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When to replace the 12V battery?

Discussion in 'Gen 2 Prius Main Forum' started by Rere1967, Aug 22, 2021.

  1. Rere1967

    Rere1967 Junior Member

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    Location:
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    Vehicle:
    2008 Prius
    Model:
    Touring
    2008 Prius Touring
    145,000 miles
    Original owner

    I may or may not have left the vanity mirror light in the “ON” position a couple nights ago and when I went out a couple days later to drive, the fob would not open the door. Seemed curious, so I used the manual key to get in and discovered the battery was dead. Read a bit on here, and decided to just jump the 12V battery first from the front (didn’t work) then from the back (worked but I inadvertently shut it off and wouldn’t work a second time) and finally from the front with success. Whew! Drove the car around the neighborhood to charge it a bit, then put the car into diagnostic mode to check the battery. 12V is at 11.5V and then when I start it, the hybrid battery is at 14.5V. I’ve checked 2 days in a row with the same results. However, my husband seems to think we should go ahead and replace the battery. I think it’s fine. What say you?
    Current battery is from Toyota and is about 6-7 years old.
     
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    With the battery age, and current voltage displayed, I’d replace ASAP. You could check it’s viability with an electronic load tester*, but odds are it’ll fail the test anyway.

    * Solar BA9 for example, is a prosumer-level tester.
     
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  3. dolj

    dolj Senior Member

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    !4.5 V is the charging circuit in the car trying to charge a severely discharged battery and does not indicate the battery's voltage.

    You need to fully charge it with an external battery charger immediately if not sooner. The longer the battery sits in a discharged state the more its longevity is damaged if it is not already past the point of no return. After the charging is complete, test the battery to confirm (or not) whether it is holding a charge, as Mendel advises above. If you are not able to charge above 12.7 V or the load tester does not pass, replace the battery.
     
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  4. Mendel Leisk

    Mendel Leisk Senior Member

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    With our once or twice weekly drives, our 2010 pretty much LIVES on a charger. And that’s ok: modern smart chargers you can do that, no ill effects.
     
  5. Rere1967

    Rere1967 Junior Member

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    Touring
    Thanks for your replies. Just so I am clear, the 12V battery should read 12.7V when fully charged? The 14.5V hybrid should read what number to be “safe”? I was thinking these readings were good, but now I see they are not, but I don’t understand what the numbers should actually be! I have an appointment at my service station for Tuesday to replace the battery. Just had to have the MFD replaced (found a used one!) so it’s been in the shop more than in use! Still love my car, though.
     
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    Both the voltages are measuring the 12 volt battery. 12.7 is the sort of reading you'd expect from a battery in good shape, measured independently of the car's dash displays.

    The 14.5 reading is meaningless, the car is showing you the 12 volt reading when the car is fully on (aka Ready), getting helped by the hybrid battery. Any car, hybrid or not, will show similar (high) volt reading when on.

    Too: voltage alone, while better than nothing, does not tell you much about the battery's "health". An electronic load tester (like aforementioned Solar BA9) will test the battery's health: you need to connect directly to the battery posts, enter some info about the battery (battery type, Cold Cranking Amp spec), then run the test. The tester applies a load, determines the as-tested CCA, and gives a verdict. The Solar's verdicts are "pass", "pass but recharge needed" and "fail". Something to that effect.

    Dealerships, and automotive retailers selling car batteries, will often have electronic load testers these days. Dealerships will likely charge 1/2 hour labour to test (for 5 minutes work...), while automotive retailers typically will do the test for free. You get more bees with honey...

    Still, bottom line, with battery of 6~7 years age, depleted due to accidental drain, just replace it.
     
    #6 Mendel Leisk, Aug 22, 2021
    Last edited: Aug 22, 2021
    kenoarto likes this.
  7. Rere1967

    Rere1967 Junior Member

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    Thank you so much for taking the time to explain. I greatly appreciate it! While I do like to do what I can, I know my limits. I am thankful that my local service station is pretty great. I’ll drop it off today and have it replaced. Thanks again! You guys are the best! I always come here first when I have an issue, and I’m never disappointed.
     
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  8. MilkyWay

    MilkyWay Active Member

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    Got mine at 140k never changed battery. I have 276k now.

    I had to jump it one time from leaving light on and was fine that was 10s of thousands of miles ago.
     
  9. Another

    Another Senior Member

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    Is it the original battery? If not how old is it? What brand?
     
  10. MilkyWay

    MilkyWay Active Member

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    I don't recall I jumped it one time probably a year ago. I bought it at 140k and that was 4 years ago. It's approaching 280k miles I really got my money worth lol. I got it for about 4k in 2017...very clean with leather.

    At 140k ("The Honey Moon Phase") I did new tires, front wheel bearings and struts. Did trans flush (total waste highly advise never ever ever doing that)....Spark plugs.

    It only needed the front wheel bearings / tires everything else was just because it was new to me.

    Since then did 1 or 2 more wheel bearings several sets of tires. And cheap struts.

    No serious issues at any time ever including today.

    For the last 100k miles it has had a P0456 "very small evap leak" and burns oil.

    And tire pressure light is faulty the last 20 or 30k miles.

    Have seriously done nothing from 200 to 280k. Can tell needs front sway bar links but not urgent. Cruises along with the check engine light on and the tire pressure light on lol and has been that way 100k miles.
     
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  11. Another

    Another Senior Member

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    If you have time, take a look at the battery and let us know if it’s OEM or aftermarket and any date code on it.
     
  12. aplus

    aplus New Member

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    With that age it could be worth replacing. If it was a brand new battery then pick up a trickle charger for about $70.

    I once made a mistake and left the light on overnight, draining a brand new battery so that it wouldn't start up in the morning. In this case, I was able to fix this by putting it on a trickle charger for a few hours - there's a special mode for it to let it work for this specific type of battery. Search for Schumacher Fully Automatic Battery Charger/Maintainer.