Replaced 12v battery but old one still reads 12v...

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Mryan5281, Aug 30, 2020.

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  1. Mryan5281

    Mryan5281 Junior Member

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    Hello, I replaced my 12v battery in my 2009 Prius about 4.5 years ago. I drive this purely as a commit car, 100 miles per day currently 210k miles. I checked the battery voltage of my 12v with system check and it was 11.8ish drop to 11.4ish with lights on. So I ordered the AC Delco (
    ) battery on amazon rather than the optima yellow top I did last time. I used a cheap multimeter to check voltage of the new batter it was 12.55 which was what it was suppose to be. I put it in the car and did system check which showed like an 11.9V with no load. I thought it should be higher?? I then tested the old battery with the multimeter and it read 12.55...

    Was my old battery fine??

    Should the new batter be reading higher than 12v with no load in system check?
     

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    #1 Mryan5281, Aug 30, 2020
  2. Patrick Wong

    Patrick Wong DIY Enthusiast

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    A fully-charged 12V AGM battery will measure around 13.0VDC. Your new battery was around 50% charged. If you fully charge the battery the open-circuit voltage will increase. However since you drive 100 miles a day the battery should get charged up within a few commute days.
     
    #2 Patrick Wong, Aug 30, 2020
  3. Mryan5281

    Mryan5281 Junior Member

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    What about my old battery reading 12.55 when out of the car? Did it needed to be replaced? Should have been fully charged so based on what you said it should have been more... correct?



     
    #3 Mryan5281, Aug 30, 2020
  4. RobertK

    RobertK Member

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    Sounds like you need to check the multimeter. I know that the car is not 100% accurate, but .65 volts is a pretty big swing on a 12v battery. Two batteries with identical voltages would make me look again as well.
     
    #4 RobertK, Aug 30, 2020
  5. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Yes, your daily commute pattern should have been enough to keep the old battery fully-charged. It did not "need" to be replaced right now, but it was not in new condition.
     
    #5 Patrick Wong, Aug 30, 2020
  6. Patrick Wong

    Patrick Wong DIY Enthusiast

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    One way to check the multimeter would be to use a new alkaline battery intended for household items. For example a 9V battery should measure ~9.6V, a new 1.5V battery should measure ~1.6V, etc.
     
    #6 Patrick Wong, Aug 30, 2020
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