Whats the best thing to do when your ebikes batteries are smoking while riding and when its turned off?
"Best" depends on your experience and ability to deal with the specific things needed to be done in the situation, so not knowing that about you or any future readers of the thread I won't try to give "best" advice, but safest (to the rider at least) is:
If they're smoking, presumably they are very hot and it's possible they could burst into flame if they're hot enough, so your safest option is to do what you seem to have done and get away from the vehicle and call the fire department.
As for why they might have started smoking, it would usually be a short circuit from battery positive to negative, causing intense heating inside the batteries (and the wiring). If there is a fuse in the system and that never blew, the short could be before the fuse. Or the fuse could be sized too large to be able to protect against this specific failure. If there is no fuse I'd add one.
Another possibility is if the batteries aren't really capable of handling the current demand of the system, they could heat up internally just from a heavy enough load during the ride, and then without any cooling airflow the heat would remain inside and if intense enough could begin causing electrolyte degradation / outgassing (even smoke), or heat up the plastic casing enough to cause it to emit smoke.
To find out the cause, if you want to troulbeshoot and fix it, you'll need a multimeter and some patience.
