2nd Gen 12 Volt Battery Charging Issue

Discussion in 'Gen 2 Prius Technical Discussion' started by Lou Fezio, Mar 19, 2018.

  1. Lou Fezio

    Lou Fezio Junior Member

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    I've got an 05 Prius that I have had an odd problem with since new. I've had to put a new 12 volt battery in this car every 3 years and this seems wrong. I suspect I've got a charging problem.

    So I just put a new Optima battery in again about a month ago and the strange thing is that when I check the voltage on the new battery it reads 12.5V. When I turn the car on the voltage goes to 13.8. If I bring the battery up to 12.9 V with a trickle charger it quickly goes back down to 12.5 after I run the car. When I run the car in diagnostic mode the voltage stays at 13.8 but the battery looses voltage until it is back at 12.5.

    I don't see how this can happen.

    Has anyone else experienced anything like this?
     
  2. bisco

    bisco cookie crumbler

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    can you test the draw on the battery with everything off, ig on and ready?

    if those readings are correct, i'm surprised you're getting three years, but you may have to wait for the surface charge to wear off, after trickle charging, then test.
     
  3. Lou Fezio

    Lou Fezio Junior Member

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    I'm not sure how to test the draw on the battery with ig on. Are you talking about the current draw?
     
  4. bisco

    bisco cookie crumbler

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    yes, i'm not either. it might be easier to start with measuring the correct voltage and checking it every morning for awhile.
    i like to pop the hood in the evening, and measure it in the morning inside the fuse box jump point. that way, you don't have to open any doors which causes current draw.
     
  5. Lou Fezio

    Lou Fezio Junior Member

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    I can do that.
     
  6. AzWxGuy

    AzWxGuy Weather Guy

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    Have you checked the integrity of the grounding point on the back hatch lip near the battery? Take a look to see if there is anything amiss there, corrosion or rust. You might even want to carefully loosen the retaining bolt and move the wire around a bit to make sure it is a good connection then tighten back up. Might see some sparks when you do this. Of course also check the battery post connections. The 12.5V you are measuring isn't too bad. A problem battery will measure less than 12.0V. The 13.8V you see with vehicle on is also correct. You could hook up a DVOM to measure off-state current draw, to eliminate the chance of a parasitic draw from somewhere. There are threads that mention what a normal off-state current draw should be (SKS and non-SKS vehicles). At what voltage do you assume you need to replace your battery anyway?
     
  7. dolj

    dolj Senior Member

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    That is my feeling too.
    That is debatable, 12.5 V for an AGM (which the Optima is) is about 60% SoC. So, all that really tells you is that it needs a charge. This is interesting though, keep reading to see why.
    That would be correct, if the battery had a higher SoC, but at 60% SoC, I'd expect to see voltage upwards of 14 V, and more like 14.4 V. So I would focus on whether the 12 V voltage sense wire is good all the way from the battery to the ECU. That is the thin white wire that plugs in to the black block attached to the +ve terminal of the 12 V battery.
    While possible, and it wouldn't hurt to test if everything above pans out, it doesn't seem likely at this point.
     
  8. SFO

    SFO Senior Member

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    How many hours per day/week is this vehicle driven or in 'Ready' mode?
     
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  9. Lou Fezio

    Lou Fezio Junior Member

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    This car is my daily driver. I use it about 45 minutes/day, more on weekends.
    This is the only vehicle I've had that with regular use still accepts charge from the trickle charger.
     
  10. bisco

    bisco cookie crumbler

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    how is the voltage this morning lou? my 6 year old battery read 12.68
     
  11. edthefox5

    edthefox5 Senior Member

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    Mine reads 12.6 too.

    First thing in am check battery before starting.
    Then with car STILL OFF turn on headlights with high beam on. Measure battery. Voltage should not drop but 1/2 volt .5. On mine it reads 12.1

    Leave the headlights on for 5 minutes. Mind did not drop at all during this event. Yours will probably tank as it’s not holding a charge very well.

    Your car is charging just fine. If it was not it would throw a lot of codes as no charge means the dc-dc converter in the inverter has failed. That’s a fatal error and the car will freak.

    Btw how many times has that battery gone dead enough that you had to jump it?
     
  12. Lou Fezio

    Lou Fezio Junior Member

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    My one month old battery reads:
    12.78 under the hood with nothing on
    12.65 at the battery with the hatch open
     
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  13. fotomoto

    fotomoto Senior Member

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    Where do you live and how many miles per year does the car operate?
     
  14. bisco

    bisco cookie crumbler

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    that sounds pretty good lou. you might try charging it up and see how high it will go. with the proper charger, it should get close to 13v, and hold around there in the car during your morning checks.
     
  15. Lou Fezio

    Lou Fezio Junior Member

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    I live in Seattle and put about a thousand miles a month on the car.

    So last weekend I drove to my son's place and had the car on for 1 1/2 hours. I checked battery voltage before and after the trip and it barely changed.
     
  16. Lou Fezio

    Lou Fezio Junior Member

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    I've checked the ground strap and it's good. So are the battery connections.
     
  17. bisco

    bisco cookie crumbler

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    sounds like you're good, but i would keep an eye on it for a few days, then once a week, then once a month until you are comfortable.
     
  18. Lou Fezio

    Lou Fezio Junior Member

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    I did this last weekend. My trickle charger brought it up to 12.9 and after I used the car it was back to 12.6. And all the time the MFD showed the system voltage at 13.8.
     
  19. bisco

    bisco cookie crumbler

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    it's possible that you have a bad battery.

    not in the sense that it won't work, but that it won't hold a full charge as a new one should.
     
  20. Lou Fezio

    Lou Fezio Junior Member

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    I don't believe it's the battery. This is the 5th battery I've put in this car and they all seem to last about 3 years. This battery is behaving like the others I've put in.

    The charging system seems not able to put a full charge on the battery. But the charging system voltage is good at 13.8 so what gives?

    Is there a spec for the voltage the charging system should be operating at? Is 13.8 volts low?
     
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