12v Battery Issues

welcome!

if you are going to do this yourself, you'll need some tools.

you have to get the spec for the quiescent draw when the car is off from the service manual.

then with a digital meter, read the draw on your battery. if it is within spec, you just need a battery maintainer between drives.
if it is outside spec, you have a major headache to track down.

an alternative is to get a free load test at an auto parts store, and a recharge if the battery passes.

then just keep a maintainer on it.

 

I can offer our experience, which suggests that all is not well with your C; a trip to Toyota or other specialist for diagnostics may be in order.

We have a 2015 with around 33,000 miles, often left in the garage for 2 weeks or more without a trip and then not often more than 5-10 miles. We are on the original battery and have not had a problem starting at all. The garage is unheated and temps have been in the 20's and 30's the last few days (last time out yesterday). I expect to replace the battery next spring, unless we run into trouble this winter.

The only thing I can think of that might run a battery down "routinely" is if the fob is kept too close to the car, and it continually 'recognizes' it. I don't know if this would be enough to run the 12v battery down (seems unlikely to me), and I would expect there to be fob battery issues as well if that is the case.

It seems more likely something is "on" and draining the battery, though I only know enough to wonder just what kind of problem could cause that. Fault in the "on" switching? Or there are a few odds and ends on the dash and under the hood that come on from time to time, one of which might be 'stuck' on.

 

Worked on my first Prius C last weekend... Was surprised at how much the 12v drained down with even something as simple as leaving the door open. But maybe as above commenter pointed out, the FOB was too close to the car? 12v voltage drains when car is off are fairly challenging. You're gonna need to become an expert on monitoring your 12v voltage and perhaps pull fuses to certain circuits when car sits to find the one that's draining. First step is to fully charge your 12v and disconnect it from everything in the car and confirm the voltage drain no longer is happening. Then hook it back up, but without the fuse for the interior lights plugged in. And just keep going through each system till you find it...

 

Given the reported history, I suggest that checking the charging voltage AND checking all the main battery cable connections also should be done before, or coincident with, the above.

 

Cool theory, and that would explain a lot of battery problems on many Priuseses, but not this one.

The c trim One and Two have ordinary metal keys that the driver has to twist to make their cars READY. Trims Three and Four have electronic keys like the big Prius.

This one will need traditional electrical troubleshooting: connect an ammeter at the battery, and monitor the readout as the car is closed, locked and 'abandoned.' And periodically afterwards too.

If the draw is beyond the norm (I don't happen to know, but for many cars it is around 30ma) then it needs further investigation: pull fuses one at a time until the extra demand vanishes to learn which circuit is hosting the vampire.