The way I understand it, 12 volt battery is maintained by the high voltage battery through a converter connected the 12 volt battery. If the car sits for too long, the converter will automatically disconnect from the 12 volt battery to prevent depletion of the high voltage battery, because if the high voltage battery gets too low there is no way for the car owner to recharge it. There is a reset button on the dashboard that re-connects the two batteries to allow the car to be powered back on. If the 12 volt battery sits too long without being maintained, the manual gives a procedure for jump starting. I think if you connected an automatic trickle charger to the jump start connections, you may be able to prevent having to jump start, but I have never gotten a confirmation whether that would be possible.