelope. Due to unexpected paremeter change during normal aging. Li batteries can withstand many thousand cycles.
At the same time NiMH batteries are much more forgiving. but are twice as heavy too. And even more expensive.