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Does a weak 12v battery affect the Hybrid battery system?

Discussion in 'Gen 3 Prius Fuel Economy' started by southtxprius, Sep 14, 2021.

  1. southtxprius

    southtxprius Junior Member

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    I discovered my 12v battery isn't holding a charge well and needs to be replaced. Would this impact the hybrid battery system and MPGs a bit?
     
  2. burrito

    burrito Active Member

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    Yes. If the 12V battery is perpetually low, the car will keep running the ICE to attempt to charge it. Others have reported that replacing their bad 12V battery gave them an increase in fuel efficiency by 5-10 MPG.
     
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  3. southtxprius

    southtxprius Junior Member

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    Thank you! I started noticing a drop off in mpg, but I also got new tires so I wasn't sure.
     
  4. burrito

    burrito Active Member

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    I just changed mine today, so we'll see if I notice a difference.
     
  5. southtxprius

    southtxprius Junior Member

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    And would a weak 12v battery cause the car to not go into Ready Mode? It's happened intermittently, and usually resolves itself upon turning the car off and back on.
     
  6. bisco

    bisco cookie crumbler

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    yes, the 12v boots the ecu's and closes relays. turning it off and on when it is going south can get enough juice to make ready
     
  7. burrito

    burrito Active Member

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    If you go to any car parts store, they'll load test the battery for free. If it tests as bad, you should replace it promptly. Of course, they'll hope that you'll buy it from them, but you don't have to.

    Incidentally, I did this recently, and the lady insisted that she had to test it under the hood. I told her repeatedly that, "the battery is right here, I've even exposed it, you can just clamp right on to the battery itself." Finally, she agreed.
     
  8. southtxprius

    southtxprius Junior Member

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    Thanks, Bisco!!!

    A dealer noticed the 12v was low when I was in for service a few weeks ago. Charged it and it tested "good." I told them about the intermittent ready mode issue. They just said to monitor it. Car didn't start this morning. Called AAA for a jump. They got the car started, but tested the battery and it failed. I drove to the dealer for a new battery, but on the initial start up at the dealer after install, it wouldn't go into ready mode.

    Car will get checked out tomorrow.
     
    #8 southtxprius, Sep 14, 2021
    Last edited: Sep 14, 2021
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  9. southtxprius

    southtxprius Junior Member

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    Thanks, Burrito!
     
  10. southtxprius

    southtxprius Junior Member

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    Dealer did a health check and said the car is fine.
     
  11. Mendel Leisk

    Mendel Leisk Senior Member

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    Including the 12 volt? They typically have a pro-level electronic load tester, the kind with a small printer, that prints out something about the size of a grocery store receipt. See anything like that? How old's the battery btw?

    New tires can also impact mpg. What did you get, and what did you have before?
     
  12. southtxprius

    southtxprius Junior Member

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    The old battery was only 11 months old. I discovered that the negative post was bent and cracked from the dealer installation.

    The health check was done the morning after the new 12v battery was installed. No Ready light issues since the first startup after the 12v install.

    Previous tires were Energy Saver A/S. Replaced after 66,624 miles. New tires are Defender T+H, so there is a definite decrease in mpg from the increased rolling resistance.
     
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  13. abdullah arslan

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    Got 5mpg decrease after installing Michelin Cross Climate 2
     
  14. Seanathon

    Seanathon Junior Member

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    Sounds like I'm not the only one, I got a Prius V, just installed Cross Climate2, mpg down from ~36mpg to ~30mpg. So it's probably my tires. How long have you had the cross climates? I imagine after a certain amount of miles the mpg would recover (or maybe just wishful thinking)
     
  15. bisco

    bisco cookie crumbler

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    part of it may be a larger circumference as well.
     
  16. abdullah arslan

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    installed in late summer and since then getting lower mpg, right now we are in the cold season of 30s here in Indiana, and gets around 40. Expecting better performance compared to its own tires during winter in terms of mpg. Btw, it has really soft threads so I tried 37 psi and gains about 2 mpg
     
  17. abdullah arslan

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    my MPG just bumped back to normal range after replacing rear brake pads and rotors. Caliper pistons were stuck so caused uneven worn and extra friction. check others things as well
     
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  18. Mendel Leisk

    Mendel Leisk Senior Member

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    This rear brake drag can be caused by misoriented piston: it has a cross pattern on the face that MUST be oriented like an X, and well seated thus.

    To check it’s done right this time: raise the rear and verify wheels are now relatively free spinning.



    more info in attachment:
     

    Attached Files:

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  19. abdullah arslan

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    Exactly!, I personally paid extra attention to leaving the piston in the X position to match the little notch on pads. I don't how much this affects traction battery but the performance really mattered in terms of MPG, driving smoothness, the car's kinda unleashed :D
     
    #19 abdullah arslan, Dec 22, 2021
    Last edited: Dec 22, 2021
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  20. Mendel Leisk

    Mendel Leisk Senior Member

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    I think you're able to judge just by seat-of-pants, but if you want to double check, try the wheel spin test. It should be pretty much last step, in any rear brake job in particular. The drill:

    1. Disconnect 12 volt neg cable.
    2. Do the brake job, taking particular care with the piston orientation, to have it X oriented on assembly. Do NOT apply parking brake.
    3. Push brake pedal multiple times, to seat the reassembled brake components.
    4. Reconnect 12 volt neg cable, and take for a short test drive. Use brakes gently.
    5. On your return, apply/release parking brake several times, gently at first.
    6. Raise rear and check wheel spin. Slight drag is normal, but wheel should free-spin 2~3 revolutions with a good push.
     
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