That describes the situation where the 12V battery is flat but the HV battery is charged. But what happens if the HV battery is flat on a non-plug in hybrid? Can it be charged from 12V?
No. Nobody is putting in a 12v to 400v dc to dc converter. There would be no point. I haven’t done the math but I’d be surprised if you got more than a couple miles out of it. If your high voltage battery is completely dead, you’re calling a tow truck.
Yeah, I started wondering the same thing as I was writing that response, as I do actually own a Lexus with the Prius drive chain.
For starters, it would be pretty hard to get into that state in the first place. My understanding is they actually report a 10% or 20% charged battery as "completely empty". And the it's completely disconnected, so it shouldn't discharge much.
Looking though the service manual, there doesn't even seem to be an error code for "HV battery low", nor any mention of the DC-DC converter being able to charge the HV battery from 12v, it only goes from HV to 12v.
So I really suspect if you do happen to end up with a flat HV battery that's too flat to start the engine, it's just going to flash the "Replace HV battery" error code. Theoretically, someone with the right tools could disassemble the battery and charge the individual modules.
And the only speculation I can find on google is that "flat HV batteries are very rare" and probably only happen when the HV battery has other issues and needs replacement.
Edit: After more googling, apparently Toyota dealers have a special charger called the THS that can charge the HV battery in non-plugin hybrids.
There is no 12v starter motor, it always just uses one of the large motor-generator units to start the engine, and in the Prius drive chain, they require 600v (boosted from 200v from the HV battery).
So if the HV battery is flat, you can't start the engine to charge it.
