What Voltage & Current Readings When Checking Battery Drain?

Discussion in 'GM Electrical Tech' started by xJohnDoe001x, Mar 8, 2013.

  1. xJohnDoe001x

    xJohnDoe001x New Member

    What Voltage and current readings should I be reading with the engine off when I disconnect the positive cable and put the positive VOM probe on the battery positive terminal and the negative VOM probe connected to the Positive Cable?

    Of course before I check I'll pull all fuses and disconnecting everything else except the ECM on my
    '90 GMC Sierra, 5.7L TBI, 1/2Ton, 2WD
    Sincerely Jon
    Last edited: Mar 8, 2013
  2. ChevyFan

    ChevyFan The Sheriff Staff Member 5+ Years 1000 Posts

    What's your goal? To determine the output of your charging system or checking for the ability for the battery to hold a charge?
  3. xJohnDoe001x

    xJohnDoe001x New Member

    Thank you for letting me know that I did not make it clear that
    I want to check the battery drain, with the engine off
  4. dpeter

    dpeter Epic Member 5+ Years 500 Posts

    If there is ANYTHING using power no matter how small, you will get the same battery voltage between positive terminal and positive cable as you get from positive terminal and the negative terminal of the battery. As for current/amp draw.... no clue but to guess it should only be in the mili amp range. I may go check mine just for a base line because I am not having problems at present. I'll get back to you.

    Well after checking, it seems theory does not match reality. Battery voltage is 12.45 and volts between battery post and cable is 10.7(cable disconnected) . Current or amp draw goes from 1.2 at first contact then to 0.6 for about 10 seconds and finaly drops to 0.07 and stays there. Measurements were take with a Fluke 89DVM
    Last edited: Mar 9, 2013
  5. xJohnDoe001x

    xJohnDoe001x New Member

    Great answer! I was stuck on the pre ECM car method where you do check for voltage and less than three volts is good when everything is disconnected.

    You are right! theoretically all battery voltage should register?

    The Engine Computer Module is always drawing a current, I suspect less when the engine is off.
  6. dpeter

    dpeter Epic Member 5+ Years 500 Posts

    Forgot to consider A/C verses D/C. There are probably capacitors in play that would cause the open circuit voltage measurement taken between cable and battery terminal to be different than battery voltage.
  7. xJohnDoe001x

    xJohnDoe001x New Member

    You helped me get my head right earlier, I am now more interested in the amps but my Fluke 77's current checking capacity doesn't work, which is why I got it for $5 at a swap meet., so I have to decide to get it fixed or buy a new cheap one from Walmart for $20.

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