1. Attachments are working again! Check out this thread for more details and to report any other bugs.

What's your 12v battery resting voltage?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Doug Pinkerton, Oct 4, 2024 at 9:41 AM.

  1. Doug Pinkerton

    Doug Pinkerton Junior Member

    Joined:
    Aug 22, 2014
    9
    4
    0
    Location:
    Lehigh Valley, PA
    Vehicle:
    2013 Prius
    Model:
    Two
    I recently replaced a two year old Diehard battery (under warranty) that was showing 11.9 in the morning on the Signal Info portion menu of the Prius . I happened to check that menu as I was experiencing some odd behavior and read that a low 12v can cause a variety of odd behavior.

    Two weeks later and the new 12v is also reading 11.9 in the morning. At the battery itself, I get about 12.2.

    According to everything I've read here on Prius Chat and elsewhere those numbers point to a poor battery.

    Having a battery "fail" after just two years and brand new one apparently having issues within two weeks, I was concerned that something funny might be going on and I decided to go my Toyota dealer to have them test the battery and check for parasitic draw.

    They told me parasitic draw was well within norm (under 50 mA). The battery failed their test. Printout from test shown here.

    However, there is a note on my work order, in reference to my mentioning the 11.9 displayed in Signal Info, that reads:

    "Resting voltage of 11.9 is completely normal on a hybrid vehicle as compared to other hybrid models"

    This is very interesting to me. While it goes against everything I've found related to healthy voltage, it does seem to align with things I've read about the Prius's charging of the 12v being low on its priorities.

    Can anyone check their resting and tell me if it aligns with what the tech said?

    While I'm happy that there does not to be something weird going on with the car, I'm not certain why my brand new replacement battery is failing and I'm confused by conflicting opinions on "normal" resting voltage.

    Thanks!
     

    Attached Files:

  2. Tombukt2

    Tombukt2 Senior Member

    Joined:
    Nov 29, 2020
    10,077
    1,738
    0
    Location:
    Durham NC
    Vehicle:
    2009 Prius
    Model:
    Base
    The statements wording of English doesn't even make sense even as poor as my English might be something about the resting rate is normal for a Prius hybrid vehicle versus other hybrid vehicles yeah okay whatever I know in my vote which is another hybrid vehicle unlike the Prius not the same kind hybrid synergy drive or whatever and I don't see anything about its 12 volt resting voltage being below 12 volt I measure at the battery I don't care what it is after it comes down the pipeline through all the wiring I expect them one volt voltage drop pretty easily I would imagine but what it's sitting in the back of the car and it's fully charged state resting state after charging whatever you want to call it needs to be 12 volts now what the car sees after it comes down the line I'm not sure if that's a bad thing or not as long as it stays right at 12 volt 11.9 etc problems generally aren't supposed to develop until we have a full voltage dropper better You see things talked about it 10 volts and 9 point something volts and so on Not in the Prius but in other 12 volt systems My car used to start a regular starter with 10 and 1/2 volts in the battery according to the fancy expensive test meter.
     
  3. StarCaller

    StarCaller Senior Member

    Joined:
    Feb 15, 2014
    1,253
    699
    0
    Location:
    Cedar Crest, NM, USA
    Vehicle:
    2014 Prius
    Model:
    Two
    12.7v since 10 years /
     
  4. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    109,690
    49,848
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    is it an agm battery? mine was always around 12.7 in the morning, until it croaked
     
  5. PriusCamper

    PriusCamper Senior Member

    Joined:
    Mar 3, 2012
    11,239
    4,588
    0
    Location:
    Pacific Northwest, USA
    Vehicle:
    2007 Prius
    Model:
    Two
    Only two problems you have is: 1) mechanic who thinks 11.9volts is normal is an idiot and that 2) Prius have a really weak charging system for 12volt batteries and over time you need occasionally put the 12v on a real battery charger, preferably a smart charger that goes up as high as 10amps.

    Also make sure there's no light on the dash indicating a door is open when you're not driving.
     
  6. Doug Pinkerton

    Doug Pinkerton Junior Member

    Joined:
    Aug 22, 2014
    9
    4
    0
    Location:
    Lehigh Valley, PA
    Vehicle:
    2013 Prius
    Model:
    Two
    Bisco: Yes, AGM
    PriusCamper: Based on the response I've so far received in this thread, I would agree.
    And yet, the tech claims that he commonly sees this number and works with Prius all day.
     
  7. Tombukt2

    Tombukt2 Senior Member

    Joined:
    Nov 29, 2020
    10,077
    1,738
    0
    Location:
    Durham NC
    Vehicle:
    2009 Prius
    Model:
    Base
    12V + at terminals If you lose close to a volt running through the car that wouldn't make me panic too much as long as I have my 12.5 12.3 whatever it is back there at the battery resting charge should be fine Is there something wrong?
     
  8. Tombukt2

    Tombukt2 Senior Member

    Joined:
    Nov 29, 2020
    10,077
    1,738
    0
    Location:
    Durham NC
    Vehicle:
    2009 Prius
    Model:
    Base
    When the fancy tester goes on the battery that's doing this 11.9 does it fail to load test well if so then they'll take care of it and get you another battery I imagine If it fails the load test on the $30,000 battery machine then probably something's wrong with the battery If it passes the load test I would think it should be reasonably good to go especially in a Prius as there's no starter or anything to do all that does is turn on low voltage computers if you will and headlights if you're got halogens in that's quite a load on the battery at night especially if you're one of those that sit in the parking lot with your lights on while you're doing whatever in the car and the air conditioner running and all that I know the air conditioner is not running the 12 volt but the fan and all that is
     
  9. ASRDogman

    ASRDogman Senior Member

    Joined:
    May 29, 2018
    6,327
    3,373
    0
    Location:
    Florida
    Vehicle:
    2010 Prius
    Model:
    Two
    What shows on the display is after things have started up, and before the hybrid system
    starts up.
    You HAVE To check the actual 12v Battery in the trunk, BEFORE opening the drivers door.
    So you leave it unlocked so you can get the trunk open to check the voltage AT the battery.
    at 12.5, it's questional.
    This statement is not correct, 11.9 is NOT normal, for a Prius.
    "Resting voltage of 11.9 is completely normal on a hybrid vehicle as compared to other hybrid models"