12V battery died again

Discussion in 'Gen 5 Prius Care, Maintenance and Troubleshooting' started by IABoy987, Mar 27, 2026 at 6:18 PM.

  1. IABoy987

    IABoy987 Member

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    Vehicle:
    2018 Prius Prime
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    Plug-in Base
    Months ago I posted about my Prime 12V battery dying, and the knowledgeable people here explained the Prime battery facts of life to me. What I should have done instead of using the accessory position, was to turn the car "fully on" ready to drive in order that the big battery keeps little 12V happy.

    So today while wife ran some errand I waited in the car. I turn the car "fully on" and verified that in fact car would actually move in F or R gear. I then pushed the park button and started reading a book. About 30 minutes later the radio went off and the dash indicator said "12V battery low, see owners manual." :( I turned off car and when wife returned much later, luckily the car started and we drove home.:)

    Obviously having car ready to drive, but not actually driving, does not "connect" big battery to keep little battery charged up.

    Am I missing something as in a work around would be to put on parking brake, keep foot brake pedal depressed and car in F position so big battery actually keeps 12V alive?
     
    #1 IABoy987, Mar 27, 2026 at 6:18 PM
  2. VelvetFoot

    VelvetFoot Active Member

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    Your profile says 2018. Is this a Gen5?

    Anyway, I have an '26 HEV, but I just went out, started the car, put it in Park with the radio on and went inside the house for a while. The battery monitor shows it was charging when in Park. Only for a few minutes though, I didn't let it go for a long time.

    The only time I got that message was when I was fooling around with my dashcam on Accessory. It wasn't too long either, so it's pretty sensitive.

    I cut and pasted a couple of things from my manual.

    This is probably a PHEV specific thing, I bet.

    upload_2026-3-27_18-35-58.png

    upload_2026-3-27_18-39-36.png

    upload_2026-3-27_18-55-3.png

    upload_2026-3-27_18-58-24.png
     
    #2 VelvetFoot, Mar 27, 2026 at 6:58 PM
  3. IABoy987

    IABoy987 Member

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    2018 Prius Prime
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    Plug-in Base
    Thank you VelvetFoot for the information. ;) A lot of good stuff, but not sure how much is applicable to my "older" Prime.
    Yes, my Prime is a 2018 so what generation category should I be looking in? I saw plug in and troubleshooting and went here thinking where my Prime is.
     
    #3 IABoy987, Mar 28, 2026 at 8:04 AM
  4. VelvetFoot

    VelvetFoot Active Member

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    #4 VelvetFoot, Mar 28, 2026 at 8:31 AM
  5. BiomedO1

    BiomedO1 Senior Member

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    Have you checked your battery voltage with a meter or ever replaced your 12V battery? Using a volt meter will tell you, if your system is working properly. Car OFF should read 12.xV, car ON at least 13.8V+. If your car isn't doing that; there's a problem with your DC-DC converter or your not in "READY" mode. If you was actually in "READY" mode, there's NO way the battery could've slid under 12V. Even if your traction battery ran low; the car's ECU would've turned-ON the ICE, same as it would while your driving. Have you ever replaced the OEM battery from 2018, it's 8 years old and probably on it's last legs. Get a free check at any auto parts store.
    There's also a misconception that there is a 12V maintenance charge while the car is plugged-in. That is WRONG; the 12V maintenance charge is ONLY ON while the traction battery is ACTIVELY charging. If your read your OM; it states to unplug your car after the traction battery is fully charge; otherwise the car will actually drain your 12V battery by doing more internal readiness testing.

    FWIW; the car will start with the battery all the way down to around 10.75V; but it may start throwing phantom error codes due to lack of power.

    IMHO; testing is the only way to nail-down an issue. Guessing usually doesn't solve anything. Good Luck....
     
    #5 BiomedO1, Mar 28, 2026 at 10:23 AM
    Last edited: Mar 28, 2026 at 2:15 PM
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