Hybrid Battery Troubleshooting

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by trevansg, Oct 12, 2025 at 7:34 PM.

  1. trevansg

    trevansg New Member

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    Hello, I'm looking for some help diagnosing the Red Triangle of Death on my 2005 Prius. The triangle came on a few weeks ago, and since then I’ve noticed a loss of acceleration power and the fan running loudly in the back. I tried running a test with Dr. Prius, but it wouldn’t complete because the battery was too cold or something. I decided to open up the battery pack and test all the cells individually, following the instructions in a video using a Harbor Freight battery tester. I recorded the results in a spreadsheet, and I’m not seeing anything that looks particularly bad, but maybe I’m interpreting it incorrectly. The lowest health status was 60%, and most were 85% to 100%. None of the internal resistances looked too high either. Would that be enough to trigger the triangle and cause these issues? My other thought is that the green corrosion on the bus bars might be causing problems, or maybe it’s a bad inverter cooling pump or dirty fan.

    thanks!!
    upload_2025-10-12_17-34-27.png
    CELL, Health, Volt, Internal Res (mOhm)
    1 83 6.5 6.23
    2 100 7.65 4.9
    3 100 7.73 4.88
    4 100 6.72 5.25
    5 100 7.65 4.86
    6 60 7.23 7.29
    7 100 7.66 4.87
    8 85 6.39 6.16
    9 100 7.78 5
    10 100 7.77 4.92
    11 100 7.62 5.31
    12 100 7.06 5.59
    13 100 7.86 4.96
    14 100 7.87 4.96
    15 100 7.66 4.92
    16 100 7.87 5.02
    17 100 6.92 5.27
    18 100 7.42 5.11
    19 100 7.67 5.01
    20 100 6.61 5.2
    21 100 7.72 5.04
    22 100 7.7 5.11
    23 100 7.39 5.11
    24 100 7.66 4.98
    25 100 7.68 5.06
    26 100 7.1 5.18
    27 100 7.51 5.31
    28 89 6.35 6.02
     
    #1 trevansg, Oct 12, 2025 at 7:34 PM
  2. PriusCamper

    PriusCamper Senior Member

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    What were the error codes?
     
    #2 PriusCamper, Oct 14, 2025 at 1:57 PM
    Brian1954 likes this.
  3. bisco

    bisco cookie crumbler

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    clean the corrosion first, and replace the sensor wire
     
    #3 bisco, Oct 14, 2025 at 2:00 PM
  4. ChapmanF

    ChapmanF Senior Member

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    ^ This.

    May as well find out for what reasons (out of the couple hundred possible) the car has shown the triangle, before just starting to do things.
     
    #4 ChapmanF, Oct 14, 2025 at 2:10 PM
  5. PriusCamper

    PriusCamper Senior Member

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    And I'm willing to look at those battery module voltage and capacity numbers, but my morning coffee hasn't kicked in so would prefer a frame of reference before going too deeply down this very familiar rabbit hole.
     
    #5 PriusCamper, Oct 14, 2025 at 2:11 PM
  6. Brian1954

    Brian1954 Senior Member

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    The voltages of the 28 modules are all over the place. A static reading of healthy battery modules should not vary by more than 0.10 volts of each other. My guess is that the trouble code that occurred was P0A80 " Replace Hybrid Battery Pack".
     
    #6 Brian1954, Oct 14, 2025 at 5:51 PM
  7. PriusCamper

    PriusCamper Senior Member

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    That's common if a failed battery pack sits for a while. As in this these numbers may be just self discharge data. As in modules that have self-discharged the most should be noted on the spread sheet to further the pack diagnosis. There's a dozen additional types of data points for evaluating a module/pack you can read about on PriusChat.

    Resting voltage isn't as good of a data point compared to module voltage loss after 2 minutes under load from a 50w/12v bulb.

    But this is just preliminary data... I'd like to see a charge and balance of the pack as well as a couple of deep cycling of the pack before drawing any conclusions beyond the replacement of failed or hot-running cells.

    All this and more is how you'll find the bad modules before they find you.
     
    #7 PriusCamper, Oct 14, 2025 at 6:09 PM
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