The 12V battery will go dead if it's not on a maintainer. I "killed" mine by leaving the car plugged into the EVSE in my attached garage while I fled to a warmer climate for five months one winter. When I returned, the car acted as though it had no batteries at all in it. When I connected a small charger to the 12V, it woke back up and reported the HV battery still at full charge...which it was. If you connect a smart charger (CTek, etc.) to the jump points under the hood, the 12V will stay fully charged. BTW, I put "killed" in quotes because my 12V did come completely back from the grave with charging, and has been fine ever since. It's still the original, too (car's production date: 02/2016). And yes, I'm also a BECM victim...I got in line about two weeks ago.