Lithium Ion battery fires usually occur when batteries are older and have reached the end of their duty cycle or were overcharged. What occurs is that one or more of the batteries in the parallel packs (4.2v) fails. That parallel pack will have a lower voltage than the rest. So When the complete battery series pack gets charged to the charger’s shut off value it has effectively overcharged every 4.2v parallel pack. More parallel packs will then fail. As they fail and get overcharged and consequently overheated they can self ignite. Lithium battery fires can cause a run-on Li-Ion fire event which is difficult to extinguish conventionally. requiring large volumes of water or specialized type extinguishers.
So it’s recommended that Li-Ion batteries be charged at low amperage rates and not left with a state of charge over 80% or under 30%.
Then just top them off right before they are used. The problem is that conventional e-bike battery chargers only charge to 100% SOC.
So specialty chargers are required or voltage monitoring during charging and disconnecting when voltage is at a lower voltage is required. 4.02v for each parallel pack is where I cut off precharging. For example I have a charger set to cut off at 52.2v for a 48v 13s e-bike battery instead of 54.6v. I also do not charge st over a 2amp rate.
Monitoring the battery’s voltage changes every time it’s charged is very important. Even just looking at the %SOC on the e-bikes display can give you hints. If it’s been showing 100% for a long time every time the battery is charged and suddenly starts progressively dropping its percentage/voltage reading after being fully charged, there’s a good chance that battery is failing and needs to be taken out of service.