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The main battery is completely isolated by a contactor when the vehicle is parked (and not being charged). The contactor is operated by the 12V battery.
There is a large DC-DC converter to charge the 12V battery and power the ancillaries when the car is on, but it’s not much use being on the wrong side of the contactor.
I have pondered whether it would be possible to have a manual override of some kind to jump start the 12V system, but it seems no manufacturer considers it worth the hassle and complexity.
grams
Voltage regulation is a thing.
The systems I work on are all powered from single battery banks; from traction to lighting inverters to computers and microprocessors.