Can someone check these 2008 Gen II 12V battery numbers for me?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by DamoRC, Nov 27, 2011.

  1. DamoRC

    DamoRC New Member

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    I have a 2008 GenII Prius, 40K miles and over the last month or so mileage seems to be dropping (based on miles obtained per tank). I think that this might just be the bladder tank holding less as it has gotten a little chilly here in SoCal. However, in trying to figure this out I came across a thread indicating that a faulty/old 12V battery can impact the MPG. So I used the test procedure documented here:

    (Oops - I cannot post the link as an actual link - but it is post 1020264 here on Priuschat)

    The numbers I got (twice) were:

    (1) No brake, power button once, enter Vehicle Signals screen = 11.7V
    (2) No brake, hit power button again to add some load - dropped to 11.1V momentarily then returned to a steady 11.4V
    (3) Depress brake, hit power button, car in ready mode, voltage displayed = 14.0V, dropping to 13.9 when ICE kicks in.

    As I said, I got identical numbers for both tests and I was checking with a voltmeter attached to the 12V at the same time (these numbers fluctuated more but were within 0.1V of the "Vehicle Signals" display. These numbers seem low compared to what was indicated in the testing thread.

    Any thoughts / comments - Do I need a new 12V?

    Thanks

    DamoRC
     
    #1 DamoRC, Nov 27, 2011
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Yes, you have less than 12 volts under all scenarios except when it is being charged by the HV battery. If it was over 12 volts except under load it might be good.
     
    #2 JimboPalmer, Nov 27, 2011
  3. bisco

    bisco cookie crumbler

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    what are your display mpg's and calculated mpg's? miles per tank is meaningless.
     
    #3 bisco, Nov 27, 2011
  4. seilerts

    seilerts Battery Curmudgeon

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    Yes. You could try putting it on a trickle-charger at 4 amps or less for a few hours, letting it sit overnight, and then repeating the test. It could just be low state of charge, but usually for those numbers, it will be dead/no start soon.
     
    #4 seilerts, Nov 27, 2011
  5. cwerdna

    cwerdna Senior Member

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    #5 cwerdna, Nov 27, 2011
  6. DamoRC

    DamoRC New Member

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    Understood. Question is regarding the battery health.

    Thanks,

    DamoRC
     
    #6 DamoRC, Nov 27, 2011
  7. bisco

    bisco cookie crumbler

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    right, and you're going about it the right way. but as a symptom, you would be best served to understand and track mpg's properly as well.
     
    #7 bisco, Nov 27, 2011
  8. edthefox5

    edthefox5 Senior Member

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    Very poor numbers under load. Death is imminent. Usually at the most inopportune time. I suspect it has been jumped before.
     
    #8 edthefox5, Nov 27, 2011
  9. andyprius

    andyprius Senior Member

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    It is recommended, even on normal car batteries to charge about every six months. Since the Prius 12V/ charge system is so anemic, this recommendation goes double. If you think I am blowing hot air, just research all the threads on Priuschat, on problematic 12V Batteries.
     
    #9 andyprius, Nov 27, 2011
  10. F8L

    F8L Protecting Habitat & AG Lands

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    Low numbers indeed. Do you have a voltmeter so you can test the battery with the car completely off?

    One of symptoms for a dying 12v battery is the engine will continue running even when stopped when it would normally turn off if the battery was good.
     
    #10 F8L, Nov 27, 2011
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