Bought Battery, It Sat on Shelf 17 Months, Installed Today: Readings are 11.4 / 11.1 / 14.3 V

Discussion in 'Gen 2 Prius Main Forum' started by PraiseDale, Sep 27, 2025 at 8:09 PM.

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  1. PraiseDale

    PraiseDale Junior Member

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    Jan 2024, my toddler and wife were goofing around in the 2008 Prius. They ran the 12V battery (installed 7/2020) down to nothing.

    Recharged it with trickle charger. Worked great since then.

    Bought a new Interstate S46B24R AGM battery from Costco produced 2/2024. Old battery was working fine, and honestly, toddler parenting is exhausting, so kept putting it off. Replaced it today with the one I bought in 2/2024.

    Did the readings in the diagnostic menu, which were.
    One-brake tap: 11.4 V
    Two-brake taps: 11.1 V
    Engine on: 14.3 V

    I understand now I shouldn't have done that, but what do you all reckon is the way forward?

    Per ChatGPT and other forum posts, I ought to do an AGM smart charge to fully charge it overnight, then do the diagnostic check and hope it gets back up closer to 12.9 / 12.6 volts?

    If so, any recommendations for a good smart charger for Gen 2 Prius? (never bought one before)
    Also, when charging, should I have the charger "bite" the metal plate thing under the hood by the fuse box, or can I connect positive terminal directly to the 12V itself?

    Any other advice / tips?

    Thanks everyone

    -Praise Dale
     
    #1 PraiseDale, Sep 27, 2025 at 8:09 PM
    Last edited: Sep 27, 2025 at 8:16 PM
  2. PraiseDale

    PraiseDale Junior Member

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    After talking to Costco people, I'm just gonna give the battery back to them, get 50% money back and get a brand new one. Then I don't have to buy a charger and always be having the "I hope the car starts" feeling with this 18-month old battery, plus it gets cold here in the winter
     
    #2 PraiseDale, Sep 27, 2025 at 9:12 PM
  3. PraiseDale

    PraiseDale Junior Member

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    Update. After a 20 minute drive, I then checked the battery levels and the display said:

    11.9 (one brake tap) / 11.6 (two taps brake) / 14.3
     
    #3 PraiseDale, Sep 28, 2025 at 1:17 PM
  4. pasadena_commut

    pasadena_commut Senior Member

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    Never a good idea to let a lead acid battery just sit for long periods of time.

    Too bad you didn't test it with a voltmeter before putting it into storage and again before putting it in the car. In theory AGM batteries for the Prius are supposed to self discharge at 1-3% per month. Would have been good to know what the rate for this brand actually was. (It also varies with temperature, more discharge with more temperature.) Anyway, lemons, but let's make lemonade anyway.

    Vmonths=Vstart*exp(months*factor).

    From date of manufacture that one is 12 + 7 = 19 months old. We will have to estimate Vmonths from the 11.4 value as 11.6V, since that power mode usually pulls down the battery voltage by around 0.2 volts. (That's with a fully charged battery, it probably comes down more for a significantly discharged one, which means the discharge rate might be slightly less than calculated because of this error.) We also have to estimate Vstart, let's use 13V.

    ln(11.6/13)=19*factor
    factor=ln(11.6/13)/19
    factor= -.005997 (which is hardly any different from...)
    factor=-.006

    One month percent loss is exp(-.006) = .994, or 0.6 percent. That is a really low self discharge rate, even for an AGM battery.
     
    #4 pasadena_commut, Sep 28, 2025 at 3:06 PM
  5. PriusCamper

    PriusCamper Senior Member

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    Should of listened to ChatGPT... Only problem is your trickle charger is inadequate and you needed to put in on a smart charger of up to 10amps or greater and that 12v would of been good as new.
     
    #5 PriusCamper, Sep 28, 2025 at 3:16 PM
  6. BiomedO1

    BiomedO1 Senior Member

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    You should have a jump box for those cold Kansas winters. You'll need to take it inside with you, because lithium batteries don't like the cold either.
     
    #6 BiomedO1, Sep 28, 2025 at 3:50 PM
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