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12v battery reading below 1v?

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by jyii07, Dec 7, 2023.

  1. jyii07

    jyii07 Junior Member

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    I noticed a decrease in mpg over the past year or so and finally got around to taking a look at my 12v battery. I just measured using a voltmeter and when measuring after the car's been cold for 12+hours, it read below 1v. When the engine turned on it measured below 3v. The battery is right around 4 years old so it is nearing the end of the life but thought it would read higher since my car still starts and has no other problems. Are these valid readings and is it possible that my car can still function with a resting 12v state of below 1v?
     
    #1 jyii07, Dec 7, 2023
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    I’d suspect the meter first.
     
    #2 Mendel Leisk, Dec 7, 2023
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  3. jyii07

    jyii07 Junior Member

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    this is actually the second meter I tested my car battery with. going to test my partner's 12v battery this weekend to see how hers looks to rule it out.
     
    #3 jyii07, Dec 7, 2023
  4. bisco

    bisco cookie crumbler

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    car wouldn't start at those voltages. maybe 11 and 13
     
    #4 bisco, Dec 7, 2023
  5. Doug McC

    Doug McC Senior Member

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    There’s no way the relays would close with that low of voltage, and highly unlikely the inverter/converter is only putting out 3 volts once the relays do close (basically nothing would be working with that low of output voltage). So either your meter is bad (interesting that two of them are though) or you are not getting good contact between the probes and the battery terminals (basically operator error), which is very possible since you have two meters showing effectively impossible voltages.
     
    #5 Doug McC, Dec 8, 2023
  6. jyii07

    jyii07 Junior Member

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    ha sorry for the false alarm guys. it looks like i didnt connect the probes to the terminal properly so I wasnt getting an accurate reading. the battery is reading at 11.95v now with the car cold. looks like its time for a replacement anyway so ill do that later today. thanks for all the responses.
     
    #6 jyii07, Dec 8, 2023
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  7. BiomedO1

    BiomedO1 Senior Member

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    The battery is fine, especially sitting in the cold for 12+ hours. Start looking for a battery when it's down to mid 10.xxVDC. Battery should still be able to close the traction battery relays.
    My Prime was reading just 11.75 VDC, 3-hours after it completed a traction battery charging cycle. Put a 10A charger on it for 15 minutes and it settled at 12.65 VDC.
     
    #7 BiomedO1, Dec 8, 2023
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  8. Doug McC

    Doug McC Senior Member

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    I agree with BiomedO1. I would take for a nice cruise at highway speeds and check it again before rushing out to replace it. Maybe have it tested and based on that result, charge it and see what you get. This time of year most of us are running the headlights more and if you also have all the interior lights set to come on when you turn the car off and stay on for awhile, combined with low temperatures, it wouldn’t be surprising to see a slightly lower reading, especially if you had made several stops during the previous use.
     
    #8 Doug McC, Dec 9, 2023
  9. Mendel Leisk

    Mendel Leisk Senior Member

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    You can get a prosumer level 12 volt battery electronic load tester, say Solar BA9, and see for yourself where it’s at. They’re around $60 USD on Amazon US.
     
    #9 Mendel Leisk, Dec 9, 2023
    Last edited: Dec 9, 2023
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  10. jyii07

    jyii07 Junior Member

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    MPG has been dipping for a while and the battery has hit 4 years in age so I decided to take the preventative measure and replace it. It's hot here in socal most of the year and i know batteries degrade faster in the heat too. Whether or not the battery could go another year, or two, or three it was such a quick fix and gives me peace of mind so I swapped it out anyway.
     
    #10 jyii07, Dec 12, 2023
    Doug McC, Brian1954 and bisco like this.
  11. bisco

    bisco cookie crumbler

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    a sensible purchase
     
    #11 bisco, Dec 12, 2023
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