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I need help Calculating power consumption.

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  • I need help Calculating power consumption.

    I would like to use my multimeter to check the total draw of my system while in different states. By states I mean ON, OFF, different power savings modes, Stand by, etc. I would then like to take the consumption rates and calculate pull on the battery per hour or how long the system will render the battery unusable to start the car.

    How do I measure the current draw?
    What are the needed calculations to measure the life of the battery.
    mobile nerd

  • #2
    The multimeter must be able to read current (not all do) and be rated high enough for your computer. You'll want to use thje amps setting on the multimeter. Amps are equal to watts/volts, so if you've got a 100 watt power supply and the battery is at 12 volts, your multmeter must be able to handle up to 100/12=~8.3 amps. Chances are you aren't going to hit the max current draw, but if your multimeter is way off, then don't try it or you'll fry your meter. (it'll just blow a fuse if it's feeling generous)

    To measure current draw, put the multimeter probes in the correct holes on the multimeter for measuring amps, then put the multimeter in series with your curcuit. Something like this:

    +car battery --- multimeter --- +power supply
    -car battery ------------------ -power supply

    Batteries are rated in amp-hours for a specific current draw. A 20 Ah battery may deliver 1 amp for 20 hours. You'll actually get a bit more or less, depending on the battery, and the total available amp-hours can change, depending on the current draw. A 20 Ah battery that runs for 20 hours at 1 amp, may run for 45 hours at .5 amps or 6 hours at 2 amps.

    I don't know how much charge a battery needs to have to start the car.

    I'm a tinkerer and don't have any electronics background, so my apologies if I am mistaken on any technical points, but the general concepts should be correct.


    • #3
      good star

      Thanks for the info. I am going to head out to the shop and check the multimeter.

      I want to make sure my total pull doesn't get out of control when I start adding more pieces to the puzzle. I also need to document some base line draw numbers for the system I have so I can make comparisons to other configurations that I may be testing in the future.
      mobile nerd


      • #4
        Question for UTAHRC

        SO if I understand what you posted correctly,

        Amps = Watt/Volts therefore

        Watt = Amps x Volts ?

        Becasue I am trying to figure out how much of a PSU my system really needs.

        For example my laptop dvd drive says on its label "5V DC 5A"
        So does that mean it uses 25 Watts? 25 watts seems high for a laptop device.

        (Maybe it was "5V DC 5mA" hmmm, better check that out)
        New site at

        2002 Toyota Tacoma Doublecab


        • #5
          Re: Question for UTAHRC

          You can use 0.01 ohm 1% resistors and measure the voltage across them. You hook them in series with the load. It will give a better measurement than using the multimeter, but will require that you build a jig that puts the resistors inline with the load.
          MPEGBOX - Plexiglass Computer


          • #6

            Watts = Amps * Volts
            Amps = Watts / Volts
            Volts = Watts / Amps

            Your DVD drive is rated for 25 Watts, though is likely the maximum draw, which may only occur momentarily, such as when a disc is spinning up. When there's no disk in the drive, it may only be drawing a few amps.


            • #7
              Well I looked at the dvd rom again, and my memory was a bit off.

              The label said "5V 1.5A" = 7.5 Watts

              Much better.

              Thanks For the help
              New site at


              2002 Toyota Tacoma Doublecab