12v battery questions

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by misterdean, Nov 19, 2018.

  1. misterdean

    misterdean Junior Member

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    because i have 2 batteries that show under 12v. is that not something to be concerned about? MPG was my initial concern, but my current focus is why is my car showing me under 12v on two batteries.
     
    #21 misterdean, Nov 19, 2018
    Last edited: Nov 19, 2018
  2. bisco

    bisco cookie crumbler

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    i thought it was 12.5 volts?
     
  3. misterdean

    misterdean Junior Member

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    those are the readings right after a slow charge, and after i drive it.

    this morning on cold start was the first time i seen this battery at 11.8v.
     
    #23 misterdean, Nov 19, 2018
    Last edited: Nov 19, 2018
  4. jerrymildred

    jerrymildred Senior Member

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    Nope. If it was, I'd always be concerned. It's not uncommon to see a reading under 12V, especially if you're reading after a load has been applied and it's been partly drained without a recharge. It's a small battery. When you open the driver's door, the brake pump runs and drains the battery. Then you start hitting buttons to enable the service menu. That drains it some more. So you now have no way to know the resting voltage. I'd be amazed to see 12+V on the battery after that.

    Never trust the readings on the service menu for checking the battery condition. Use a multi-meter and unlatch the hood by going through the passenger side door so you don't activate the brake pump. If you have two batteries that have tested good (hopefully with a proper tester), why would you think they were both bad? If you think the tester is bad, use another one.
     
  5. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    That's really low. A fully-charged, brand-new battery, fresh off a charger with "surface charge", should be flirting with 13.0.

    If they have an up-to-date electronic load tester yes. Dealerships also have these, but will charge something I'd think.

     
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  6. jerrymildred

    jerrymildred Senior Member

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    If your foot is on the brake, it's ready mode and then off. If your foot is off the brake then it's ACC and then On. [edited to add the last few words of that line that didn't get posted the first time]
    I think that will depend on the charger.
     
    #26 jerrymildred, Nov 19, 2018
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  7. misterdean

    misterdean Junior Member

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    you are the first guy to say seeing under 12v is not a cause for concern. i know getting in to service menu uses battery but didn't think it would drop the voltage instantly from 12.3v to 11.8v.

    only the used battery that was tested good by goodyear. i was really conflicted about buying a new battery when a pro tells me there is nothing wrong with the one i have. but seeing 11.9v on my service menu was spooky and i was desperate for an answer to my MPG drop.
     
  8. misterdean

    misterdean Junior Member

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    watched the video. 13v with the volt meter, 14v with the load tester, 12.5v with the hydrometer.

    three different tools. three different results. this makes my head hurt.

    EDIT - i called a shop near me. they have a machine that tests volts load and CCA. will bring it in. gonna place my bet and say the battery passes all tests.
     
    #28 misterdean, Nov 19, 2018
    Last edited: Nov 19, 2018
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  9. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    I'll bet the CCA is a bit below spec, which is not good. A brand new battery typically will have CCA at least 20~25 above spec.

    Any more details btw: what's the battery, and what charger are you using.
     
  10. ChapmanF

    ChapmanF Senior Member

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    Jerry might have been the first in this thread, but I've said it before and I'll say it again. When you read the voltage on the MFD service menu, you consistently see a good fraction of a volt, around 0.6 in my experience, less than what you would see at the same moment with a voltmeter on the battery. The MFD service menu is showing you a voltage measured at the MFD, which is downstream of the battery by a handful of fuse links, fuses, and connections that all have slight voltage drop. It will be dropped a little more in ON, a little less in ACC, given the different amount of current being drawn in each case..

    To some, the moral is "never trust the MFD service menu!!!1!! always measure at the battery!".

    To me, it's more like, use the MFD service menu whenever you want, just remember the offset. It's dependable and consistent.

    -Chap
     
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  11. misterdean

    misterdean Junior Member

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    this makes sense and everybody seems to agree. but i think my MFD is showing more than a little difference where a brand new battery should be.
     
  12. jerrymildred

    jerrymildred Senior Member

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    You can think all you want, but you won't know until you measure it with a known good multimeter.
     
  13. 05PreeUs

    05PreeUs Senior Member

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    Spot on.

    The ONLY way to test a battery.... is AT THE BATTERY. Voltage is an indication of "potential", but capacity (A-Hr) is what you really need to know. To know if a battery is good or bad, it needs to be:
    1) Fully charged (in a Prius, best done with an external source, like a charger)
    2) Load tested

    Having said that, it is true that a resting voltage of 12.2v no-load measured at the battery terminals, is essentially at, or near death.
     
  14. jerrymildred

    jerrymildred Senior Member

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    I generally agree, but @ChapmanF makes a good point that you can interpolate the voltage from the MFD. But personally, I wish they'd never put that function in the MFD because so many people misuse it.

    True if the battery has just been charged and nothing has drawn current from it. Most likely to happen on a workbench rather than in a car with the cables connected. Remember that the Prius battery isn't much more than a motorcycle battery.

    For what it's worth, here's my anecdotal story. For weeks, my '05 was taking an unusually long time to charge the brake accumulator when I'd start it up. It was otherwise running fine and I was getting around 50 mpg and no codes. I thought the pump was giving out, but found out that the 12V battery was down around 8.5 volts even after driving it for a ways. Got a new battery from Toyota. Pump worked like a new one. Gas mileage stayed the same.
     
  15. bisco

    bisco cookie crumbler

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    we have needed a battery testing sticky for 15 years
     
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  16. misterdean

    misterdean Junior Member

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    i know guys. i will get it done.

    at least i'm not completely chasing shadows. my readings of 12.3v and 11.8v are not healthy. big question is how accurate is my MFD.
     
  17. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Since this is your original concern I am wondering the following:

    1. You have owned the car two years.
    2. You only changed the engine oil once.
    3. What is the viscosity of the newly installed oil?
    4. Are you aware mpg will decline with very cold ambient air temps vs. moderate temps.

    BTW, even if you only drive 3,500 miles annually, the oil should be changed annually at minimum to get water condensation out of the crankcase.
     
    #37 Patrick Wong, Nov 20, 2018
    Last edited: Nov 20, 2018
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  18. ChapmanF

    ChapmanF Senior Member

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    It's pretty accurate. It just accurately measures the voltage at the point where it plugs in, not the voltage upstream of the connections and fuses between it and the battery.

    -Chap
     
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  19. misterdean

    misterdean Junior Member

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    i have no idea what the viscosity is. i'm using mobil1 full synthetic.
    i'm in florida. very cold is not a factor.

    changing oil annually is another first time hearing something like this. everything i read says go by the manual which is in miles not time frame. i also hear the manual is conservative to get people in the shop more often and if using full synthetic you can get by longer. i understand everybody has personal preferences but i definitely won't be changing annually with the amount i drive it.

    this is what i'm assuming. granted, mine could be goofy. but overall i've noticed car guys tend to look down on the MFD in favor of manual testing. everything i read says it's not as accurate as tools but it is within a reasonable margin.
     
    #39 misterdean, Nov 20, 2018
    Last edited: Nov 20, 2018
  20. 05PreeUs

    05PreeUs Senior Member

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    Toyota specifies 5k miles or 6 months as the minimum service interval for a G2 Prius.

    Quote:
    "Miles or Months?
    Toyota recommends obtaining scheduled maintenance for your vehicle every 5,000 miles or six months, whichever comes first.
    For example:
    • If you drive 5,000 miles in less than six months, you should obtain maintenance at 5,000 miles — don’t wait until six months.
    • If at six months you have driven less than 5,000 miles, you should obtain maintenance at six months — don’t wait until 5,000 miles.
    Be sure to keep an eye on your mileage so that you obtain maintenance when recommended. If you are a low-mileage driver, mark your calendar to remind yourself to obtain maintenance every six months."
     
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