12v Battery - Time for replacement?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by roverguy78, Jun 20, 2010.

  1. roverguy78

    roverguy78 Elite Lurking Member

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    I'm starting to think that my original 12v battery in my '06 (28K miles) may be getting weak. Below are the voltage readings using the MFD.

    12.2 @ 1st press of power button
    11.9 @ 2nd press of power button
    14.1 with car running

    I recently took a 600 mile road trip and averaged 43 MPG, which seems low. I had been averaging 47 - 49. This also seems to coincide with the increased temp and humidity here in South Texas, which is another reason I'm thinking the battery may be tired.

    I've noticed that the windows do roll up more slowly in accessory mode, and the dome lamp is dimmer than when the car is on/running. I had never compared before, so I'm not sure if it has always been like that or not.

    My car was made in March '06, so the battery is over 4 years old now.

    I have never left any lights on to run the battery down. Have never required a jump start.

    What do you guys think? Time to replace it? If so, I would be going with the Optima conversion kit.

    Thanks for any input!
     
    #1 roverguy78, Jun 20, 2010
  2. eestlane

    eestlane Member

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    Replace it! Voltages are on the low side, and a defective 12v battery can indeed decrease your gas mileage due to needing constant charging. I replaced it in my 07 recently with an Optima because the doors would not unlock reliably by touching the door handles. My voltage readings were similarly low.
     
    #2 eestlane, Jun 20, 2010
  3. roverguy78

    roverguy78 Elite Lurking Member

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    Update: I checked the voltage directly at the battery with a digital multimeter...

    Car off: 12.4
    Power press 1: 12.2
    Power press 2: 12.0
    Car on: 13.9

    I'm pretty impressed with how accurate the MFD readings are in comparison.

    From what I have read here, the 12.4v reading with the car completely off means that the battery is about 50% charged. Does this alone indicate that the battery is nearing the end of its life? The last trip in the car (Friday) was about 250 miles, so you would think it should be fully charged, right?

    I recently had my windows tinted, and when I picked up the car, I noticed they had used the radio as it was on a different station, and the climate control had been used to some extent. I now wonder if they did so while the car was not actually on. This was also directly before I noticed a decline in MPG. Maybe that explains the potential diminished battery capacity.
     
    #3 roverguy78, Jun 20, 2010
  4. NoMoShocks

    NoMoShocks Electrical Engineer

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    I don't know of any reason your 12 V battery should have a 50 percent charge unless someone has been using accessories without running the car. So, I would replace it. A good fully charged battery should have a voltage of nearly 13.2 V.

    I have the same problem in my 3-1/2 year old 2007 prius. It is the number of years the battery has been in use, not miles on the car, that is the overriding factor.

    My numbers are:
    12.2 @ 1st press of power button
    11.9 @ 2nd press of power button
    11.6 @ turn on headlights after 2nd press
    14.6 with car running

    So now the question I have is what after market batteries might I choose that would be superior to the Toyota offering which I have heard to be in the neighborhood of $150. Looks like a lawnmower or motorcyle battery so I should be able to get a good one for around $40, or do I need one specifically for Prius. Does it have some kind of temperature sensor built into it? There is a sensor wire connection to this battery.

    What is an Optima?
     
    #4 NoMoShocks, Jun 20, 2010
  5. roverguy78

    roverguy78 Elite Lurking Member

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    #5 roverguy78, Jun 20, 2010
  6. direstraits71

    direstraits71 Member

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    #6 direstraits71, Jun 20, 2010
  7. Patrick Wong

    Patrick Wong DIY Enthusiast

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    More like 13.8 when the car is READY, which is the expected output of the DC/DC converter.
     
    #7 Patrick Wong, Jun 21, 2010
  8. NoMoShocks

    NoMoShocks Electrical Engineer

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    Well at any rate, my local dealer quoted $165 for OEM replacement, so I guess I will check out Optima or other on line source.
     
    #8 NoMoShocks, Jun 21, 2010
  9. roverguy78

    roverguy78 Elite Lurking Member

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    I went ahead and ordered the Optima kit. 4 years seems like a reasonable life expectancy for a battery in this climate. I'd rather take care of it before it becomes a real problem.
     
    #9 roverguy78, Jun 21, 2010
  10. roverguy78

    roverguy78 Elite Lurking Member

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    Update: After having run through two tankfuls of fuel since replacing the battery, it certainly looks like the original battery with its reduced capacity did indeed cause the mileage hit. Previous 2 tanks with the old battery only averaged 42 & 43. After replacing the battery, the next two tanks both returned 47 average. So a 4-5 MPG hit from the weak 12v battery.

    So the moral of this story is that if you leave your Prius in someone else's hands (like I did with the window tinting shop), warn them not to run power accessories without having the car in ready mode. While my battery was over 4 years old, I was not having any problems with it before then as I never ran it down.
     
    #10 roverguy78, Jul 9, 2010
  11. bruceha_2000

    bruceha_2000 Senior Member

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    The OEM is expensive, the Optima has much more capacity in the (basically) same size case.
     
    #11 bruceha_2000, Jul 21, 2010
  12. vertex

    vertex Active Member

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    It sounds like it was on the edge, and would have gone soon anyway.
     
    #12 vertex, Jul 21, 2010
  13. roverguy78

    roverguy78 Elite Lurking Member

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    I wanted to update this thread with something important that I hadn't previously mentioned. When I replaced the battery, I maintained the 12v to the car with a portable jump-starter battery. So, the immediate MPG increase I observed was not because of any ECU being reset, which could easily result in the MFD displaying a higher average. The new 12v battery clearly made a difference in my case.
     
    #13 roverguy78, Sep 18, 2010
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