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Series resistance of battery pack?

Discussion in 'Electronic Basics' started by Joe, Feb 25, 2004.

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  1. Joe

    Joe Guest

    Is there a way to measure the series resistance of a battery, or battery
    pack? I am using LTSPICE to try and duplicate the results of a breadboard I
    have constructed, but I have no idea what the series resistance of a battery
    is. Maybe there's a ball park figure I can use? OR an equation?

    TIA,
    Joe
     
  2. CFoley1064

    CFoley1064 Guest

    Subject: Series resistance of battery pack?
    Choose a resistor which will load the battery pack to something near
    recommended maximum current. After charging the battery pack, measure no-load
    voltage with a DMM (Vnl). Place the resistor across the battery pack, and
    measure voltage across the resistor (Vfl). Calculate current across your load
    resistor

    (I = V/R).

    Then calculate internal resistance by

    R = (Vnl - Vfl) / I.

    Good luck
    Chris
     

  3. Measure the battery's voltage without any load. Call this value V1. Connect
    an external load resistor across the battery and measure the battery's new
    voltage and the current flowing through the load. Call these values V2 and
    I. The battery's internal resistance is given by:

    Ri = (V1 - V2) / I

    Make sure you use a resistor that generates enough current to cause a
    significant drop in the battery's voltage.

    This assumes a battery model consisting of an ideal voltage source and a
    resistor. For a more accurate measurement you'll need to plot a graph of
    battery voltage vs. load current and evaluate the slope of the graph at the
    operating point you need.

    cheers,
    Costas
    _________________________________________________
    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying
     
  4. Joe

    Joe Guest

    Chris,

    Thank you for the reply. I am not sure what is meant by recommended maximum
    current. I have a battery chart that shows the mAH for different types of
    batteries at a certain current drain value, eg, for AA alkaline manganese
    dioxide batteries, the chart shows 2000mAH at a typical drain of 50mA.
    Is that what I use (50mA)?

    Joe
     
  5. Joe

    Joe Guest

    Thank you Costas, a significant drop in battery voltage, like maybe it will
    drop the voltage by one half? Would that be enough?

    Thanks,
    Joe
     
  6. That is a reasonable load, especially if it is close to what your
    future load will be. You might measure the voltage of the pack before
    during and after applying that load, and average the before and after
    as the unloaded voltage. Subtract the loaded voltage from that
    average and divide that difference by the load current (in amperes,
    ..05 in this case) to calculate the approximate battery resistance.
     

  7. One half sounds a bit too much. How about setting the current to the nominal
    current of your circuit? That would give you a more accurate Rin masurement
    as the relationship may not be linear depending on the type of battery, etc.

    Costas
     
  8. David Wood

    David Wood Guest

    It might be interesting to compare your results with reference data
    from the cell manufacturer. For example, the datasheet for the
    ubiquitous Duracell Alkaline-Manganese AA shows 120 m-Ohm at 1kHz:

    http://www.duracell.com/oem/Pdf/MN1500.pdf

    Why would they reference a frequency rather than DC in their chart?
     
  9. Joe

    Joe Guest

    Thanks John, I measured a rechargeable 9V (actually 7.2Volt) NiMH battery
    to start with. I came up with about 6ohms. Is that about what you would
    expect? I measured it with 70mA (approx 100 ohm) load. It is not fully
    charged for probly a month or so, just been sitting on my bench, but it has
    not been used much either.

    I am going to be using a 12volt lawnmower battery to power one of my
    projects and that is on the charger right now, so I can measure it when it
    is at full charge and keep track of it as it discharges.

    Joe
     
  10. Joe

    Joe Guest

    Hi Costas,

    I did that and came up with about 6ohms for a rechargeable NiMH (7.2V). I am
    using a different battery for my application so I will measure that one
    when it is fully charged and possibly plot the values for a learning
    experience. Thanks for the tip

    Joe
     
  11. Joe

    Joe Guest

    Hi David,

    I was wondering that myself. I went to the site, thanks for the link. The
    graphs they show are very instructive, but I don't know what's up with the
    impedance at 1Khz, maybe someone else on this forum can explain it...

    Joe
     
  12. That sounds quite reasonable to me. That would correspond to a >1 amp
    short circuit current.
    That one should measure well under and ohm.
     
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