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12V Battery Drain Even After Replacement

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Alex G., Sep 30, 2015.

  1. Alex G.

    Alex G. Junior Member

    Joined:
    May 13, 2014
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    Location:
    Lindenhurst, New York
    Vehicle:
    2007 Prius
    Model:
    II
    After noticing rapid power drains in my original 8 year old 12v battery (with AC running and the Prius just idling it would drain from green to black in just a few minutes) I brought my car to the dealer to get a replacement Toyota 12v battery. Being that I had some other work to be performed, I was able to get a good deal on the labor for the install vs me purchasing one myself and doing the install. Upon diagnosis of the original 12v battery they said they found many bad cells and it was basically only functioning at 47% efficiency.

    I expected that with the brand new battery installed, my Prius would be running as it was when the battery was brand new again -- I could have the car idle without rapid power drain and my gas mileage would improve. Yet now even with the new battery, I still find my Prius struggling... the battery charge still rapidly depletes if the car isn't moving and as a result, my gas mileage is pretty abysmal because my Prius is always utilizing power to recharge the battery. My AC has been running, but only at medium fan speed -- I can only imagine how quick the power drain would be if on high.

    What could be going on here? I don't understand what could be draining my brand new battery so rapidly. I could take it back to the dealer and have them go over everything again, but I wanted to see if anyone here has experienced battery/gas mileage issues even after a 12v battery replacement.

    If a Prius is idling with a good battery and the AC running, shouldn't the fully charged battery not be drained so rapidly? I never remember it doing that. The poor gas mileage I've been getting is still making me think something is wrong here.
     
    #1 Alex G., Sep 30, 2015
  2. carbon6

    carbon6 Junior Member

    Joined:
    Oct 25, 2009
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    Location:
    Beaveron, OR
    Vehicle:
    2009 Prius
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    N/A
    Your battery display is showing you the status of the high voltage battery bank, not the 12 volt battery. The 12 volt battery wont' affect your mileage.

    Mike
     
    #2 carbon6, Sep 30, 2015
  3. Alex G.

    Alex G. Junior Member

    Joined:
    May 13, 2014
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    Location:
    Lindenhurst, New York
    Vehicle:
    2007 Prius
    Model:
    II
    Thanks for the clarification Mike. Is it normal for such a rapid discharge of the high voltage battery while the car is sitting idle and the AC running? I recently got a work vehicle for my commutes and so my prius definitely hasn't been getting much use lately other than very short trips. I'm assuming this is what's taking the toll on my gas mileage.
     
    #3 Alex G., Sep 30, 2015
  4. RRxing

    RRxing Senior Member

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    Other Hybrid
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    Limited
    Correct. Plus, stop and go traffic with the A/C running will drop the HV battery down to about 2 bars, then the ICE will start to charge it up a bar or two. Perfectly normal.
     
    #4 RRxing, Sep 30, 2015
    epoch_time likes this.
  5. bisco

    bisco cookie crumbler

    Joined:
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    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    it sounds like your hybrid battery is headed south, how many miles on her?

    are you sure they diagnosed the 12v and not the hybrid battery? never heard of such a thing.
     
    #5 bisco, Sep 30, 2015
    Mendel Leisk likes this.
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