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  • Calculate internal battery resistance

    Posté le 27th septembre 2008 leucos Pas de commentaires

    Battery internal resistance forms a divider with external load.
    So to calculate internal resistance, take those steps :

    • measure the battery voltage without load (Vnoload)
    • measure again with a resistor of RloadΩ attached (the lower ohm value, the better, but watch outfor power ratings !), you have a difference with previous measurement (Vdrop)

    The battery internal resistance is then :

    Rbattery = Vdrop*Rload/(Vdrop+Vnoload)

    Real world example on AA :

    Vnoload = 1.421v
    Rload = 460Ω
    Vdrop = 0.002v

    Thus :

    Rbattery = (0.002 x 460)/(0.002 + 1.421) = 0.65Ω

    Since my metter has a milliVolts resolution, we just have a range between 0.001 and 0.003 for Vdrop.
    So Rbattery can be anything between 0.33Ω and 1Ω.

    Short-circuit current for this AA battery would be somewhere between 1.421/0.33 = 4.33A and 1.421/1 = 1.42A
    Mesasurement made, it is around 4.2A. Watch out for your metter current rating if you do this. Some batteries have a much lower internal resistance.

    electronics ,

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