I'm new to the Prius, so please forgive this very basic question. This evening, long after sundown, I found myself stuck on an unmoving expressway with no sign of traffic starting up again (the radio reported a vehicle fire ahead, all lanes blocked). Time dragged on, and I started worrying about the possibility of the headlights draining the 12V battery , assuming that they do indeed run off the 12V battery. In a conventional car this wouldn't have been a problem because the idling engine would have maintained enough battery charge, but of course here the ICE wasn't running. So I turned off the power button to shut down the lights (it's a Canadian car, so there's no way of turning off the headlights with the power button on). That was safe in this particular situation because there were plenty of stopped vehicles behind me. It definitely would not have been safe if mine had been the last vehicle in the lineup. Was I worrying unnecessarily? In other words, would the ICE have started up if the system had sensed that the 12V battery was getting too low?
Yes, the ICE cycles on and off automatically to maintain the high voltage traction battery, which in turn maintains the 12v auxiliary battery. People have even camped/slept in the Prius overnight with the Prius in Park/Ready and the AC on. SCH-I535
Just leave the car in Ready mode. The traction battery will keep the 12V battery charged, and the ICE will start as needed to charge the traction battery.
You are better off to leave the car running as it takes care of all you are worried about. The ice will start if the pack needs charging. The 12v is taken care of by the big pack. The 12v basically takes care of the PC's. H
There isn't an alternator run by the engine like a conventional car, the 12v battery is instead charged by a DC-DC converter that runs off the HV battery (which is charged my the motor-generators in the transmission and the computer makes sure it stays charged). So don't worry about it.
And yes, all the 'Normal Car" stuff is 12 volt. The A/C compressor is 201v AC, and the Motor/Generators are 550 v AC, but pretty much everything else is 12v DC.
Actually there are -two- alternators. MG1 and MG2. And they are turned by the engine (if running). But it's a different system than an obsolete type vehicle. As partially explained above.
