slow current leak-- cannot find the source

Discussion in 'Gen 3 Prius Technical Discussion' started by tennessee3, Aug 8, 2025 at 10:11 AM.

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  1. tennessee3

    tennessee3 New Member

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    I have a 2012 Prius that runs fine-- can go on a 300 mile journey, but if it sits unused for 10 days, the lead acid battery is dead and the car will not start. Recharge the battery and it works fine. The battery has been replaced several times-- that is not the problem, nor is the charging system the problem. Toyota did the usual check for a current leak and could find nothing. The lithium battery seems fine-- the display shows it charging and the car intermittently runs on the Li battery as it always did.

    If I remove all the fuses from the passenger compartment, the leak goes away. The battery remains charged and the car starts after two weeks or more.

    I know that the IGN and ECU-ACC circuits kill the battery, because if I add them back to the passenger fuse box, the battery dies in a week.

    Each test takes about 2 weeks to run, to confirm that the battery is not going to die.

    Are there any Prius electrical system experts out there with ideas? Any better way to identify the source of the leak?
     
    #1 tennessee3, Aug 8, 2025 at 10:11 AM
    Last edited: Aug 8, 2025 at 10:25 AM
  2. Tombukt2

    Tombukt2 Senior Member

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    I hope it's AGM 12 volt
     
    #2 Tombukt2, Aug 8, 2025 at 12:03 PM
  3. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Put an ammeter in series between 12 volt battery's neg post and ground, see what milliamperes you get with the car closed and having sat half an hour, with all fobs well away.

    Specifically, I disconnected the 12 volt battery neg cable at the body, connected a jumper wire to it, connected another jumper wire to a ground point, and ran both wires out the hatch threshold, gently closed the hatch on them, and then connected my multimeter to the two jumper wires. After a half hour wait, set the meter to amps first (just in case there's a massive draw), then to milliamperes.

    @ChapmanF's got a slightly more complicated connection method, with some benefits; he may notice this and post a diagram.

    Doing this I was seeing about 20 milliamperes, with spikes to 40~50 every few seconds, which could have been the blinking security icon on the dash.

     
    #3 Mendel Leisk, Aug 8, 2025 at 12:49 PM
    Last edited: Aug 8, 2025 at 12:56 PM
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