In the case of Teslas, yes. The power drain on the 12V system when parked from all the computers etc is apparently so high that they have to recharge it periodi

In the case of Teslas, yes. The power drain on the 12V system when parked from all the computers etc is apparently so high that they have to recharge it periodically from the main battery. The service life of the 12V battery is pretty terrible because of this. They also have battery heaters that switch on periodically to keep the main battery warm, which I think are powered by the main battery. Basically, if you leave your Tesla sitting in a garage for a few days you can expect the main battery to be connected and used at least some of that time (maybe all of it) even though it has a contactor to disconnect it entirely.