Battery connection

Discussion in 'General Electronics Chat' started by stroker, Jan 25, 2011.

  1. stroker

    Thread Starter New Member

    Jan 25, 2011
    3
    0
    Here is the question.
    Problem car battery has a load, draw when key is off. I want to verify the proper meter connection to find the load.

    I say disconnect the POS cable and attach the meter in series, battery > meter> POS cable.

    Amp scale selected on ohm meter.

    I am not where I can try this so this is why I am asking here.

    The other argument is that this can be also done having the meter attached, in series on the ground side , ground cable> meter> battery

    Unable to try this, and would not like to give out an incorrect answer.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you try to use your meter set to read amperes for troubleshooting auto battery drain problems, you will very likely blow the fuse in your meter. Just opening the car door will turn on a number of lights in the interior of the car, and they act as a momentary dead short until their filaments heat up.

    You are better off to use a high-wattage 1 Ohm resistor between the negative battery terminal and the negative cable, and measure the voltage drop across the resistor using your multimeter. Since I=E/R, your reading in Volts = Amperes.

    You could also use a 100 Ohm power resistor; Radio Shack carries some rated for 10W. No danger of exceeding that power rating with a car battery, as maximum current would be 130mA even if there was a dead short somewhere.

    If you have more than a few mA drain with the key out and the doors closed, you have a problem somewhere. I've had capacitors go bad in radio/tape deck amplifiers twice in different vehicles which caused the power drain.
     
  3. stroker

    Thread Starter New Member

    Jan 25, 2011
    3
    0
    Very good some things I had not considered. I was taking for granted that the draw was small and did not think about the meter fuse.. Nice one thanks.
     
  4. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    The last one I worked on had a problem with the trunk light. All I had to do was open the trunk and touch the bulb, it was hot.
    The fixture slid down the hinge, causing the light to stay on. If there is a light under the hood I would check that also.
     
  5. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Back in the day, when most things were mains powered, we'd stick a 100 watt lamp in series with the incoming 110. If the lamp lit brightly there was a large drain somewhere. You can do the same thing here too. Use a 55/65 watt headlamp instead.
    How long does your battery last with this drain? That'll give you a real rough idea too.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I had a problem with a trunk (boot for you UK and Downunder folks) light as well. I found that an adhesive chair leg floor protector stuck to the underside of the lid was just the right thickness to keep the switch depressed the correct amount to keep it turned off when the trunk was closed.
     
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