2010 Prius, 12v battery or key fob battery? "Key fob not detected"

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by wheelsup, Jun 26, 2025 at 8:35 PM.

  1. wheelsup

    wheelsup Junior Member

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    Key fob not detected message.
    • 12v battery measures ~12.2v-12.3v resting after sitting for three or four days. It's ~10 years old. AGM. Tests ~50% capacity.
    • Key fob battery is 3v. Measures 2.8v under 470Ω load.
    Wondering if this is the 12v battery or key fob...while I know I could replace the 12v I'd hate to do it without a reason.

    Normally key fob says "key battery low" IIRC, so this has me questioning the 12v.

    Also my local dealer is around $170 for their battery, claiming it fits the 2010 Prius. Is this correct and a decent price?

    upload_2025-6-26_20-37-31.png
     
    #1 wheelsup, Jun 26, 2025 at 8:35 PM
  2. bisco

    bisco cookie crumbler

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    sounds like either/both, but might as well start with a fresh fob battery. 10year old 12v? i would replace it if it were me. idk if that is the right size, but it is a great price.
     
    #2 bisco, Jun 26, 2025 at 8:40 PM
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  3. ASRDogman

    ASRDogman Senior Member

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    Best to replace the 12v battery, it will soon start giving you hybrid errors.
    Make sure you charge it for 4-5 hours at 5 amps or less to give it a full charge before
    installing....
    Key fob batteries are inexpensive, won't hurt to change it...

     
    #3 ASRDogman, Jun 26, 2025 at 9:01 PM
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  4. wheelsup

    wheelsup Junior Member

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    I have a Noco Genius 5, so will put it on that before putting it in the car. Thanks.
     
    #4 wheelsup, Jun 26, 2025 at 9:06 PM
  5. Brian1954

    Brian1954 Senior Member

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    You need to replace the key fob battery. 2.8 volts for that battery is very low. A new CR1632 battery should measure about 3.3 volts.

    I just replaced my wife's key fob battery today because she got the same "Key fob not detected" message. She said she had to hold her purse close to the dashboard for the car to detect the fob. I measured the old fob battery at 3.00v, and the new battery I installed was 3.3v.

    The 12v battery you show in your post is the correct one. That is a very good price for the battery.
     
    #5 Brian1954, Jun 26, 2025 at 10:41 PM
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  6. Paul Gregory

    Paul Gregory Senior Member

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    One of my pet peeves is "keep replacing parts until it works."
     
    #6 Paul Gregory, Jun 26, 2025 at 11:13 PM
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  7. wheelsup

    wheelsup Junior Member

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    OK I can replace, as I have spares. But just to confirm, that is 2.8v with the 470Ω resistor across it, it's 3v without a load.

    My new ones are around 3.27v as you described.

    That being said, I'd think 12.2-12.3v resting should be more than enough for that 12v to continue service.
     
    #7 wheelsup, Jun 26, 2025 at 11:17 PM
  8. Brian1954

    Brian1954 Senior Member

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    I agree. If you already carry a jumper pack in your car, I would wait to replace the 12v battery. If you do not have a jumper pack, I would not hesitate to replace the 12v battery, especially at the price you show in your first post.
     
    #8 Brian1954, Jun 26, 2025 at 11:36 PM
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