12v battery questions

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

  1. misterdean

    misterdean Junior Member

    Joined:
    Jan 10, 2017
    49
    12
    0
    Location:
    FL
    Vehicle:
    2006 Prius
    Model:
    Two
    MPG dropped from 52 to 45. so far i have tried new tires oil spark plugs air filter 12v battery. old 12v battery tested good but service menu was showing 11.9v with accessories. new battery showed 12.3v. MPG didn't change. today i went back in service menu before my first start and it was showing 11.8v. after driving it i checked again and saw 12.3v. is this normal? everything i hear says under 12v something is wrong. wasn't expecting to see a brand new battery at 11.8v. i'm wondering if something else might be going on. the only thing i can think of is alternator but it did go back up so not sure what to think.
     
  2. jzchen

    jzchen Senior Member

    Joined:
    Apr 9, 2012
    1,372
    371
    0
    Location:
    Arcadia, CA
    Vehicle:
    2012 Prius v wagon
    Model:
    Five
    12.3 V seems a little low. 12.5 V is a fully charged AGM battery. If you have a multimeter test the voltage at the battery terminals to verify the accuracy of the monitor you are using.
     
  3. JimboPalmer

    JimboPalmer Tsar of all the Rushers

    Joined:
    Apr 14, 2009
    11,838
    6,052
    0
    Location:
    Greenwood MS USA
    Vehicle:
    2012 Prius v wagon
    Model:
    Three
    The Prius has no alternator, it converts 201v from the HV Battery to about 13.8v (on my Gen 2) whenever the car is Ready.
    This is really slow to charge the 12 battery. (My v charges at 14.4v, much faster)

    If you have secure place to park it and a full tank of gas, I would leave it in Ready for 12 hours.
     
  4. misterdean

    misterdean Junior Member

    Joined:
    Jan 10, 2017
    49
    12
    0
    Location:
    FL
    Vehicle:
    2006 Prius
    Model:
    Two
    it shows 12.6v on the first press of the power button and 12.3v on the second press. this is ready then accessory mode right? when the car is on it's 14.5v. whats the purpose of leaving it in ready mode for 12 hours?
     
  5. Mendel Leisk

    Mendel Leisk Titanic Social Director

    Joined:
    Oct 17, 2010
    36,183
    24,988
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    Get at the least a digital multimeter, test the battery first thing in the morning, with the car off. The car should have had a good drive the previous day.

    A brand new AGM battery in 100% health should be reading 12.7 at least. 12.6 would be ok. But reading just the voltage, is kinda like feeling for something in the dark: better is to get it checked out with an electronic load tester. For DIY, Solar BA5 has been around for a while, what I use.

    I've got a hunch you were sold a stale battery, but get it tested properly.
     
  6. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    83,650
    36,565
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    leaving it in ready mode is another way of charging the battery. same a if you were driving, but easier if you have a secure place to leave it that long.
    better to take it back and have it charged, or buy a trickle charger.

    a new 12v should test around 13 volts when new and fully charged

    but the 12v may not be your problem. with a 12 year old car, you have to get back to basics.
     
    #6 bisco, Nov 19, 2018
    Last edited: Nov 19, 2018
  7. misterdean

    misterdean Junior Member

    Joined:
    Jan 10, 2017
    49
    12
    0
    Location:
    FL
    Vehicle:
    2006 Prius
    Model:
    Two
    this is the first time i'm hearing this. all the threads i've read said around 12.5v.

    i will also add that i slow charged this battery before installing it. 12.6v is the highest i've seen.
     
  8. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    83,650
    36,565
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    12.5 is a minimum, and fine for an older battery. every new battery fully charged will read about 10% more than the rating, before putting a load on it
     
  9. misterdean

    misterdean Junior Member

    Joined:
    Jan 10, 2017
    49
    12
    0
    Location:
    FL
    Vehicle:
    2006 Prius
    Model:
    Two
    appreciate the clarification.
     
  10. Mendel Leisk

    Mendel Leisk Titanic Social Director

    Joined:
    Oct 17, 2010
    36,183
    24,988
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    For an AGM battery, brand new, 100% healthy, 12.8~12.9 is the at-rest voltage you'll see, first thing in the morning. If it's just come off a charger, it'll read higher, around 13.0 due to what they call "surface charge". But a day or two later it'll settle back to 12.8~12.9.

    All this though is rearranging deck chairs on the Titanic: if you want a definitive diagonsis, get it checked with an electronic load tester.
     
  11. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    83,650
    36,565
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    how many miles on her?
     
  12. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    83,650
    36,565
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    you may need throttle body work, and other things that might not throw a code.

    does she burn any oil?

    have you checked/cleaned/changed the air and cabin filters?
     
  13. Mendel Leisk

    Mendel Leisk Titanic Social Director

    Joined:
    Oct 17, 2010
    36,183
    24,988
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    If you bought the battery from a local retailer, there's a good chance they would have a pro-level electronic battery tester, and will do a gratis test. If it fails, you're at the right place for refund or replacement.
     
    jerrymildred likes this.
  14. misterdean

    misterdean Junior Member

    Joined:
    Jan 10, 2017
    49
    12
    0
    Location:
    FL
    Vehicle:
    2006 Prius
    Model:
    Two
    i did buy a slow charger. it charged only a few hours then stopped and said healthy. after a full charge it showed 12.5v.

    120K

    to be honest, i'm a complete nub. i have no knowledge if it's burning oil. the only response i can give is the oil was just changed, i have never seen it leaking oil, i've never seen the oil light on, i haven't had it long enough for multiple oil changes. this was my first oil change. the engine air filter was changed. not the cabin filter. i don't use the heat or AC.

    bought from ebay. if i bring the battery to a shop, can they tell me the same thing as buying a electronic load tester / digital multimeter?
     
    bisco likes this.
  15. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    83,650
    36,565
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    yes, an auto parts store should have a load tester and do a free check
     
  16. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    83,650
    36,565
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    you should learn how to check your oil and tires pressure, it could save you from disaster
     
    jerrymildred and 05PreeUs like this.
  17. 05PreeUs

    05PreeUs Senior Member

    Joined:
    Oct 1, 2016
    1,001
    488
    0
    Location:
    MSP
    Vehicle:
    2005 Prius
    Model:
    N/A
    MPG is miles driven divided by gallons used (station pump #) and not the MFD "lie-o-meter", that is a GUESStimate at best. But yes, it should not normally change much, unless your driving did.

    How long have you owned the car?

    As others have posted, the Prii do not have an alternator, the 12v is charged from the High Voltage pack, but the system is not designed to be a 12v battery "charger", but only a maintainer. Therefore, if the 12v becomes discharged for any reason (lights left on etc) it can take nearly forever for the internal system to "recharge" the 12v. That is problematic because 12v batteries (actually most batteries) do not respond well to being in an under-charged state for any length of time (say a day or more), before they start to permanently lose capacity.

    In some cases, adding extra 12v loads like high performance stereos, laptop chargers and such can over draw the power available to charge/maintain the 12v, causing it to be continually under-charged.

    Also, reading the voltage from the MFD, while a good initial step, really does not tell you about the battery for sure as there are multiple potential electrical connections between the battery and the measurement hardware that can cause a reading lower than at the battery itself; which is all the really matters.
     
  18. misterdean

    misterdean Junior Member

    Joined:
    Jan 10, 2017
    49
    12
    0
    Location:
    FL
    Vehicle:
    2006 Prius
    Model:
    Two
    i will bring it to a shop. i have a feeling they will tell me the battery is fine cause the old battery was tested and i was told it's good. it's very unlikely i have 2 suspect batteries. i think either the service menu is showing me numbers that aren't accurate, or there is something else going on in the car that is effecting the battery.

    i know how to check the oil and tire pressure. guess i should be thankful i'm not that helpless.

    around 2 years but i've only put around 7K on it. no stereos or charging devices.
     
    bisco likes this.
  19. jerrymildred

    jerrymildred Senior Member

    Joined:
    Oct 28, 2016
    6,360
    7,948
    0
    Location:
    Tampa, FL
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Premium
    Given this information, why are we still fixating on the 12V battery? There are still lots of other possibilities.
    Tire pressure
    Tire brand and type
    Dragging brakes
    Driving style and traffic conditions
    Air conditioner efficiency and time of usage. (Most of you are not using it now, but we still are here in FL.)
    Weather
    HV battery condition
    Etc.
     
    bisco likes this.
  20. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    83,650
    36,565
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    3,500 miles a year is really low, even lower than me. but that shouldn't be a reason for a sudden change in mpg.
    without a trouble code it will take patience and a process of elimination.
     
    jerrymildred likes this.
Loading...