Parasitic aux battery drain

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by djimbuser, Oct 2, 2025 at 10:26 PM.

  1. djimbuser

    djimbuser Member

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    Location:
    Henderson, Nevada
    Vehicle:
    2007 Prius
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    N/A
    I've read somewhere on here that ~57mA is acceptable battery draw when the car is off. This car seems to drain its battery if not driven for about four days.

    During this troubleshooting session the aux battery has had a battery charger connected at all times so its voltage level is fine.

    I've tried disconnected the brake control module in the back and saw no change in the current draw disconnecting or reconnecting.

    I've tried removing relays.. all of the relays under the driver's side dashboard and all of the relays under the hood. Doing that didn't seem to make any difference.

    I located the J6 connector and tried disconnecting the CM and Body ECU wires one at a time and that didn't seem to make a difference. I tried disconnecting the radio.

    What DOES make a difference is disconnecting the DOME fuse however that's not an acceptable solution.

    Watching the current draw it's ~160 mA for typically 24 seconds then it drops down to ~55mA for about 2.5 seconds then goes back up to ~160mA.

    Here's a video to help illustrate this: https://youtube.com/shorts/UUIRHCe7Dco

    Any help is well appreciated.
     
    #1 djimbuser, Oct 2, 2025 at 10:26 PM
  2. dolj

    dolj Senior Member

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    It is not outrageously high, although it is still 20-30 mA higher than it should be.
    This would indicate to me that your battery has seen better days. A proper load test will confirm. If your battery test is okay, you might have some a/m accessory that is always on, drawing those few mA.
    If your key fob is more than 3 yards away from the car, then you are not waiting long enough for the car to completely quiesce.
     
    #2 dolj, Oct 2, 2025 at 11:42 PM
  3. VelvetFoot

    VelvetFoot Member

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    "Proper" being the key word.
    My Harbor Freight (cheapo) digital battery tester showed my lawn tractor battery was good, but it failed to start even after being on a battery maintainer. Not sure if all digital units are like that.
     
    #3 VelvetFoot, Oct 3, 2025 at 9:18 AM
  4. djimbuser

    djimbuser Member

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    The car had been off for more than ten minutes before I recorded that video.

    I'll load test the battery today although I don't think the battery is the root cause even if it tests bad.
     
    #4 djimbuser, Oct 3, 2025 at 9:23 AM
  5. VelvetFoot

    VelvetFoot Member

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    Just curious, do you know how old it is?
     
    #5 VelvetFoot, Oct 3, 2025 at 9:25 AM
  6. djimbuser

    djimbuser Member

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    Location:
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    The aux battery? About nine months.
     
    #6 djimbuser, Oct 3, 2025 at 9:26 AM
  7. VelvetFoot

    VelvetFoot Member

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    Yeah, that's not right.
     
    #7 VelvetFoot, Oct 3, 2025 at 9:27 AM
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