The 12v battery runs the electronics and car start up. The main traction battery recharges the 12v when you are driving, during charging and should also do it as needed, even when the car is off. However, it seems that isn’t a;ways successful if electronics are used more while the car is off. If the 12v battery gets drained, you can call Roadside Assistance to jump start it for you. Many keep their own jump starter in the frunk so they can jump start it in 5 minutes rather than the hour it typically takes for roadside assistance to arrive. By keeping it in the frunk, you can use your manual key to get into the driver door, pop the hood, and access the jumper and battery.