If they short out, they can overheat and burst into flames. About 10 years ago a dive boat off Santa Barbara Island burned to the waterline, killing all but one or 2 that were sleeping on deck. The cause was determined to be the Lithium Ion batteries that the divers were re-charging in the main cabin while they slept. There have been numerous fires involving E-bike batteries and EV’s (cars). E-bikes are banned from high-rise apartment buildings in NYC.