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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  6. Hayslayer

    Hayslayer Member

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    You have some misunderstandings as to how the 12v system works in the Gen 2.

    First, your 12v battery is very weak right now, whether that is from a failing battery or from not enough use is yet to be determined. What are your normal use habits for the car? Even a brand new battery will drain almost completely in about 6-7 weeks of low/non-use due to normal shutdown current draw of the cars systems. This can be extended by deactivating the SKS system using the button under the steering wheel labeled "KEY".

    Second, when you see the 14 volts, that is not the 12v battery, that is the DC-DC converter powering up and taking over the 12v power system. This happens when the car goes to READY mode, where the HV battery is placed on-line and HV power is sent to the inverter systems. Similar to how alternator in an ICE car comes to life when the engine starts running.

    The pending P3014 means "Block 4 becomes weak", which means block 4 (of 14) (or module 7 and 8 of 28) in the HV battery at one point in the recent past exceeded the threshold for the code. If it happens again within a certain timeframe, it will RTOD and go to a limp mode style operation. Usually, this "pending code" is an early clue of upcoming HV battery failure.
     
    #6 Hayslayer, Sep 21, 2025 at 5:43 PM
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  7. pumpanddump

    pumpanddump Junior Member

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    Got the battery tested underload and they said its bad. It wasn't the place i bought it from...going there on monday since its under 3 year warranty.

    Yes,she doesn't drive the car long distances. Its a grocery getter / hair appointment assistant.

    Unfortunately the main computer in the dash is becoming unreadable, except at night, even then its toast looking...meaning..i cant really observe the regeneration of energy on screen.

    The codes will reset,I will check techstream again, within 48 hours. After the new accessory battery is installed.

    Maybe i neec to get a trickle charger for this accessory battery? Is this an option, any brand suggestion, models? Cost?
    Thanks.
     
    #7 pumpanddump, Sep 21, 2025 at 6:24 PM
  8. JC91006

    JC91006 Senior Member

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    1. If you lasted 2 years without doing anything to the battery, then you probably don't really need a trickle charger. You just need a battery charger that can top off the battery once or twice a year. Hook it up at night, by morning, it'll be fully charged. Do it Jan and June of every year and you shouldn't have any issues. I use a very high quality one from CTEK. It's a trickle charger and battery charger all in one. And it has the important AGM charging function, for all the Prius batteries (and other cars) that are AGM

    CTEK - 40-206 MXS 5.0 Fully Automatic 4.3 amp Battery Charger and Maintainer 12V


    When you get your new replacement battery, try to charge it fully before installing. Then it'll be fully charged from day 1
     
    #8 JC91006, Sep 21, 2025 at 7:08 PM
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