[Carac]

Why isn't there a "jumper" backup system built in? i.e. instead of having to lug around a sub-1kWh jump starter battery from Amazon why can't the 212kWh battery in the car "jump" itself? I'm assuming it already has to step down a charge without an alternator to keep the 12v up anyway.

 

[Carac]

The HV battery is stabilized and disconnected most times unless you are moving or charging, most of the cars systems run at low voltage other than HVAC, and Propulsion.

There is a DC to DC converter that charges the 12V battery, every OEM sets this up a little different. Tesla used to have lot of 12V failures when Model 3 first came out, but now is better.

If I was GM, I would be careful parking EV's for long periods of time outside in the Summer, its only a matter of time until something goes haywire and they get an angry HV battery.

Assuming the system is monitoring the charge on the 12v battery, it seems a simple solution would be to have the HV battery come back on to top off the 12v when it drops below a certain level, just as a backup, then shut back off. First priority would be to find the vampire sucking the 12v dry though.