2005 - Dying Accessory Battery... wait a minute... huh?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by pumpanddump, Sep 21, 2025 at 2:50 PM.

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  1. pumpanddump

    pumpanddump Junior Member

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    Ran diagnostic screen from the dash with car off.. got into setting and see the Accessory battery is at 10v.

    Why was I checking - because Red Triangle of death and multiple DTC some are bullsh*t but here they are: All of these have a "X" checkmark in the current column of techstream Images attached below:
    P3000, C1259, C1310, C2318,

    However -Stayed in the diagnostic screen, gave the Prius a full start - and it went back to 14v.
    Now I understand how the main battery would recharge this.. so yea, I get how that works. Somewhat of a good sign. Maybe.

    So is it dead / dying Accessory battery? Can I get it reconditioned or am I looking at buying a new battery to resolve all issues present, it might be under warranty since it says free replacement .
    I purchased this accessory battery in 2023!

    Right now its completely disconnected, put the multimeter set to 20v, and it's reading 11.51v
    Is that enough of weakness to make the system throw a Red Triangle of death?

    The only DTC on the hybrid battery was "section 4 is weak". ABS Regenerative issues: C1259 / C1310

    Thanks in advance!
     

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    #1 pumpanddump, Sep 21, 2025 at 2:50 PM
  2. JC91006

    JC91006 Senior Member

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    You have to first address the low charge on the 12v battery before you can diagnose any other problem. So if you have a battery charger that charges at low amperage, you can charge the 12v to full and then see if it'll hold a charge. At the current state of under 12v, it's basically a dead battery
     
    #2 JC91006, Sep 21, 2025 at 2:52 PM
  3. pumpanddump

    pumpanddump Junior Member

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    Wow... the battery only lasted 2 years!?! thats insane. I'll bring it up to the shop and see if this battery can hold a charge.
     
    #3 pumpanddump, Sep 21, 2025 at 3:02 PM
  4. JC91006

    JC91006 Senior Member

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    you could have allow it to drain by not driving long distances, short distance driving (like a mile or 2 a day) will not charge the battery properly
     
    #4 JC91006, Sep 21, 2025 at 3:04 PM
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  5. bisco

    bisco cookie crumbler

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    could be under warranty, get a free load test at most auto parts stores
     
    #5 bisco, Sep 21, 2025 at 3:08 PM
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