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I don't drive much. How can I keep my 2013 Prius c 12v battery in good shape?

Discussion in 'Prius c Care, Maintenance and Troubleshooting' started by RTW, Nov 9, 2023.

  1. Mendel Leisk

    Mendel Leisk Patron saint of newly poured sidewalks

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    By aux outlet, do you mean like cigarette light socket? If you put a positive volt meter probe on the positive pin of your Battery Tender wiring harness, and neg probe connect to car body, and have the car off, you should be getting accurate read of the battery volts.

    To get a more in-depth read of the battery condition, some sorta electronic load tester*, connected directly to the battery posts, is what's required. I appreciate that's a pita with the c though.

    * Solar BA9 would be my pref. They look to be out on Amazon.com though.
     
  2. RTW

    RTW Junior Member

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    Update: For context, today is Sunday. I got the new battery installed last Monday, charged overnight, and removed the charger (Battery Tender Jr.) on Tuesday. The car has been sitting since then, so for 5 days, with only one short trip on Thursday. This afternoon, I tested the voltage of the battery at the terminals with my multimeter, and it read 12.38. I also tested by putting the red lead from the multimeter into the positive part of the quick connect harness, and touched the black lead to the metal door latch (the body of the car didn't work, likely because of paint?), and it, too, showed 12.38, so now I know that I can, indeed, successfully test voltage via the harness without having to pull out the battery. Yay!

    I also tested the voltage again in the cigarette lighter with the key turned to Aux, and it again showed 12.2. So, from this, I'm guessing I can surmise that when I test in the cigarette lighter, I can add .2 to the reading to get the real voltage, which is even more convenient.

    I'm guessing/hoping that a voltage of 12.38 on a new battery that has effectively been sitting for 5 days is okay?

    I also want to take this opportunity to thank everyone who offered help with this. I really appreciate it!
     
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  3. bisco

    bisco cookie crumbler

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    it's hard to say. a better test would be to charge it up until charge complete, then test it at the harness the next morning.
    12.38 is low for a new battery, so you want to know how much the car is draining it each day when not being driven
     
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  4. RTW

    RTW Junior Member

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    Well, the weird thing is that the first reading I took, at the terminals, showed, I believe, 12.52. Then I shifted the battery a little, checking the vent tube and ground connections, and then I took more readings. It was 12.39, and then after that 12.38 a few times. This all in a matter of a few minutes. Very strange.

    I'm assuming it will be fully charged tomorrow, and I'll check voltage again. And I guess I'll check once a day for a few days to see if it's draining for some reason, though I can't imagine how/why, since nothing is on, or plugged in, and doors are closed and locked, and no inside lights are on.

    How long should a battery hold a full 12.6 or 12.7 charge when sitting in 35-55 degree (F) weather?
     
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  5. Mendel Leisk

    Mendel Leisk Patron saint of newly poured sidewalks

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    Was 12.52 fresh after a charging session? If so, should be more like 13.0.
     
  6. RTW

    RTW Junior Member

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    No, it was after sitting for 5 days, being driven once to the store and back in the middle of that, about 10 min. each way. After its first charge, it showed 12.7. But I don't know if I can trust that 12.52 reading, since the subsequent ones were lower. But maybe the 12.52 was the real voltage. I need to do more testing this week...
     
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  7. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    I would support this assessment.
     
  8. ColoradoBoo

    ColoradoBoo Senior Member

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    My wife's 2017 Prius is lucky to have 2,000 miles on her in 6-months (she retired) so I make sure it's the only car we drive around on the weekends but sometimes don't even get much use so I'll take her on a 20-mile run to help keep both the 12-volt and traction battery happy.
    About once a month, I'll check her 12-volt with my batter tester and hook up the charger for a few hours if it says it's low.
     
  9. Mr.Vanvandenburg

    Mr.Vanvandenburg Active Member

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  10. bisco

    bisco cookie crumbler

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  11. bisco

    bisco cookie crumbler

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    There is a constant drain on the battery in modern vehicles due to electronics.
    A new agm 12v should be at least 12.7 fully charged.
    It should be higher, but you never know how long they have been sitting on the shelf
     
  12. Mr.Vanvandenburg

    Mr.Vanvandenburg Active Member

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    Don’t know. This guy talks about it more than I want to read about it. It’s about right, close enough for a car owner to know when to charge.

    I avoid all of it and I can read what I need to know from my battery monitor in real time, plus a graph, on my cell phone. It shows voltage differences when the car is doing different things as I drive, or when car is off.
    It’s like the multimeter is a wagon, and I have a Tesla.

    https://modernsurvivalblog.com/alternative-energy/battery-state-of-charge-chart/#:~:text=The%20%E2%80%98State%20Of%20Charge%E2%80%99%20%28SOC%29%20of,you%20need%20to%20know%20its%20condition.&text=The%20%E2%80%98State%20Of%20Charge%E2%80%99,to%20know%20its%20condition.&text=Of%20Charge%E2%80%99%20%28SOC%29%20of,you%20need%20to%20know
     
  13. Mendel Leisk

    Mendel Leisk Patron saint of newly poured sidewalks

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    I think AGM’s should read about 0.1 volt higher. Also CTEK chargers often have a charging setting with a bit higher voltage, recommended for extreme cold conditions and for cylindrical cell AGM (like Optima Yellow Top).
     
  14. RTW

    RTW Junior Member

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    Update: Okay, so I left the battery charging overnight. This morning the light was solid green. I disconnected the Battery Tender Jr., and when I tested with my multimeter at the harness, it showed 12.9 (which was a relief). Then I opened the front door, and the car made that usual whirring sound (which surely uses a bit of power from the 12V battery), and I turned the key to Aux (maybe a little more power?), and took a reading with the cigarette lighter gizmo (drawing a little more power from the battery), and that read 12.5. So, maybe that's a "full battery" reading for this gizmo. Time will tell with repeated tests like this one. Then I went back and tested with the multimeter again at the harness. This time it showed around 12.8, which I'm guessing makes sense, given everything I just did.

    It's hard to know how long this supposedly new battery sat on the shelf before Toyota gave it to my mechanic to install. I also can't easily find the manufacture date on the battery, given how hard it is to maneuver in its installed location. But, I'm hoping that these readings indicate that the battery is healthy. Out of curiosity, I'm going to take a reading at the harness every day for the next few days while the car is sitting, to see how much the battery loses each day. Unless I'm mistaken, I'm guessing it's normal for that to happen (since it's drawing a tiny amount of power from the car all the time). I mean, one should not expect a battery to remain at full charge voltage as days tick by, right?
     
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  15. bisco

    bisco cookie crumbler

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    well done, that is correct. a new battery shouldn't lose much, but it will lose a bit each day.

    i used to go away for 2 months in the winter, and it would always start, until it didn't.

    i still got 5-6 years out of the battery, but now i put a tender on while we're away.

    idk if the c battery is any smaller than the lift back, or if the draw is the same with the car off.

    you don't have sks, which should be a help.
     
  16. RTW

    RTW Junior Member

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    So, here's a family straightforward question: After fully charging a Prius AGM battery with a battery charger, what should a multimeter show after sitting for a couple hours? How about overnight? And then after sitting for 2 days? After 3 days? After 4 days? What should these voltage readings be for a newer, healthy battery (keeping in mind that one might be remotely unlocking and locking the doors before taking measurements each time, which will draw a bit from the battery as well)?
     
    #76 RTW, Nov 22, 2023
    Last edited: Nov 22, 2023
  17. Mendel Leisk

    Mendel Leisk Patron saint of newly poured sidewalks

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    12.8~12.9

    Still, voltage is a relatively untelling stat. The electronic load testers (such as Solar BA9) impose a load on the battery, determines it's current Cold Cranking Amps (CCA), or other similar values if you want, an assesment, specifically "good", "good but needs recharge" or "fail".

    I've found in-practice a battery in good shape will have tested CCA well above spec. If the CCA has fallen below spec, it's likely on it's way out.

    The OEM third gen Yuasa battery is 325 CCA if I'm not mistaken. And maybe 40~45 amp hours?
     
    #77 Mendel Leisk, Nov 22, 2023
    Last edited: Nov 22, 2023
  18. RTW

    RTW Junior Member

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    But I've been reading that that will actually be a residual "surface charge" from the charger, and that if one leaves it for a few hours, or overnight, the reading should be a bit less.

    Anyway, I think you're right that just checking voltage can't necessarily tell you the health of a battery. I'm getting an oil change soon, and I'll ask them to test it then, since I don't have the equipment.
     
  19. Mendel Leisk

    Mendel Leisk Patron saint of newly poured sidewalks

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    I have optima yellow top. Any downtime it’s on a CTEK 4.3. Then a day comes we have a good drive. Get home, it sits overnight, off charger. Next morning I’ll see 12.8~12.9.

    fresh off charger i rarely measure, usually over 13.
     
  20. bisco

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    12.7 or higher is acceptable. should only lose .1 volt or so per day at most, unless there's a drain from the car that shouldn't be there, or the battery is not healthy.

    an overnight test with the negative disconnected can tell you if the battery is a problem.
    a load test will tell you if there's a capacity loss.

    when it is below 12.6 in the morning, put the charger on the next night.