When to Replace 12v?

[dude7]

For safety reasons (and probably regulatory too), the HV battery has to be isolated when the car is "off". Which means you need a power source to run whatever mechanism connects it to the rest of the system (a high-current switch of one type or another).

OTOH, it's not like there's no circuitry on the battery side of the HV switch; all the battery management system is there. So maybe you could find a mechanical or optical way to communicate to the HV side that you want to close the main switch, and it could do it using its own power. You'd have to think about the safety implications, but it might be possible.

I found out the hard way that I can't "jump start" my Model Y with jumper cables. In an ICE car, you can simply use another 12v source, e.g. another car, by attaching jumper cables to turn the starter motor. There's no equivalent functionality in my Model Y where I can attach another 12v source, e.g. my other car, to get my Model Y to get the HV switch to contact. Tesla built in a mechanism to open the frunk by using a 12v battery, but no corresponding function to start the HV battery. Maybe I should have tried to disconnect the 12v battery after charging it and reconnected it, but again, this happened when my car was parked outside during a historic, national newsworthy rainstorm. In any case, if this happened away from my house, I would NOT have the tools to disconnect the 12v battery anyway, so it's a half-useless point.