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Even / odd battery voltages!

Discussion in 'Gen 2 Prius Main Forum' started by Colin McEwan, Sep 17, 2022.

  1. Colin McEwan

    Colin McEwan Junior Member

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    Well here's an odd one. Went to start the car this morning (last used a few days ago) and got a triangle warning light and no "Click" from the main HV relay. Pulling out Torque and Dr Prius, they both claimed no ECU fault codes, and then I did a double-take when I noticed THIS

    upload_2022-9-17_11-50-40.png ​
    All the odd numbered blocks reading as about 10v, all the even numbered blocks reading as about 16v.

    Has anyone seen anything like this happen to a battery pack? I can't see how it's possible for a battery pack to end up like that, so my only thought is that it must be a problem with the voltage sensors in the battery controller.
     
    #1 Colin McEwan, Sep 17, 2022
    Last edited: Sep 17, 2022
  2. PriusCamper

    PriusCamper Senior Member

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    I've seen that before on here... Can't remember what the resolution was? But usually strange stuff like the best place to start is to clean corrosion inside the battery ECU and replace the voltage sensing harness, aka wire frame no.2 and in the process any loose or failed connections will be noticeable.
     
    #2 PriusCamper, Sep 17, 2022
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  3. bisco

    bisco cookie crumbler

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    How is the 12v?
     
    #3 bisco, Sep 17, 2022
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  4. pasadena_commut

    pasadena_commut Senior Member

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    It is either a problem in the pack or it is a logic glitch. Since the latter is the easier of the two to resolve, start there. Unplug the 12V battery, and then check it with a DVM. If it measures OK then after 5 minutes or so (to let any residual charge elsewhere in the electronics drain) reattach. Try to start the car again. Still a problem? Remove, inspect, and reinstall the pack disconnect. Try again. If it is still messed up then you will have to open the pack, after pulling the disconnect, minimally to verify what the internal voltages are. Most likely they will all be within range (what mechanism could cause a serial set of cells to discharge/unevenly charge every other one like that?) and the problem is somewhere in the sensing wiring or circuitry.
     
    #4 pasadena_commut, Sep 17, 2022
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  5. Another

    Another Senior Member

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    Reboot
     
    #5 Another, Sep 18, 2022
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  6. Colin McEwan

    Colin McEwan Junior Member

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    Solved!

    I'd assumed it would be a faulty battery ECU, as I've had issues with it previously (corrosion and a cracked pin on the voltage probe plug) so I went ahead and bought a "battery ECU" on eBay to replace it with and resolved not to bother opening the battery up until it arrived.

    Meanwhile, the 12v battery level dropped to useless (there's a phantom current draw that I really ought to get round to tracking down...) so I dug it out to charge. The ECU arrived today, so I took the fully charged 12v battery out to the car and, noting the possibility of a logic glitch, decided to try plugging it back in just in case. Lo and behold, normal battery voltages!

    So, the cause was either (or a combination of)
    - battery ECU getting glitchy and needing disconnected and reconnected (this surprised me -- I wouldn't have thought the battery ECU would be powered with the ignition off?), or
    - low 12v battery level causing problems for the ECU (dropping too low to provide a reference voltage for a DAC input?)

    Just as well, as it turns out what the ebay selled had sold me (and physically labelled) as a "battery ECU" was in fact a hybrid control ECU so I would be sending it back anyway :D

    I'm going to be trying to track down the phantom current draw, and I'm still suspicious of that battery ECU.

    Thanks for your suggestions!
     
    #6 Colin McEwan, Sep 24, 2022
  7. Mendel Leisk

    Mendel Leisk Senior Member

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    I know the Owners Manual cautions to remove the 12 volt before charging, but seems like overkill. FWIW, ours is on a charger more than off, never remove it.
     
    #7 Mendel Leisk, Sep 25, 2022
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