Why does the 12v drain so easily?

The 12v.battery is about the same as a motorcycle battery so it doesn't have much capacity. It's use during start up is to just power up the computers so it doesn't need much juice. If it goes dead in a week it's probably shot, just replace it.

 

Not sure about all Prii, but Gen2 has a relatively slow 12v charge rate when the car is running, so you really need a good hour or more to really bring it up if it got too low.

 

(1) Use a battery-maintainer when the car is parked for a long time without running.
and/or
(2) invest in a jump-starter pack

There are lots of threads on PriusChat about (1) and (2) - search can be very useful! (I'll leave that as a good exercise for you, since I already know how to use search!)

BTW Welcome to PriusChat!

 

Welcome to PriusChat!

Cold is an enemy to all batteries, not so much because the battery is weaker, but because everything is harder. Oil is thicker, Transmission fluid is thicker, etc. The fans are all running, often the wipers are too.
The 12 volt battery does not turn the engine, but it powers the computers. The computers decide if they will allow the use of the HV Battery, which starts the engine. You really want all the tests to come back perfect before you allow high voltage out of that battery.

 

the battery was likely defective when you purchased the car. many dealers let them go dead on the lot, which severely damages them.

my 3 prius have been left up to 7 weeks in the winter with no starting issues when we got home from vacation.

 

I am having similar problems but I live in Oregon where it's typically very mild. I use my Prius C for errands, like grocery store, etc. It's a 2014 with low mileage and ran fine until just before Christmas 2017. I had it jumped then and it ran until February 8. I had it jumped again and took it to the dealership. The first guy I encountered in service just said I don't drive it enough. That's ridiculous. Even if I don't drive it for a few days, I go out and run the engine. I'm painfully careful not to leave anything on at all. The battery indicator shows it's topped off.

So last Friday, March 2, it didn't start. again. I have noticed that when the car doesn't start, the events are as follows: I push down on the brake as far as it goes, but it doesn't go to the floor. Then I push start and the heads up display flashes, the brake depresses to the floor, then everything dies. The car just whines for awhile after that, even when I turn it off again. Now it's completely inert in the garage.

The service guy didn't even offer to check the battery, probably because it's awkwardly located under the back seat which I didn't realize until then (since the jump point is under the hood). However, when I bought the car brand new, it had to be jumped in the showroom. My husband asked if it was going to be a problem for it to sit awhile between trips since he knew I'd probably only drive it once or twice a week around my area. They said there would be no problem with that, it was just low from being looked at without the engine turned on. If I had known about the battery issues, I would have bought a Corolla. I've complained to Toyota USA but they've just passed me to the dealership who has yet to show any interest. Now since the battery has been jumped at least 3 times that I know of, it's probably shot. Oh, and the heads up display never gives any hints that the battery is low or otherwise problematic. We have a 2011 Prius that never has any problems (and others before that). It seems it's the Prius C that doesn't work right. :-(

 

welcome!
the battery indicator is for the hybrid battery, not the 12v. get a new battery, and you'll be fine.

 

Yes, that's what I originally thought but could never find anything specific about the indicator. There certainly doesn't seem to be any warning or idiot light for the 12 V. I took the car in to replace the battery last Wednesday and it's still there. Why? Because there's a battery shortage and they have to wait for the factory to build new ones! Unbelievable. I can only surmise that these batteries need replacing a lot. Now I wonder if I should trade it while it has low miles.

 

trading it might be a good idea. in the meantime, you can get a battery online, or from a local auto part store.
the o/m has a section on the battery icon, but you are correct, there is no warning for a bad 12v.

 

This is a Catch-22, if there was such an indicator, it would be powered by the same 12 bolt battery it is designed to test.

Toyota Prius Battery Sealed AGM - (2010-2015)


TOYOTA Prius V Battery. S46b24r. Prius, efi, dohc - 2880021171 | Village Toyota Parts, Homosassa FL


https://www.ruralking.com/51jis-auto-agm-battery.html?gclid=Cj0KCQjw7Z3VBRC-ARIsAEQifZRQn-7foEKMJRHVkToIYopr05BRFAyC6odTdjzYWHMVyj1EVb4zC1UaAnfhEALw_wcB

amazon.com/ACDelco-ACDB24R-Advantage-Automotive-Battery/dp/B010GKJ8F6

Optima DS46B24R Optima battery direct fit replacement for Prius 2004 to 2015 , Prius V & Plug-in Prius with Instructions

There are other batteries. Locally a NAPA or OReilly's auto parts store may have them

https://www.napaonline.com/en/p/NBP9851P?searchCategory=11.1&searchIndex=App

https://www.oreillyauto.com/detail/super-start-platinum-5003/battery-accessories-16452/battery---automotive-16864/battery-12836/battery/prius/4742707

 

As I suspected, there are lots of options. I suspect they are holding my car hostage in hopes I will trade it in. I left a stern message with the service manager and next time I will replace the battery myself. I will get a tickle charger too. Thanks so much for your help.

 

My 12v is going almost 5 years now, with the past 3 years spent in Wisconsin cold winters. Hopefully it'll go for another year or so.