voltage drop from battery to obd port?

Discussion in 'Gen 3 Prius Accessories and Modifications' started by StarCaller, Jan 10, 2015.

  1. StarCaller

    StarCaller Senior Member

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    on my battery poles I measure like 12.8v, my scangauge reports around12.4v/
    I have an additional voltmeter connected to the cigarette lighter cables & same thing there: 12.4v
    why would the voltage go down soooo much?
    my first thought was that my battery is not getting fully charged, but after measuring directly at the poles, it's obvious that the voltage drop happens on the way/
     
  2. xliderider

    xliderider Senior Member

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    Try measuring the voltage at your power port with the same DVM you used to measure the battery voltage at the battery posts/terminals.
     
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  3. Mendel Leisk

    Mendel Leisk Sand Pounder

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    The only way you get voltage at the cigarette light is by having the car on, if I'm not missing something. Maybe that accounts for the voltage drop? Have you tried checking voltage at the underhood jump start point, with a multimeter?

    FWIW, I've got one of these, has obdII with 12 volt pins at one end, 12 volt plug at the other, could check it out myself in a bit.



    Capture.JPG
     
  4. Easy Rider 2

    Easy Rider 2 Senior Member

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    All wiring has some resistance and thus some voltage drop when current is flowing.
    The more current, the more drop.
    Then there is potentially more voltage drop across connection points; more if corroded, less if clean and tight.
    The difference might actually be on the ground side, IE not a really good ground.
     
  5. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Ok, got this:

    At battery posts: 12.39 (volts)
    At OBDII: 12.34 (measured at 12 volt plug on cable shown above)
    At JumpStart point, underhood: 12.49

    That last one kinda weird, not sure why. Good quality RMS meter too. Ambient temp 4C.
     
  6. StarCaller

    StarCaller Senior Member

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    so the common opinion here would be that I'm ok, right?
    maybe I'm making an elephant out of a mouse..... ;)
     
  7. Easy Rider 2

    Easy Rider 2 Senior Member

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  8. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Anyone have a good rule-of-thumb for what ambient temps do to voltage readings? All my readings are kinda low, wondering if at least some it is due to 4C ambient temp?
     
  9. StarCaller

    StarCaller Senior Member

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    I measured again:
    battery poles: 12.47
    jump start under the hood: 12.37

    outside temperature: 35° F
     
  10. xliderider

    xliderider Senior Member

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    As Easy Rider stated above, a good ground point is really essential to getting an accurate reading. Don't use any old bolt on the sheet metal.
     
  11. StarCaller

    StarCaller Senior Member

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    no old bolts on a '14..... :)
     
  12. Mendel Leisk

    Mendel Leisk Sand Pounder

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    I rechecked my measurement at the battery. I got 12.39, both at a bolt adjacent to the battery, and directly to the negative post of the battery.
     
  13. xliderider

    xliderider Senior Member

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    It was a figure of speech. SMH
     
  14. StarCaller

    StarCaller Senior Member

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    I know, that's why the smiley is there /
     
  15. kc410

    kc410 Active Member

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    Car and Deep Cycle Battery FAQ, Battery Manufacturers and Brand Names List, and
    go down to Temperature Compensated Battery State-of-Charge (SoC) Tables
    you can also find the same document at
    Battery-State-of-Charge

    Use the far-right AGM data.
    According to the chart 12.39V @ 4°C puts your battery at around 60% charged.

    Keep in mind the accuracy of the voltmeter doing the measuring & what sort of temp compensation the meter may have.

    The chart should be accurate - but it may not match your specific battery. Still, it should be pretty close.
     
  16. fuzzy1

    fuzzy1 Senior Member

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    For more thoroughness, measure the 12V outlet (provided the probes have insulated sides so they don't short together) and the OBDII connector port with the same DVM you used at the battery and hood locations.

    Measure all four location with the ignition in the same operating condition. Because the 12V outlet isn't powered when the ignition is Off, put the car in ACC mode before measuring.
     
  17. Easy Rider 2

    Easy Rider 2 Senior Member

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    Batteries are less efficient when cold.
    4C isn't THAT cold and there should be very little difference due to that.
     
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