Checking/maintaining my 12 battery

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by NewHybridOwner, Apr 6, 2022.

  1. NewHybridOwner

    NewHybridOwner Active Member

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    I decided it was time to check the voltage of my car's original 12v battery, since the car has done mostly only very short rips for the last year or two. 12.07 volts at the battery, with only the internal lights on. Hooked up a "smart" charger (that has user-selectable 2A and AGM settings) to the jump points under the hood, and now, after an hour or so, the charge current is down to 1.2A and the voltage is 14.9, which seems a little high but not outrageous.
     
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  2. tankyuong

    tankyuong Senior Member

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    I would replace it
     
  3. sam spade 2

    sam spade 2 Senior Member

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    You need to leave the charger connected for a LOT longer.......like 24 hours maybe.
    And then disconnect the charger and disconnect the battery from the car and see how quickly the voltage falls.

    BUT.....given that the battery is about 7 years old and has some HARD duty recently, you probably should just start shopping for a new one.
    Likely that it's on it's last legs and no amount of "maintaining" will help at this point.
     
  4. bisco

    bisco cookie crumbler

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    something wrong with that reading, unless the charger is still connected. when you're done charging, turn the headlites on for a few minutes and take a reading
     
  5. NewHybridOwner

    NewHybridOwner Active Member

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    Yes, the 14.9V was with the charger still connected. IIRC, the typical max. charge voltage for AGM 12V batteries is 4.7V.
    Less than an hour later I had to go out, and when I went to disconnect the charger it was already in "Float Charge" mode.
     
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  6. Mendel Leisk

    Mendel Leisk Underfoot

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    Get it tested with one of the new gen battery testers. Automotive retailers selling batteries usually have one, and will test for free. Dealerships have them too, but will likely charge you $50~100, for the 5 minutes the test takes. Something like Solar BA9 is a DIY level option. If that's not in the cards:

    Let the charger run to completion. Use the car for a few days. Pop the hood in the evening, Check voltage first thing in the morning, at the under-hood jump point.

    If it's back to 12.07 it's time. Assuming this is original battery, it's time anyway.
     
  7. NewHybridOwner

    NewHybridOwner Active Member

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    Would the battery have to be tested at the battery itself (pain to remove stuff to get at the terminals), or can that be done at the under-hood jump points?

    And I noticed that in an earlier thread you reported that you had connected charging leads under the hood. Where did you attach the positive lead? is there a convenient point for one of those eyelets in some obscure location?
     
  8. Mendel Leisk

    Mendel Leisk Underfoot

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    Checking at the battery itself will eliminate any issues with loose connections. Checking voltage at the jump point is usually ok though. The modern automotive 12 volt testers (like aforementioned Solar BA9) require connection directly to the posts.

    This is where I connected the positive wire of my quick connect (for a Clore CTEK 4.3):



    Very good idea to disconnect neg lead at battery before doing this.

    And for the neg wire, I noticed an unused, threaded hole, a little lower and towards the front from the fuse box. I had a few metric threaded bolts (M6 coarse thread, pretty sure; 10 mm socket size), found one that fit:

    3D62C08D-1182-468E-A252-2685067DFC32.jpeg

    (Neg connection pretty much dead-Center of pic.)
     
    #8 Mendel Leisk, Apr 7, 2022
    Last edited: Apr 7, 2022
  9. johnHRP

    johnHRP Member

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    12.07 V is not the end of the world. Just charge it until it is full. Typically charger will stop when the reading voltage is 14.3V or above, depends on which brand. For AGM the cutoff in smart charger
    NOCO5 14.6 V
    ALDI 5A 15.5 V
    LIDL 5A 15 V
    CTEK 5 ~ 14.9 V
    Some dumb charger will continue charge it at low current ~ 70mV or lower nonstop.
    Battery last longer when it is never fully discharge >11.8V and not over charged. Overcharge can couse hydrolysis and electrodes erosion
     
  10. NewHybridOwner

    NewHybridOwner Active Member

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    Well, after having charged the battery yesterday until the charger switched into "Float Charge" mode, the battery is now, after "resting" for about 20 hours, at 12.2V when checked under the hood, so my guess is that the battery is on its way out and should be replaced before too long.
     
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  11. johnHRP

    johnHRP Member

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    Yes, it is not new anymore but if it still hold the charge without running to READY mode in 2 or 3 days, it is still usable and may last couple more years. If you leave the cars for few weeks, and rarely used, A new battery may be the best way for your peace of mind. Always get AGM battery for safety reason (accident). Mine is read 12.3V since 2 years ago and still works just fine although it is more than 6 years old. I can leave the car for more than 1 week without issue.
    What charger brand do you have? The charger will go to float stage

    What is important is how much voltage drop under load. Try to put on ACC mode, and turn the headlights. If it is less than 10.5V then you may need replacement. Otherwise, you can still use it for a few more years.
     
    #11 johnHRP, Apr 8, 2022
    Last edited: Apr 8, 2022
  12. sam spade 2

    sam spade 2 Senior Member

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    You now have enough "data" to change it from a guess to a conclusion.
    And if you do not keep the battery tender on it when not in use, that "before too long" might be a lot shorter than you think.
     
  13. NewHybridOwner

    NewHybridOwner Active Member

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  14. NewHybridOwner

    NewHybridOwner Active Member

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    The charger is a "Vector" device that I've had for at least a decade.
     
  15. Mendel Leisk

    Mendel Leisk Underfoot

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    Yeah, I would start researching replacement.
     
  16. Paladain55

    Paladain55 Junior Member

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    Get her swapped. We are just lucky with Priuses that they don't need high amps from the battery because clearly, that would fail the amp tester. I think as long as the system gets 12v ish it will go into ready mode.
    Yup the charge to done and sit for 24h and measuring resting voltage is a relevant test. And yours is in the 75% ish dead zone. Probably one more time leaving the interior light on overnight and it will be done.
    You can get creative with an aftermarket AGM battery swap or get the Prius specific battery for a bit extra cost.
     
  17. NewHybridOwner

    NewHybridOwner Active Member

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    I don't see the aftermarket AGM batteries (of a reputable brand) being significantly cheaper than the one from the dealer -- and often with an inferior warranty. And some dealers offer discounts from the list price, making them no more expensive than the after-market ones.
     
  18. Paladain55

    Paladain55 Junior Member

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    Pretty much. The oem battery is extremely high quality. I did an odyssey extreme since I already had it and they on average last forever as well (with my experience so far. My oldest is 6 years old and resting voltage is still 12.7)
     
  19. johnHRP

    johnHRP Member

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    Get only original Toyota or Yuasa AGM battery. They are still the best normalised to usage time. Prius only need 60 Amps to start to Ready Mode. Even some Outlet charger with 75A rating can start it without any issue. Generally, all battery degrade over time and it is up to us when we need to replace it. It is hard to tell, my solution after 5 y old is simply getting cheap Li pack jump starter for about $20. Once it starts giving problems i can still use the car for couple days before new battery come.
     
  20. johnHRP

    johnHRP Member

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    It is not about how old the charger but how it maintain the charging state without overcharging it. Most charger with microcontroller will stop charging completely at certain voltage and can do float charge once the voltage drop at certain limit.
     
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