Gen 2 12v battery went from 12.8v down to 5.8v overnight. Just had it load tested yesterday.

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Longinus876, May 27, 2023.

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  1. Longinus876

    Longinus876 Member

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    Gen 2 12v battery went from 12.8v down to 5.8v overnight. Just had it load tested yesterday. It's also got a P0A80 code. Is there a connection between the two?
     
    #1 Longinus876, May 27, 2023
  2. PriusCamper

    PriusCamper Senior Member

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    You didn't mention the results of the load test?

    And while there's not a specific connection, in general you don't want to spend any time figuring out the P0A80 code until you have a healthy 12v because weird things happen with the car when the 12v goes bad... Maybe try charging the 12v for 24 hours on a smart charger?
     
    #2 PriusCamper, May 27, 2023
  3. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Well, a Prius with P0A80 "check hybrid system" can't go into READY mode, which means it can't charge its 12v battery. That would suggest that you've been discharging the 12v battery and it hasn't had any chance to recharge. That's a way that they are connected.
     
    #3 Leadfoot J. McCoalroller, May 27, 2023
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  4. bisco

    bisco cookie crumbler

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    if it went down with the car off, then you have a quiescent load that is placing a large drain on it.

    you can try removing fuses one at a time to find the offending circuit.
     
    #4 bisco, May 27, 2023
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  5. Longinus876

    Longinus876 Member

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    #5 Longinus876, May 27, 2023
  6. bisco

    bisco cookie crumbler

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    exactly
     
    #6 bisco, May 27, 2023
  7. Longinus876

    Longinus876 Member

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    I started by pulling the orange handle on the HV batt and the drain went to zero.
     
    #7 Longinus876, May 27, 2023
  8. bisco

    bisco cookie crumbler

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    sounds like you've got a short. any trouble lights?

    when the car is off, the hv battery is supposed to be completely isolated for safety.
     
    #8 bisco, May 27, 2023
  9. Longinus876

    Longinus876 Member

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    With the triangle lit, the 12v batt was charging at 14.4v across the terminals. Today I used a jumper pack to pull the 5.5 volts up to 12.4 and I pulled the neg battery cable off to stop all the drain. I put the meter across the chassis and neg on battery and it was pulling 12.4v again so I pulled the orange handle out and it went to zero. This is all with the key off so I'm kinda wondering the hybrid battery is pulling the 12 v down like that. No wonder it can't hold its charge. Does any of this make sense?

    I had to pull the orange handle to keep the HV batt from pulling down the 12v battery. I agree that the HV battery should be totally isolated from the rest of the car when the car is off.
    Trouble lights include the Red Triangle and the ABS warning and I think there's another one to the right of the ABS but it's not on all the time and I don't remember what the heck it is. Sorry.
     
    #9 Longinus876, May 27, 2023
    Last edited by a moderator: May 28, 2023
  10. Brian1954

    Brian1954 Senior Member

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    You need to read the trouble codes by using a scan tool compatible with a Gen 2 Prius and then come back to this thread and list all the trouble codes to get help from the members of this forum. I think that you will have more than one trouble code.

    Posted via the PriusChat mobile app.
     
    #10 Brian1954, May 27, 2023
  11. Longinus876

    Longinus876 Member

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    There's a cable w/ white connector (charge cable from the dc to dc to the 12v). If I pull that off for 10 seconds or so, the red triangle goes off and the car is drivable for awhile. Then not.
     
    #11 Longinus876, May 27, 2023
  12. Longinus876

    Longinus876 Member

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    I had them pull the codes and it's P0A80 only on a Foxwell scanner.
     
    #12 Longinus876, May 27, 2023
  13. Longinus876

    Longinus876 Member

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    Load test was fine. All green lights.
     
    #13 Longinus876, May 27, 2023
  14. Longinus876

    Longinus876 Member

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    Yeah, I know. I don't know which one to get. What do you all use?
     
    #14 Longinus876, May 27, 2023
  15. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    That doesn't make any sense to me, but it also isn't a normal step in troubleshooting or repairing. Good luck!
     
    #15 Leadfoot J. McCoalroller, May 28, 2023
  16. SFO

    SFO Senior Member

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    Most use techstream (toyota diagnostics SW) running on a winblows device, or you can emulate the OS/hardware. You'll also need a cable to connect to the vehicle, either a low end 'mini-vci cable', or a better middle of the road Openport 2.0. Spending more may not be needed.

    There are some other options to consider in this thread : Gen2 OBD2 app review | PriusChat
     
    #16 SFO, May 28, 2023
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  17. Brian1954

    Brian1954 Senior Member

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    How old is your 12V battery?

    Posted via the PriusChat mobile app.
     
    #17 Brian1954, May 28, 2023
    Last edited: May 28, 2023
  18. Longinus876

    Longinus876 Member

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    Yeah, I know. Interesting. Thanks for your help.
     
    #18 Longinus876, May 28, 2023
  19. Brian1954

    Brian1954 Senior Member

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    You need to take it to a Toyota dealership or other competent mechanic for diagnosis since you have not been able to do it yourself.

    Posted via the PriusChat mobile app.
     
    #19 Brian1954, Jun 12, 2023
    Last edited: Jun 12, 2023
  20. rjparker

    rjparker Tu Humilde Sirviente

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    I am pretty sure your diagnosis has an incorrect assumption or result. First a 12v battery discharged to 5-7vdc is 98% guaranteed to be bad.

    To properly load test it, you take it out, charge it (yourself or at an autosupply) and then have an autosupply load test it. They have a machine that puts a high current load on the 12v battery which then evaluates the resultant voltage drop.

    Again a 12v battery that has 5-7 vdc is almost always bad.
     
    #20 rjparker, Jun 12, 2023
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