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Brand new battery voltage low 11.9 to 12.0, why?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by chinna, Mar 19, 2019.

  1. chinna

    chinna Member

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    2006 Prius
    2006 Prius with HID lights.

    I replaced original battery with Optima approx 8 years ago and now that battery was weak.

    Last week replaced with Bosch Premium Performance battery Group Size 51 AGM from pepboys. It was charged before putting in at 12.9v.

    Now a week later, I was checking battery voltage on the Screen, it seems low for charged battery.

    When I turn on Ignition (without brake) and check the voltage it is only showing 12.2v. No other loads.
    When I turn on Headlights(HIDs are only 35W each) voltage dropped to 11.9 and stabilized back to 12V.
    I thought for a brand new battery and fully charged it should have been higher than that. I was expecting close to 13 volts.

    Later I turned on car(with brake) and checked voltage and it is showing 14.2 v (it jumped to 14.3 but stabilized at 14.2) with headlights also.

    I am puzzled why battery is not charged fully and why it is not showing high voltage. There are no known loads/short . All lights are off when I was seeing 12.2v without headlights. Neither there was any incident where Lights were left on before this voltage measurement. Pretty much headlight were also off most of the driving last week.

    Any ideas, if this is normal, and I should be looking for some issue?

    Thanks
     

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    #1 chinna, Mar 19, 2019
  2. RobertK

    RobertK Member

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    The car should be charging the battery at 13.8 volts if the 12 volt battery is fully charged. The 14.2 volts you are seeing indicates a low 12 volt battery. You should recharge the battery and see if the values change. If the battery can't hold around 12.9 volts you either have some drain on the battery or the battery is defective. Since the battery is brand new you might just have the store test it for you.
     
    #2 RobertK, Mar 19, 2019
  3. chinna

    chinna Member

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    I charged battery overnight, and it took lot of charge from my 6AMP charger before using. It was still charging when I checked in the early morning. Battery voltage was showing 12.9 after disconnecting the charger.

    Instead of battery voltage increasing as battery gets charged from car, it went down. I am pretty positive about not having any external discharge when car is off. But I can check/monitor as I have Clamp On multimeter if there is any drain after the car is turned off.

    I was more worried about if there is any chance that charging circuit is going bad.
     
    #3 chinna, Mar 19, 2019
  4. RobertK

    RobertK Member

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    Given that you are getting 14.2 volts while in Ready mode, I think your car's charging circuit is OK. I would charge it again with your battery charger until the charger indicates that the battery is fully charged. This will probably be best if done overnight.

    How old is your new battery? If it is more than six months old it might be the cause of your problem.

    Out of curiosity, how did you install the Group 51 battery in the Prius, given the different terminals on that battery? Did you replace the terminals on the cables? I'm wondering if you might have a bad connection that is keeping the battery from fully charging.
     
    #4 RobertK, Mar 19, 2019
  5. Mendel Leisk

    Mendel Leisk Senior Member

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    Let the car sit overnight, with the hood ajar. Check the voltage first thing in the morning, at the underhood jumpstart point, with a digital multimeter. All the readings using the dash display are kinda meaningless.
     
    #5 Mendel Leisk, Mar 19, 2019
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  6. chinna

    chinna Member

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    This is same size as Standard AGM battery with Smart Key. Terminals are same size and has usual vent elbow to connect.

    Here is the battery
    Bosch Premium Performance Battery Group, Size 51 | 647478 | Pep Boys
     
    #6 chinna, Mar 19, 2019
  7. Mendel Leisk

    Mendel Leisk Senior Member

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    Pep Boys should an electronic load tester on hand. They really should have tested it before sale but anyway:

    drive down there, have them test it, and if it's a dud take advantage of the warranty?

    upload_2019-3-19_16-30-16.png
     
    #7 Mendel Leisk, Mar 19, 2019
  8. johnHRP

    johnHRP Active Member

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    I am afraid it is an old tread but we should correct the information here. There is nothing wrong with that battery. It is practically new with 12V under IGN ON and Headlight On.
    Typical normal working battery with Halogen Headlights ON 18 Amps, is 11.5V or more.
    Voltage reading alone gives incomplete picture about the capacity of the battery. Running All Accessories: headlight, blower, defroster etc. give better picture if the aux battery is still good. If it is below 10.5V, get ready with Ultracapacitor jump starter. When it is below 10V, you will start seeing triangle.
     
    #8 johnHRP, Jan 25, 2022
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  9. bisco

    bisco cookie crumbler

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    still, a new fully charged agm should measure close to 13 volts with everything off and no surface charge.
     
    #9 bisco, Jan 25, 2022
  10. johnHRP

    johnHRP Active Member

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    I doubt you ever have 13V once it is connected to the car. New battery often only read 12.9 V, from the display in ACC, you may get 12.8. Light load is not a good parameter. Turn to IGN ON and headlights On, it should be 11.5V or higher in a new fully charge battery
     
    #10 johnHRP, Jan 26, 2022
  11. bisco

    bisco cookie crumbler

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    agreed, that's why i said 'close to 13 volts
     
    #11 bisco, Jan 26, 2022
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