12v Battery Is Discharging, Re-Charge Now !

Discussion in 'Prime Care, Maintenance and Troubleshooting' started by Rob43, Apr 18, 2020.

  1. Rob43

    Rob43 Senior Member

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    Recently I've been monitoring the 12v batteries in my different vehicles, what I've found is very interesting.

    In a nutshell, my older Non-Computer cars have virtually no measurable 12v battery drain over a week or two. But when you apply the same exact experiment to both my Prius Prime and Prius v you get completely different results.

    The Prime and the Prius v have an absolute measurable daily parasitic drain (Prime is the worse), because of this drain, your fully charged 12v battery will drain off to an unusable point within a somewhat short time.

    ************************

    For those of you that would like to add your own testing results to this thread, here's what you need to do:

    1) Fully charge your 12v battery using some type of external 12v battery charger.
    2) Leave your car locked, BUT unlatch your hood so that you can access your 12v battery later without waking up the Primes computer. (Keep your key fob away)
    3) Once each day after your initial full charge, check and measure your 12v battery voltage with a voltmeter and log your results.
    4) Assuming you have other cars to drive, test for as many days as you conveniently can, the longer the better. For battery safety reasons, terminate your testing at roughly ~11.9 volts which is about ~40% for a 12v battery.
    5) Re-Charge your 12v battery with your external 12v battery charger and bring it back up to a full charge.
    6) Your testing is done, please post up your daily logged testing info.

    ** All of this assumes your Prime is not plugged into your OE EVSE unit because many of the Prime owners can't leave their Prime plugged in 24/7.


    Rob43

    Screenshot 2020-04-18 at 2.30.34 PM - Edited.png
     
  2. schja01

    schja01 One of very few in Chicagoland

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  3. Blue-Adept

    Blue-Adept Active Member

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    After one day my Comma Two reports 11.9 volts.

    Will trickle charge when I get home.

    Usually drive 500 miles a week. Now. 50 to 100

    See attached pic
     

    Attached Files:

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  4. Rob43

    Rob43 Senior Member

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    ^ That's an unexpected big Wow...but not in a good way.

    Absolutely put your Prime on an external 12v charger, hopefully you'll get a better result*.


    Rob43

    * Your best most accurate result comes from a "Sleeping" computer that's been off for at least one hour.
     
  5. bisco

    bisco cookie crumbler

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    what is yours reporting rob? i don't see any drop in my pip. in fact, i used to leave it for 2 months in winter, and it would only drop a few tenths of a volt
     
  6. Rob43

    Rob43 Senior Member

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    That's very good news for pip owners ! Your pip seems to operate much more like the 2008 non-computer car I tested.

    Like all of us, the pandemic/stay at home orders hit first....then I started to think about 12v battery testing. So out of my two tested Prius's, my Prius v is the most likely car to sit for a while. The Prius v had been sitting at least 2 weeks (maybe 3 weeks) before I thought to go out and test it. When I did I was shocked to find 11.81 volts* (~32%) on the 12v battery. I immediately installed the permanent ring adapter from one of my Battery Tenders so that I could easily check & charge when needed. Now almost a week later, it's sitting at ~72% charged.

    My Prime does get driven on a quasi-regular basis, so testing my prime under the same testing guidelines is tougher. What I did test on my Prime starting about ~10 days ago was this:

    1) I unplugged my 10 watt / 12v Solar battery charger.
    2) I fully charged my Prime using a Battery Tender Jr. which floated the 12v battery at 13.10 volts. (Full Charge)
    3) I would leave my hood unlatched so that I wouldn't wake up the computer or dome lights.
    4) After several days of no driving, no usage, & no EVSE plugged in, I recorded a low voltage of 12.04 volts. This LOW voltage reading was not long after my full charge of 13.10 volts, 12.04 volts is roughly a ~49% charge.

    Now imagine what some people are experiencing after possibly 3 plus weeks...



    Rob43

    * The Prius v was not charged up first, so there's no available record of beginning voltage.
     
    #6 Rob43, Apr 18, 2020
    Last edited: Apr 18, 2020
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  7. bisco

    bisco cookie crumbler

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    wow, that's a huge drop. is the 13.10 volts after the surface charge wore off?

    we've seen a lot of bad gen4 batteries here, but not prius v, iirc. but maybe because it is 7 years old? i had to replace mine after 8 years.
     
  8. Rob43

    Rob43 Senior Member

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    The 13.10 volts is measured with the Battery Tender Jr completely finished (finished for hours) and still connected. Once the alligator clips were removed I still recorded 13.10 volts in the first minute.

    Normally a healthy 12v car battery will fall back to roughly 12.6 to 12.7 volts once it has stabilized, the problem is the Prime keeps sucking down the voltage based on its parasitic draw.


    Rob43
     
  9. benagi

    benagi Active Member

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    Last year we went on a road trip for six weeks. Left my prime in the garage, not plugged into the wall. No tender on the 12 V battery. When we got home, it started no problem.
     
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  10. Rob43

    Rob43 Senior Member

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    That's good luck. (y)


    Rob43
     
  11. Trollbait

    Trollbait It's a D&D thing

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    How do you rule out the battery being abused before you got it, or it simply being defective?
    Test it disconnected from the car?
     
  12. bisco

    bisco cookie crumbler

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    i would have to think a majority of gen 4 are not dropping like that on a daily basis, or they'd all be dead
     
  13. Salamander_King

    Salamander_King Senior Member

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    My 2020 Prime 12V battery has never been charged from an external charger yet. It's only two months old. Even adding time sat on the lot since manufacturing, it is less than a year old battery. After opening the driverside door, it was 12.38V by a multimeter and 12.60V by a battery tester today. I am charging the battery now. I will keep the hood open and take a measurement again tomorrow and maybe later.
     
  14. Trollbait

    Trollbait It's a D&D thing

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    Father had a bad battery replaced under the bumper to bumper. Defective ones do make it into new cars.
     
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  15. Rob43

    Rob43 Senior Member

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    We look forward to your findings.

    Even if you can't follow the exact daily testing that I outlined due to the need to drive your Prime, some testing is certainly helpful.


    Rob43
     
  16. Salamander_King

    Salamander_King Senior Member

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    Here is the first overnight reading.
    The battery fully charged in ~5 hours. After removing the charger, the battery voltage dropped to 13.06V from 14.25V while being charged.
    After 12 hours, the voltage is 12.88V by Battery Monitor II reading. The reading is slightly higher than what I see on a multimeter reading 12.77V.

    I'll try to keep the car not driven for a week.

    Screenshot 2020-04-20 at 7.47.04 AM.png
     
  17. bisco

    bisco cookie crumbler

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    that makes more sense to me, for a healthy battery
     
  18. Salamander_King

    Salamander_King Senior Member

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    Still only overnight. I intend not to drive my car this week. Will see how low it gets in 7 days.
     
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  19. Rob43

    Rob43 Senior Member

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    Yep, like Bisco said, that's totally normal so far. I'm going to guess with no external 12v charger and no EVSE connected, you'll drop down more than you want.


    Rob43
     
  20. bisco

    bisco cookie crumbler

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    there is also a gen4 thread on this currently. some people are having issues, and others are not.
     
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