[drcross]

I think you're wrong. Most electric cars also have a 12 volt regular battery for the in car electronics and the motor battery is isolated from the rest of the car. With that said I don't know how it's charged.

[mikeash]

You could leave the car "on" to avoid that problem.

Teslas will top up the 12V battery from the main battery even when the car is "off," but I don't know how other cars do it.