I don't know on a full EV, but i know my PHEV has a relay for the big battery. When the car is off, the big battery is disconnected, and the 12v will drain (and would tend to die). I had to jump it a couple times, it just needs enough power to get the relay closed, the big battery usually has enough to run the car and start the engine as needed from there. Would have been nice if they didn't bury the 12v in the trunk, underneath the big battery though.