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12v battery keeps getting discharged

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by D Huynh, Jan 15, 2014.

  1. D Huynh

    D Huynh New Member

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    I got a new Prius Plug-in 2012 and when the cold weather came this winter, which isn't that cold at 40F, the 12v battery seems to get discharged very often. This week, I must have jumpstarted the car twice already. The 12v battery seems to get drained when I park outside in the (not so) cold, or when I park underground for hours. I also notice that the traction battery gets filled up; it's as if the 12v battery drains into the traction battery.

    (I leased a Prius 2010, not plug-in, for 3 years and this never happened even if I parked it for days in even colder weather.)

    Is anyone experiencing this? Or do I just have a lemon? Thanks.
     
  2. Michael33

    Michael33 Member

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    The 12 volt battery can't be charging the traction battery (it would be like a thimble refilling a swimming pool), but it's apparently not unusual for a failing 12 volt battery to cause glitches in the car's other systems, like the one that monitors state of charge of the traction battery. Have the 12 volt battery checked.
     
  3. D Huynh

    D Huynh New Member

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    Thanks for your reply, Michael33. I understand what you're saying, but I also observe that the traction battery gets mysteriously filled up overnight (say, from completely empty to a range of 6 miles). Then I can actually drive in EV mode for that many miles.
     
  4. Michael33

    Michael33 Member

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    The range was there all along if the car isn't being charged. Again, the car is failing to correctly calculate state of charge, and the 12 volt battery is the first thing to check.
     
  5. giora

    giora Senior Member

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    The 2012 PIP is new to you but I suspect it is a second hand?
    As Michael33 said - check the 12 V battery first.
     
  6. bisco

    bisco cookie crumbler

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    o/p, there is a lot going on here, but start by testing your 12v per method in other threads and replace if necessary. i could ask so many questions, but don't want to confuse you.
     
  7. Bill Collins

    Bill Collins Junior Member

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    Have a mechanic check to see if there is a draw on the battery when the car is turned off? I think something is left on
    I had this happen to my 2013 PIP when I had somehow missed that I had left the interior light on and it was daylight and I could not see it glowing. You can do this test yourself if you have a 12 volt test light .By holding the positive to the battery positive and the Negative to something metal (not the Battery Neg post) and see if the light goes on. If it does theres a draw If no light then you =need a new battery.
     
  8. -Rozi-

    -Rozi- Member

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    I wonder if this works for 2012 model year too:
     
  9. kenmce

    kenmce High Voltage Member

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    Any metal part of the car should always be connected to the negative terminal. Your positive probe goes to the positive post, the negative indirectly connects to the negative post, if the battery has any charge at all the tester should always light, leak or no leak.
     
  10. Michael33

    Michael33 Member

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    That's right. You'd have to connect a meter in series with the battery's positive terminal to find a phantom power drain. Most cars have them, and Priuses have been known to 'go dead' after several weeks of sitting, damaging the 12 volt battery. This may have happened with the previous owner.
     
  11. rogerv

    rogerv Senior Member

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    One thing to check- there is a small light in the cargo area with a toggle switch. If it is in the "on" position it might be running down your battery.
     
  12. D Huynh

    D Huynh New Member

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    Thanks, everyone. I brought the car to a Toyota service center and they replaced the battery. The old battery failed various tests but the mechanics didn't really know what was going on; they simply replaced the battery. I'm all set, though it was a curious incident. (And FYI, I bought the car new from a Toyota dealership.)

    BTW, I also noticed the cargo area's light, but I figured it's only on when the back hatch is open. If that's the cause then it's certainly user-unfriendly because the car didn't alert me to that light being left on. I'll keep an eye on it with the new battery installed.

    The most intriguing thing was how the EV battery got recharged overnight even when I didn't plug in. I asked the Toyota guys about this phenomenon and they just said something got damaged inside the battery and they didn't know what exactly.

    Thanks again, everyone.
     
  13. Michael33

    Michael33 Member

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    The EV battery didn't really recharge - that's impossible. The car measures the charge level, and the dying battery gave the car's battery manager a case of delerium. Technically speaking. ;-)
     
  14. bisco

    bisco cookie crumbler

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    yes, when it comes to leaving lites on, the car is 'unfriendly'.:)
     
  15. Michael999

    Michael999 Junior Member

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    Hello I'm new to the Prius chat. I had a similar problem. I have a 2013 pip and the 12V failed late Dec 2013. Turns out that the 12V was the source of low EV range as the traction battery charges the 12 V by DC-DC conversion. My EV jumped from 14.2 km to 22.8 km after replacement of the 12V battery. I had though the slow decay in EV distance from Oct-Dec was the result of colder temperatures. Location CANADA. Typical overnight temp is -30C evn in the garage.
     
  16. Michael33

    Michael33 Member

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    My Leaf powers down most (maybe all)interior lights left on, after 10 minutes or so.
     
  17. bisco

    bisco cookie crumbler

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    i think the pip does too, but not sure.(n)
     
  18. retired4999

    retired4999 Prius driver since 2005

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    I think the dome lights stay on. I may be wrong tho. :)
     
  19. 3PriusMike

    3PriusMike Prius owner since 2000, Tesla M3 2018

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    The dome lights stay on -- design flaw, IMO.
    I left one on about 9 months ago and went on a 7 day vacation...came back and had to jump start the car. Then last week my 12v battery wouldn't hold a full charge anymore. Luckily, the warranty covered it. Hard to say if it was just a marginal battery to start with or if it was the one-time multi-day dome light light on that caused the problem.

    Mike
     
  20. Michael33

    Michael33 Member

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    Just one full discharge will substantially shorten the life of a lead-acid battery.
     
    -Rozi- likes this.