You are correct. The 12 volt battery is charged when the car is in use.

The manual says the 12 volt battery depletes after 14 days of non use. I can see this with the ECU being in it's normal sleep mode when the car is not driven.

I see lots of talk about jumping from another car to start a discharged 12 volt battery. Looks like the use of a 1 or 2 amp charger with intelligence would work as well. I see this battery is a very slow charge and discharge type AGM (absorbed glass mat) battery.