In the app: Battery→See the 12V history
From the guide: Sleep and the 12V battery
What the app says about 12 V voltage: resting voltage is read after the car has slept, before it starts charging the 12 V. 12.4 V or more is healthy. Resting under 12.4 V means the 12 V battery is weak or something draws power while parked. Under 12.0 V the car may not wake up. Over 13.2 V the car's DC-DC converter is charging the 12 V, which is normal while the car is awake. A healthy car loses only a few hundredths of a volt a night.
From the guide: The car
The Ocean has a conventional 12 V battery that runs the car's computers, locks and contactors. If it goes flat the car cannot start its high-voltage system. The high-voltage battery tops the 12 V up while the car is awake or charging, not continuously while asleep.