Random notes & other stuff... » resistance http://leucos.lstilde.org/wp ...about Linux, electronics, digital photography, whatever... Wed, 16 Dec 2009 00:29:27 +0000 http://wordpress.org/?v=2.9.1 en hourly 1 Calculate internal battery resistance http://leucos.lstilde.org/wp/2008/09/calculate-internal-battery-resistance/ http://leucos.lstilde.org/wp/2008/09/calculate-internal-battery-resistance/#comments Sat, 27 Sep 2008 15:11:31 +0000 leucos http://leucos.lstilde.org/wp/?p=14 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.

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