Turning on after shut down without key in car.

The 12V 'boots' the electronics when you first push the Start button. The HV battery starts the motor. If your 12V was pulled down too far, you could get all manner of wierdness. I suggest taking it to an auto parts store for a test and a recharge. I bought my Prius with a low 12V batt, and learned the car will not top off the 12V. Instead it will only maintain the charge it already has. I had to replace the 12V, and even the NEW batt came with low voltage (presuming it was stored for long time before being shipped. I purchased a smart charger

reconditioned it, and have not had any 12V issues since.

Note: There are less expensive smart chargers than the one I purchased. I like that this one not only had a setting for 6 V, but also AGM batteries. I use it for the Prius when the cars sits for a while, as well as re-conditiioning UPS batteries.

 

agree^ test the 12v, even though new. if that's not it, replace the combo meter.

 

i had some similar issues, just wanted to post for posterity my solution

the positive battery terminal was loose and connection was intermittent which apparently fooled the electronics.

what I did was take the terminal out of the car, take a metal file, and grind down the "lips" where the screw goes through. This made the circumference of the terminal slightly smaller, so that it could grip the post on the battery better.

Since I did this and re-installed the terminal, i have not had the phantom restart issue.

 

The normal course of action is to back off the nut on the terminal clamp so that it is right at the end of the bolt, then use a large screwdriver (or similar) to prise the clamp open nice and wide. This allows the clamp to drop right down to the bottom of the battery terminal. There should be plenty of room now to use the ring end of a box wrench to tighten the clamp all the while holding the clamp firmly down.

Your clamps were not standard SAE ones rather than the smaller JIS ones for the stock JIS battery terminal post, by any chance?

 

Bought my 2009 prius used with 15,000 miles on it.
I had an issue where I'd come out and the car would be dead as a doornail. If I tried to get in, then waited like 30 minutes, then tried again, things would work.

I did 2 things:
- had the dealership completely replace the 12v battery
- one of my keyfobs was damaged and I stopped using it

That fixed the issue for me, never happened again, has 200,000 miles on it now. do not know if it's related, but, fyi. Probably worth just completely replacing the battery to try to fix it. The dealership told me the battery tested just fine but they didn't have any other ideas and I was like "battery is the only easily replaceable thing it could be just replace it". Like I said the issue went away.

 

i partially replaced the 12v two years ago, seems to have done the trick.

 

Partially replaced?

 

you have to read post 6