too much current drain on 12v aux battery

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by LKasdorf, Jun 6, 2018.

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  1. LKasdorf

    LKasdorf Junior Member

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    My 2008 prius is in a mode now where it tends to drain the 12v battery down. It did this for the first time 3 months ago, and I replaced the battery, which was the original I believe. It was due.
    Yesterday, it did it again. I drove somewhere and noticed that the rear hatch opening was a bit sluggish. When I was ready to go home a few hours later, the hatch wouldn't open at all (smart key system), and the car wouldn't boot up. It went into the low voltage nutty mode where every dash light is on.

    Jumping to a friend's car got me going and I got home. Today, I took the battery back and they are checking it, but I think it will turn out fine.

    I hooked up a temp battery and I have an ammeter in series with the battery to watch current draw. I'm not sure what I'm seeing is good on not. Generally I see .15a. But sometimes, I see .6a, or 1.6a. This is with the car just sitting there off, all doors shut, all interior lights off.

    I just now looked again, and it was at .01 - .02a. That looks great. I open the passenger door, and even with domes lights off, it goes up to .98a then I shut it and it drops to .6a for a while.

    The problem is, I don't know what is normal. Are there usual suspects for vagrant power draw?
     
    #1 LKasdorf, Jun 6, 2018
  2. edthefox5

    edthefox5 Senior Member

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    Thousands of posters come here with same complaint. The mystery’s of the 12 volt battery.

    They all have the same thing in common they never arm the alarm every single time they get out of the car especially at the end of the day and in the garage. It will alert you too a door ajar and light on. It’s a very small 12 volt battery and doesn’t take much load To kill it.

    You probably got a soft 12 volt replacement. You never checked the voltage on it when you bought it.
    And I bet you don’t drive a lot either.

    I guarantee it will check out fine no matter how bad it is. Do this simple test. Measure the 12 volts at th front jump points. Doesn’t matter what it measures at the battery it matters what it measures at the front jump points/fuse box.
    With car off measure there. Then with car off turn on headlights on high beam and leave them on for 5 minutes. Measure again after 5 mins.

    My 1 year old yellow top voltage does not drop at all after 5 minutes.

    Btw if you killed the replacement battery enough that it need a jump it’s probably damaged sulphation wise. It’s not big tough regular car battery.

    The car has a pretty good quiescent load on it probably .20 unless you approach the car with the fob in your pocket and the cars sks antennas wake up and there will be a current spike.
     
    #2 edthefox5, Jun 8, 2018
    Last edited: Jun 8, 2018
  3. edthefox5

    edthefox5 Senior Member

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    Btw you should be getting concerned about the hybrid battery too.
     
    #3 edthefox5, Jun 8, 2018
  4. Mendel Leisk

    Mendel Leisk Senior Member

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    That's high. Normal would be somewhere between 15 and 50 milliamperes. Looked it up: there's a thousand milliampers in an ampere. I've done that test on ours (3rd gen) and was seeing around 18 milliamperes, with sporadic spikes to 40.

     
    #4 Mendel Leisk, Jun 8, 2018
  5. sam spade 2

    sam spade 2 Senior Member

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    Did you charge the new battery before putting it into service ?

    Did you check the voltage while the vehicle is "running" ?
    It's possible that the charging system is not working.

    While not impossible, it is pretty rare for something like this to be caused by a parasitic drain, except maybe with non-oem equipment added. Leaving a scan tool plugged in all the time is one example that will sap the battery; aftermarket alarms will too.
     
    #5 sam spade 2, Jun 8, 2018
  6. bisco

    bisco cookie crumbler

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    how many miles on her?
     
    #6 bisco, Jun 8, 2018
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