prius loses all power when shutting off

If you connect a voltmeter to front jumpstart terminal (and ground) and meansure the voltage there what does it read when fault happens? If normal the problem is not with 12v battery connections. If low you can easily check where the problem is.

 

I'm sure it would read zero volts because everything in the car is dead at that point... I think there's a relay that's the first thing to fail when you have a weak circuit that's powering down. Something about the circuit that shuts the vehicle down, but maintains 12v power to interior lights, door locks, sks etc...

Today it even started happening when SKS first sense my hand on door handle and lost power soon as the door locks powered up. Lol... And again, the most solid fix is opening and closing the right rear door. So now I'm trying a new strategy where I turn the car off with the right rear door open, which seems to work. Too funny!

 

Then if the voltage goes down you just need to find where the problem is.

One way to check for a problem in the ground side is to connect a jump start cable from negative battery clamp to body ground. In a Prius it's easy by just removing the cargo box and connecting to hv-battery case. If that doesn't fix it problem is between battery clamp and battery, posive cable, or in eather fuse box and all those are easy to check.

Of course you could just use voltage meter to measure voltage between to get the same information.

 

There is no relay and measuring voltage at the terminals can never be zero. If it gets down to around 10.5 volts the computer starts going wonky, but the dome light will still work. Actually go check instead of just saying "I'm sure.."