My 12V Battery is dying

Discussion in 'Gen 5 Prius Care, Maintenance and Troubleshooting' started by priusasa, Dec 13, 2025 at 11:39 PM.

  1. priusasa

    priusasa Junior Member

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    Today I woke up with 2 warning light:
    20251213_113039.jpg 20251213_113038.jpg
    "Shift system malfunction see owner's manual"
    "Sub-Battery system malfunction. Visit your dealer"

    Then I took my multimeter and measured the 12V voltage, after it was sitting few hours. I measured 11.85V

    When the car is on, I measured 14.39V. So the DC-DC conventer is fine. Only the 12V is dying and I have some questions about it.

    I heard that this is common problem with the 5 gen, did anybody replaced under warranty? My car just hit 60k days ago, I dont know if I should even try.

    Second important question, can I drive car like this untill 12V dies? would it damage anything? This is very important because I would not want to take risk. (Chatgpt says it wont damage the car but..)

    If anybody had that issue, how did you solve it and made sure it will not happen again? like, is it the battery or car problem?
     
    #1 priusasa, Dec 13, 2025 at 11:39 PM
  2. BiomedO1

    BiomedO1 Senior Member

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    That's kinda normal for my gen4 (11.85V); it still starts and I believe that it will continue to do so down to about <10.5V. I've notice that it's pretty cold there; the snowflake symbol in your dash. Battery may be fine, just cold.
    Whenever I find my battery that low, I put it on a 6A charger for an hour or two - then it's fine for another 6 months. It's been worse since I retired and the car is no longer a daily driver.

    Walmart has a 140R AGM battery currently on-sale (flash sale) for around $106.

    Good Luck...
     
    #2 BiomedO1, Dec 14, 2025 at 12:16 AM
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  3. CooCooCaChoo

    CooCooCaChoo Senior Member

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    You're way outside of warranty for the battery. Time for a new one.

    As for driving it like that, chances are you'd get stranded in a parking lot because it's only going to get colder. Cold kills a battery fast, even if it inside the cabin.
     
    #3 CooCooCaChoo, Dec 14, 2025 at 3:27 AM
  4. priusasa

    priusasa Junior Member

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    This will cost too much then. I'll be looking for which battery I should get. Ah, another surprise..
     
    #4 priusasa, Dec 14, 2025 at 6:02 AM
  5. VelvetFoot

    VelvetFoot Active Member

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    Where do you live?

    My (limited) understanding is that warmer temperatures tend to cause the car to not charge the battery as much, and thereby causing premature failure. I usually get 4 years on a 'normal' car battery.

    I bought a BM-2 battery monitor the other day, and it's a fun toy. In the winter here so far, charging voltage on my new-ish LE hasn't gone below 14.4 V. I'm assuming in the summer if it shows that it's dropping, I'll put a battery maintainer on it once in a while to hopefully extend its life.

    I think the main dips are for the EVAP pump (if my terminology is correct).

    upload_2025-12-14_7-1-50.png
     
    #5 VelvetFoot, Dec 14, 2025 at 7:02 AM
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  6. BiomedO1

    BiomedO1 Senior Member

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    Yep, looks like you dipped below the 12V threshold in 12/12 and 12/13; about where the OP is. Your correct, the EVAP & ABS pumps draws a lot of current for their tests.
     
    #6 BiomedO1, Dec 14, 2025 at 10:44 AM
  7. priusasa

    priusasa Junior Member

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    I live in Boston. So here is cold and just snowed. Car still works but I need to do some research about which battery to buy. I am new to that and I want to find some "cheap but performance" battery.
     
    #7 priusasa, Dec 14, 2025 at 10:54 AM
  8. Mendel Leisk

    Mendel Leisk MMX GEN III

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    You've piled on the miles pretty quickly, are there any times the car just sits, say2 weeks at an airport?

    What's your parking situation? Do you have a secure garage with AC outlets, or?
     
    #8 Mendel Leisk, Dec 14, 2025 at 12:03 PM
  9. priusasa

    priusasa Junior Member

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    I bought just a year ago. It was 48k mileage. Previous owner did all mainteances but, battery should not just die in 2-3 year like that. No matter if you made 24k mileage per year.

    I park on the streets.

    There is a post in Reddit that some people got their battery replaced for free because their battery was faulty, I think they sit in the shelf without being used. Something like that.

    I got appointment from where I got my car and I will push them to check if my battery is faulty too. I know they will just say its out of warranty etc but I will try my chance.

    I just need to collect some data from other users.
     
    #9 priusasa, Dec 14, 2025 at 12:20 PM
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  10. BiomedO1

    BiomedO1 Senior Member

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    Well if the dealership says: 'there's nothing wrong with it' - I pickup a cheap jump pack, while they're still on-sale.
    That'll ensure that you won't get stranded for a dead battery. They're going for about a third of the price of a new battery.
    Beware; those lithium jump packs don't like the cold either - you may have to warm them up in your jacket; if you keep it under the seat of your car. Your better off placing it in your backpack or brief case and taking it inside with you - so it works anytime you need it.

    Good Luck.....
     
    #10 BiomedO1, Dec 14, 2025 at 12:55 PM
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  11. bisco

    bisco cookie crumbler

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    yes, jump pack, even if you get a new battery
     
    #11 bisco, Dec 14, 2025 at 2:08 PM
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