2012 Prius No Power until dome light is turned on

Never heard of this .... could be a short , bare wires toughing somewhere?
Have you done any mods to your Prius, lately?
Have dealer run a diagnostic on it!!

 

Always check the 12 volt battery when weird things start to happen.

 

Yes, there are TWO batteries and the small one is not monitored like the big one is.

The 12 V batteries are often neglected on the dealer's lots and early failure in pretty common.

Maybe you can even get it replaced under warranty........maybe.

In any case, giving the 12 V battery a good slow overnight charge will probably make it all better........for a while.

 

doesn't the power button run thru the dome light fuse?

 

This has the sound of a bad ground somewhere

 

Have seen equally weird stuff in a non hybrid car with a loose / corroded battery cable connection or a broken cell connector inside the 12V battery.

 

I wouldn't trust maintenance mode. I measured my 12V with maintenance mode and with a multimeter at the same time, and they differed by half a volt. Not much in the grand scheme of things, but if you're using it as an indicator of 12V health, that's pretty much the difference between a new battery and a dead battery.

That said, if you're going to be using maintenance mode, the $64-dollar-question is whether you were in Ready mode, or Ig-On, or ACC. In Ready, the battery will read 13-14V, because it's being charged by the car's DC-DC converter (off of the HV battery, even if the engine is off). Obviously, that tells you nothing about the battery. In Ig-On or ACC, there are a number of loads on the battery, so the reading you get in Maintenance mode (even if it were accurate) is not representative of the state of charge, which calls for an unloaded battery. It at least tells you that your battery isn't TOTALLY dead, but not much more than that - and the Prius can do funky things when the 12V battery is in a marginal state.

I'd suggest disconnecting the battery for 15 minutes some time, to effectively reboot all the myriad computers in the car. I'd also recommend finding a better method of checking the battery, and while you're in there, it's probably a good idea to check for corrosion or loose connections.

If it's not the 12V battery, then it's probably going to require a trip to the dealer to resolve.

 

Well, for the most accurate reading, you'd want to disconnect the battery and measure it directly (after letting it rest for at least 15 minutes). But that's a pain and I'd never do it; it's pretty much just as good to pop the hood, then close all the doors and leave the car for at least 15 minutes before measuring the voltage under the hood. You can measure between the positive jump start post in the fuse box and the negative somewhere on the frame or a bolt or something. Bonus points if you pop the hood by going in through the passenger door, so the brake pump doesn't operate from you opening the driver's door.

Basically, the point is to use the battery as little as possible just before and while taking the measurement. So take the key out of your pocket, too, so that the interior lights don't come on, and the car's not actively talking to the key.

 

So, if I pop the hood the night before, it should be good to check first thing in the morning?

 

yes, it should read 12.9 or fairly close.

 

12.9 is pretty high; even Toyota says that fully charged is only 12.6-12.8V. Mine has consistently been 12.5 for the past 4+ years. 12.0 is what I would consider low; Toyota's specs say discharged is 11.5-11.9V. If it's getting down there regularly, you'd want to consider buying a new battery, weighing the cost/waste of replacing the battery somewhat early against the inconvenience of having a dead battery.

Personally, I'll probably deal with the inconvenience and not replace the battery until it's really dead, no matter what the voltage is. But I'll keep checking the voltage from time to time to gauge the health... Others may prefer to replace the battery every 3 or 4 years, whether they need to or not, just to be certain that their battery doesn't die.

 

From the repair manual:

 

agreed, but better safe than sorry. too many people post 'my battery is 12.0, so it's not the battery, it must be something else. two weeks and 40 posts later, it is the battery.