There is a module referred to as the APM that convertes the near 400v HV system down to the 12v LV system for the control modules and maintanes the 12V battery state of charge. the 12V battery is needed to power up the modules that unlock the HV system. Any time the car is in READY mode, the 12v battery gets monitored and charged by the APM, however since the HV system is not active in ACC, it does not in ACC. IF the 12v battery state of charge gets below 10v, the car has a REALLY hard time entering Ready mode (that current draw is about 200 Amps!) and can not charge it self further. Additionally the On-Board Charge Module (OBCM) requires the 12v system to unlock the HV battery for charging, and since it runs off 12v it self, plugging in the car to 110/220V will not power that module if the cars 12V system is down. Additionally once the OBCM shuts down because charge is full and the HV system is locked off, the APM can not charge the 12v because it lacks the HV needed to do so.
For storage of a few weeks at a time: attach a 12v battery tender/maintaner/trickle charger and you should be good.
For storage of a few weeks at a time: attach a 12v battery tender/maintaner/trickle charger and you should be good.