the car has a 12V lead acid battery and a ~380 (??) volt lithium battery. when the car is on, the high voltage battery is connected and a DC/DC converter provides 12V to the auxiliary systems and charges the lead acid battery. when the car is off, the high voltage battery is disconnected. only the 12V lead acid battery is there to run your auxiliary ssytems, which includes all the computers/electronics of the car. if the 12V battery is dead, the car is dead, even if you have a full charge in the lithium ion high voltage battery. the 12V battery is constantly monitored and periodically topped up from the high voltage battery to prevent a dead battery/car. If the 12V battery is bad, this top-up wont help. I also don't know how often the 12V battery is checked, maybe if you have a high enough drain on it it might become fully emptied before the car checks it for topping up. or perhaps your circuit that charges the 12V battery is defective.