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How to know its time to change 12v battery?

Discussion in 'Gen II Prius Technical Discussion' started by paycee, May 5, 2012.

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  1. paycee

    paycee Junior Member

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    Hi PChatters,

    How will I know when to change 12v battery? I checked it early in the morning and it shows 12.1 charge. When radio is turned on it shows 11.9 and when engine is turned on shows 14. something. I get 50's on my MPG though. I am just curious if I can still get better MPG than what I am getting now if I have a new 12v battery installed. What do you guys think?

    What should be the optimal charge when checking early in the morning, when radio or lights are turned on and when engine is on?

    I would appreciate answers from PChatters who are well versed with this.

    Thank you in advace :)
  2. David Beale

    David Beale Senior Member

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    Looks to me like your battery is fine. I doubt very much that your mileage would improve if you replaced it.

    Your mileage is better than average, which is around 48 MPG US for a GII.

    A full test of the 12V battery might consist of power on without the brake pedal and turn on the headlamps. The battery should be able to hold 11 to 12V for perhaps 30 min. I wouldn't perform this test, however, as it may slightly shorten the life of the battery.

    If your battery is original and hasn't been fully discharged, it may have another year left in it. Typically they last from 4 to 5 years.
    1 person likes this.
  3. paycee

    paycee Junior Member

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    Thanks David :)
  4. paycee

    paycee Junior Member

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    I guess Im just trying to find ways how to squeeze my mileage as much as I can. Ive been a member since 08 but didnt really read on this forum until this year only. Im looking at scangauge as another way to get higher MPG. Do yo use one?
  5. uart

    uart Senior Member

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    You really need some better data points to be sure. Try it again in the morning, but make sure that you've turned off all accessories (radio fan lights etc) when you shut down the previous night AND make sure you power on to only Acc mode and not Ig-on. This is your base no load measure.

    Then you need one or more standard load measures (radio is not enough). Turn the headlights on for about 10 seconds and record the voltage drop. Then turn off the lights and put the Prius in Ig-in mode for about 10 seconds and repeat the measure.
  6. hyo silver

    hyo silver Awaaaaay

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    Hmm. I've got about 97,000 km on an original 2004 12V. Should I get a new one, even though I'm not aware of any problems?
  7. xs650

    xs650 Senior Member

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    Yes, your battery is living on borrowed time.:rockon:
  8. Mendel Leisk

    Mendel Leisk Senior Member

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    What's unclear to me is how you're checking you battery voltage. Is it via some onboard diagnostics screen?

    In my experience using an external digital multimeter, a healthy battery will show around 12.6 volts, or higher, maybe up to 12.7. The battery should not have been recently charged, via some external charger, as that will give a falsely high reading.

    If that's the case, you'd want to turn the head lights on for a few minutes, or just drive it for a day or two.


    Some people just routinely replace the battery, say every 4 years, just for insurance.
  9. RRxing

    RRxing Active Member

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    From the 2010 repair manual:

    Check Battery Voltage.PNG

    As mentioned above, it should be between 12.5 and 12.9.
  10. dhanson865

    dhanson865 Expert and Devil's advocate

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    If you want to squeeze every penny out don't change it until you notice odd behavior on the dash.

    My dash looked odd once or twice a month for several months before I had my first couldn't start the car incident.

    First time it happened I couldn't get the car in ready mode but it would power on and the dome light, dash lights all looked normal.

    I spent the money to switch the battery eventually and didn't notice any change in MPG, in fact I was getting 65-70 MPG on a dead 12v battery for a couple of months last summer.
  11. David Beale

    David Beale Senior Member

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    I have a Scangauge II. I don't think it will help you get better mileage. Learning how to drive economically will. I think there are instructions on this site somewhere or links to other sites on this. But you are doing pretty well if you are "driving normally". If your trips are mostly highway your mileage will be a little lower, and if they are short the same applies. If you drive in the city or on slower secondary highways (speeds under 55 MPH) on longer trips, say longer than 30 minutes, you can get pretty impressive mileage. Also cold weather (Northern states) will cause poorer mileage. Mine is terrible in winter, with short trips and cold weather combined giving me mileage in the 6 l/100km range (39 MPG US). Now that it's warming up I'm running 4.9 l/100 km and dropping (improving).

    Oh, and Pearl's original 12V battery is still going strong. I added water to it last summer as it appeared to be getting weak. It improved. It may crater this year. We'll see. It's never been fully discharged so I'm keeping my fingers crossed. ;)
  12. derechbennoach

    derechbennoach Junior Member

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    On this note. I checked my battery and it showed 12.1 volts. Lights and fan on 11.6. So I figured it needed changing. My MPG was in the high 40's.
    I put a 2 amp trickle charge on it for 6 hours. Now my MPG is in the low 50's? I have not checked it again - did not want to mess with even a temporary success. I am convinced this little battery can make a difference.

    What about installing one of the 1.5 amp battery minders and plug it in every few days or so?

    Sent from my Xoom using Tapatalk 2
  13. sandystevees

    sandystevees New Member

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    The 12V battery is charged at a 1.4 amp rate though the inverter when the Prius is in READY mode, so it's not going to affect how often the ICE runs. At 12.15V, it's time for a new 12V battery. Some people replace the OE battery with a Optima Yellow Top or Odyssey battery. These are deep discharge batteries that can withstand many total drains. They require a kit to mate the terminals to the Prius because the Prius uses JAL standard connectors. They also require padding in the battery holder because the physical size is different.
  14. Patrick Wong

    Patrick Wong DIY Enthusiast

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    This is incorrect. The 12V bus voltage is regulated by the DC/DC converter (typically operating at ~13.8V) and the battery will take whatever charging current it needs given its then-current state of charge. The lower the SOC, the greater the charging current.
  15. 2009Prius

    2009Prius A Wimpy DIYer

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    Your battery measured similar to mine since I bought my car new. See
    http://priuschat.com/forums/gen-ii-...-trip-record-fun-early-problem-detection.html
    for my data.
    Either I got a weak battery to start with or your battery is fine. I have never had any "weird dash" or non-start problems.

    MPG can vary a lot depending on driving conditions. Also the computer will reset (and give different MPG number) if the 12V is disconnected.

    I bought and tried BatteryMinder and could not detect any benefit, though some people claimed to have good experience with it.
  16. derechbennoach

    derechbennoach Junior Member

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    Thanks, It is probably a feel good. I did have the radio on while I was waxing it a week before I charged it. I'm going to test it today and put the charger on it later.

    --john

    Sent from my Xoom using Tapatalk 2
  17. stillageek

    stillageek Junior Member

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    The new Yellow top battery requires no kit. A direct and easy replacement
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