100 miles per week to keep the 12V battery from failing

Discussion in 'Gen 5 Prius Technical Discussion' started by thompsam, Jul 7, 2026 at 10:55 AM.

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  1. thompsam

    thompsam Junior Member

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    I have a 2025 Prius that I have owned for 15 months. I am on my 4th 12V battery. I live pretty close to most of the places I drive to and therefore don't typically put a lot of miles on the car unless I am driving out of town.
    This week the car's "Low battery" light came on and I took the car to the dealership because the last 2 times this happened, the battery failed shortly afterwards and the car wouldn't start.

    Once again, the dealership's service department told me that the battery tested bad and they replaced it. But then I was told that the reason the battery failed was that I don't drive the car enough to keep the battery from failing again, I was told I need to drive the car at least 100 miles per week. 100 miles? Seriously?

    Has anyone else had this situation and been told this horse hockey by the dealership? Has anyone found a way to get the Toyota corporate or the dealership's service department to acknowledge this is a defect with the vehicle and fix it?
     
    #1 thompsam, Jul 7, 2026 at 10:55 AM
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  2. Mendel Leisk

    Mendel Leisk MMX GEN III

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    What's your parking situation, would you be able to reliably hook up a charger?

    Another option, in particular if a charger is impractical, is some sort of cut-off switch, to isolate the 12 volt battery when the car's off. It is a bit of a hassle, and may lose some settings, say radio presets, for one.

    You could check what the "background" load is on the battery, basically you put an ammeter in series between the negative battery post and the car body, with the car fully "asleep". Around 20 milliamps is what you should be seeing, if there's grossly more, figure out why and deal with it if possible.
     
    #2 Mendel Leisk, Jul 7, 2026 at 11:39 AM
    Last edited: Jul 7, 2026 at 11:45 AM
  3. BiomedO1

    BiomedO1 Senior Member

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    Not really horsehockey; frequent short hops will kill a regular car battery too. The real problem is that new ECU dependent cars draws more electricity, with key-less entry and push-button starter constantly scanning for your FOB. OEM's has also been putting smaller batteries into their cars at the same time. Your dealership could be mishandling those new batteries too. They are heavy and a lazy technician or stock clerk can easily slam them around; chipping away at the longevity of them. New batteries are sent out only half charged; so if they install a new battery without placing a full charge on them and your doing short-hops; not running the car long enough to recover the startup and down time between use - your slowly chipping away at the battery. It's like plugging in your cell phone for two minutes and expecting it to run for a week with that short recharge interval.

    Solutions:
    Place the car on an RV trickle charger when your not using it or at least overnight.
    And/or install a BM-2 battery monitor. This is a Bluetooth device, with a phone app that will give your battery status when your in range.
    https://www.ebay.com/itm/800183124112?_skw=bm2+battery+monitor&itmmeta=01KWYNJF5E7AJ15EQMPQESWAJ6&hash=itemba4ea18090:g:hEQAAeSw4OBpbJwn&itmprp=enc%3AAQALAAAA0LB2JXbe%2Bh2pdWBqbR2c2DvDrkIgYaInyZ6mJW0KcFTc7VuyTjpbaE%2FMrtSWVuC0AIfjeMfdlh3i1Uc1HfojpX5ilZs7iVQ8n2o0wbcDxlP6MwvBk1iBFu5FeTHvrjcF8XhtYiLiUwtEl%2BjOBLIBHvZebQ930gPfhUvQeQXe2ZIgPHu3lHJX1KgwHiLY9atpLSr12Px2%2BmWGNY9g9eNo9otFfAuo6dCtkrI1bnRDzAv6wGaXDFACyiJaGlLq0uQvTnq3TMeLgp%2FERw42eWQgojo%3D%7Ctkp%3ABlBMUPTyydXnZw
     
    #3 BiomedO1, Jul 7, 2026 at 12:12 PM
  4. thompsam

    thompsam Junior Member

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    Thanks, but to me, the external charger is a way to get around a defect with an in warranty vehicle, not a fix to the root cause..

    it’s like if I picked up a nail in a tire and the tire dealership told me the solution to my problem was for me to go buy an air compressor and add air every 50 miles I drive. The air compressor does not address the root cause of the problem, which is the nail.
     
    #4 thompsam, Jul 7, 2026 at 12:28 PM
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  5. bisco

    bisco cookie crumbler

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    there are a few threads here regarding this problem that many are experiencing. read through them, there might be a helpful software update iirc, but it may not be the final solution.
    unfortunately, toyota dropped the engineering ball on this and is ignoring it and its customers.
     
    #5 bisco, Jul 7, 2026 at 1:38 PM
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