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Measurement of Internal Resistance of Battery

Discussion in 'General Electronics' started by Animesh Maurya, Aug 3, 2003.

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  1. Can I measure the internal resistance of a freshly charged battery by
    directly using a Multimeter. Or it will damage my Multimeter.

  2. Eric Immel

    Eric Immel Guest

    This will wipe out an analog meter in a big hurry. A digital multimeter
    is probably protected against such an event, but it will not give you an
    accurate measurement.

    If you have the continuous current draw specification for your battery,
    divide the measured voltage by that value to get a comfortable design value.

    The internal resistance of a battery can be found experimentally by
    measuring its unloaded voltage (Vu), then loading it with a resistor
    that is small enough to cause the battery voltage (Vl) to sag and and
    dividing the difference between them by the load current(Il). (Vu-Vl)/Il
    To get good resolution the load resistance should be about three times
    the estimated internal resistance. The manufacturer's web page or data
    sheet or someone with experience with the type of battery you have can
    probably tell you how much current it can deliver. From that you can
    approximate the internal resistance and pick your load resistor.
    Remember that unless your battery is very small, the load resistor will
    dissipate a great deal of power, so select one with an appropriate rating.

    This method is dangerous for large batteries (like car batteries)
    because the amount of current needed to cause the voltage to sag can be
    enormous (hundreds of amps). The battery may also be damaged if the load
    is connected for more than a few seconds.

    I have done this many times with bench power supplies. I have never
    tried it with a battery. You should definitely get a second opinion WRT
    safety and potential battery damage before you try it.

  3. You said that internal resistance can be measured with Digital
    Multimeter with some lack of accuracy, so what will be the percentage
    error in this measurement.

  4. Eric Immel

    Eric Immel Guest

    Wildly inaccurate. Don't do it. Use an indirect method or the
    manufacturer's spec. How big is the battery? What type of battery is it?
    What kind of load do you hope to drive?

  5. Iam using a 1.2V NiCd battery and the indirect method is working fine.

    Animesh Maurya

  6. Resistance calculated by indirect method yields 1.17 Ohms, and that
    measured by DM gives 798 Ohms ! Really Wild. But now a new problem
    arised in front of me that when I connected the +ve terminal of DM
    with -ve terminal of Battery it displays 798 Ohms, but in other case
    when I connected +ve terminal of DM with +ve terminal of Battery it
    always tends to exceed the range until I reached the maximum limit of
    2M Ohms. Why is this happening so ?

    Animesh Maurya
  7. Rich Grise

    Rich Grise Guest

    (Animesh Maurya) wrote in message
    You are very lucky to have not already destroyed your meter.

    Rule one of using an ohmmeter is NEVER use it on any powered circuit,
    and a battery is about as powered as a circuit can get! Even if it
    didn't destroy the meter, the reading would be meaningless because
    of the 1.2V. With the "right" polarity, the battery is essentially
    in series-aiding with the internal battery, so the ohmmeter thinks
    that the resistance is whatever value gives a reading of 1.2V at the
    probes, evidently 798 ohms (meaning the meter tries to apply a test
    current of whatever 1.2/780 is). When you connect it in reverse,
    the batteries oppose each other and the meter doesn't know WHAT
    to do.

    But yes, you can deduce the internal resistance with some load and
    a few meters.

    But the question that's really at the top of my mind is, Why? In
    what context, besides the classroom [we get uppity about homework
    questions], does anyone ever need to know the internal resistance
    of a battery? The battery maker will tell you its amp-hr capacity
    and maximum recommended discharge current; what else do you need?

    Good Luck!
  8. miso

    miso Guest

  9. Eric Immel

    Eric Immel Guest

    [Rich's wise warning and succinct explaination excised]
    A few possibilties as to why (pending Animesh's actual reason):

    To calculate power dissipated within the battery. This would be useful
    in derating discharge rate based on an enclosure's thermal behavior.
    What is the maximum operating temperature for NiCad?

    If the battery maker is unknown, the internal resistance and a power
    calculation would provide guidance for a maximum discharge current. I'd
    bet that someone experienced with batteries can probably look at the
    physical size and make an adequate guess.

    And, my favorite reason: curiosity. The quest for information for its
    own sake.

  10. Eric Immel

    Eric Immel Guest

    But was wrong to write that, so to right that: Current flow is
    *INVERSELY* proportional to resistance. Resistance big, current small.
    Resistance small, current big.

  11. Rich Grise

    Rich Grise Guest

    Remind me not to dislocate my shoulder. ;oD
    Thanks! That makes a lot of sense!

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