Not starting....then starting!

Definitely check your 12V as you will not get a warning to change it. All the 12V does is boot the computer enough to allow the high voltage battery to take over which then starts the car.

 

Most likely cause is the 12V battery. You can prove by using a jump pack or jumper cables but that will just prove you need a new battery. Many auto parts stores will replace in the parking lot for free.

 

You are not alone with being confused by this kind of behavior.

NO, there is no "low battery" warning because it would have to be produced by a computer board and THAT is what is not booting up right to allow it to start.
It is not unusual for repeated attempts at "booting" the computers to finally get far enough to actually run.

Get a new 12 V battery......and/or check the main battery connections.

Note: Charging is only a bandaid. Might only help for one trip or a few days.

 

All this does is show how many volts the charging system is putting into the battery. It's like checking the voltage on a battery in a conventional car while the engine is running. It doesn't say much about the battery itself. For that, you need to shut off the car, let the battery rest a few hours, and then check it with a multi meter.

 

Prius doesn’t run a starter from the 12V battery, like normal cars.

The 12V battery powers electronics and closes relays which engage the high voltage traction battery. It is this battery which starts the engine. That’s why the Prius can start up strong even with a marginal 12V battery.

 

That is NOT the battery voltage but the voltage of the battery CHARGING SYSTEM.
That proves nothing about the battery itself.

 

the engine doesn't charge the 12v, the hybrid battery charges it through the inverter.

there's no altenator, the engine charges the hybrid battery through the tranny generator and inverter.

you can do a 12v check without the car 'ready', which would show the battery charge minus whatever electronics are powered up

 

What @bisco said. The engine has nothing to do with charging the 12V battery. If the car is "READY" it's maintaining the aux battery voltage. Always.

 

You still aren't thinking in the "hybrid mode".

The "system" is running even if the gas engine is NOT.
That includes the "inverter" that charges the 12 V battery from the HV hybrid battery.
The gas engine does NOT have to be running to do that.
Any time the system is in READY mode (running) the inverter produces about 14 V, independent of what the gas engine is doing.

I tried to explain why it works sometimes and not others.
Summary: For the system to start (not the gas engine), several computer boards must boot up completely and properly.
If the 12 V battery voltage is marginal, sometimes they will boot OK and other times not. The exact reason isn't important.

 

Good luck.
You likely will need it.
I hope it doesn't fail at the "outer end" of your trip.
Once in this condition, no amount of charging will make it "well" again.

 

pleasure!