12V battery keeps dying

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Melisande, Oct 27, 2022.

  1. Melisande

    Melisande Junior Member

    Aug 3, 2021
    Orlando, FL
    2007 Prius
    The 12v battery in my 2007 Toyota Prius keeps dying. I had it replaced at the dealership in early September after my car failed to start, got a jump start, drove to the supermarket, had the car die again, and then had it towed in.

    A month later, in early October, it dies again …again in a parking lot. I had started it earlier in the day, but after two hours of sitting in a parking lot … dead again. Got it towed to the dealership again. Got another new 12v battery. I suggested that it could be a parasitic drain issue (my broth-in-law is a car repair hobbyist and suggested this to me). They supposedly tested it at the dealership for the parasitic current draw, but found nothing.

    Today, no more than 23 days after the last battery was replaced, it is dead again!

    I only drive the Prius every other day or so and some people have suggested that this might be the issue. I need to drive it more. But this doesn’t make any sense. I’ve had the same low usage pattern for about ten years now with this car and it never had this problem before. So why now? Also, why would it die just after I used it and not first thing in the morning?

    I have two options at this point:

    1. Bring it back to the dealership for a third time. I spoke w a manger on the phone and he agreed that the issue was more than merely a faulty battery (pretty obvious now) and that he would oversee the diagnostics to make sure that they found the problem. I mentioned that I was thinking of bringing the car to an auto electric specialist and he said that no one else could solve problems that they couldn’t. He said that he thinks it is not a parasitic draw, but the inverter. Does this make sense? He also said that in his 30 years as a repair person he had never once seen a Prius with a problem with a parasitic draw. What to make of this statement? Doesn’t this seem fishy? I mean besides the fact that it is impossible to have 30 years of experience with Toyota Prius (LOL). Maybe he has never seen an issue with a parasitic draw in a Prius because they don’t test them thoroughly? Oh I don’t know. But anyway, he seemed pretty stuck on the inverter fail possibilityand very very dismissive of the parasitic drain theory.

    2, Bring it straight away to a auto electric specialist (who wouldn’t be a specialist in Priuses though)skipping the dealership.

    In other words, I am trying to suss out whether it is more likely to be an issue with the inverter (hence back to the dealership) or a parasitic draw issue (hence to the auto electric specialist).

    What do you think?
  2. Tombukt2

    Tombukt2 Senior Member

    Nov 29, 2020
    Durham NC
    2009 Prius
    What I would do at this point is hook my fluke test meter. Hook it up to the battery with everything turned off and see what kind of voltage is moving when the car is just sitting there like after 20 minutes when everything is nothing hood up I'm not touching anything the remote's not near the vehicle nothing's whining whirring nothing that battery is losing voltage for a reason going to ground going to a device something or the battery is not up to snuff all five of them or however many you've had It doesn't take a lot but seriously you need to find out what is slowly or not so slowly draining the battery in 2 hours of sitting in the parking lot a dead 12 volt 40 amp an hour battery should not be a thing something's on something's hunting for electricity the whole time the car is sitting whether your SKS system is messing up or what have you something is sucking up some power Will it do this if you disconnect the 12 volt positive terminal the minute you park the car I keep my tray there where I can move it almost instantly or it's not there or it's been trimmed so I can lift it up immediately to get at it undo the terminal go do your shopping come back stick the terminal on did the car start it's not the battery's fault
  3. alftoy

    alftoy Senior Member

    Mar 16, 2009
    Saskatchewan, Canada
    2008 Prius
    Maybe try Tampa Hybrids
  4. Kenny94945

    Kenny94945 Active Member

    Mar 24, 2016
    2016 Prius v wagon
    FWIW. If you go back to the dealer, it sounds like they may treat you well.
    But, they are getting you ready for an inverter issue/replacement.
    I am unsure of the cost.
    Are you on warranty? If so I'd would return to the dealer.

    Me? I would first do as suggested and begin with a simple battery test.
    Just need a multimeter to gather results.
    Voltage fully charged should be over 12.6. Turn headlights on for 2 minutes, voltage again should be over 9.6...from my memory. This is one with the engine not running :)

    If you find a parasitic raw, that is where an electric shop could help.
    One way is to remove fuses until the affected circuit is found.
    Then the question is, who will do the repair? Will the shop have all the Toyota specialty tools for example?

    Two batteries, drive every other day, yes something is affecting the 12v battery system. Any aftermarket systems in the car, stereo, alarm, seat massager LOL ?

    As for 30 year comment. That can be interpreted. Experienced man, whose time at Toyota hasn't shown any Prius para draw issues.

    Good luck.

    MCCOHENS Member

    Oct 3, 2019
    Chalfont, PA
    2006 Prius
    This is a pretty easy issue and finding the cause may take time but not a lot of skill for anyone experienced with auto electronics. I have no idea if an inverter issue can cause battery drain, might be interesting to research that first.