# testing for battery internal resistance

Discussion in 'General Electronics Chat' started by strantor, Apr 28, 2012.

1. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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This is excerpted from another thread, and as I read it, I think I stumbled onto a way to measure the internal resistance of a battery...

With a perfect voltage supply, he should have gotten 6A, but only got 3.5A. So total circuit resistance is 1.7Ω, the other .7Ω attributed to the batteries. 0.7Ω/4 = 175mΩ per cell.

Is that correct? Is that how one tests for battery internal resistance?

Is there a better to do it, when the battery may be capable of several hundred amps and/or may explode if you use too low resistance? Imagine testing 48V 5AH 130C hobby lipo pack - scary, and where do you get a 30KW resistor to test it? There has to be a better way.

2. ### #12 Expert

Nov 30, 2010
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This document says a AA NiMh battery has "less than 50 milliohms" of internal resistance.
I haven't measured it, and I expect some off-brands won't do as well, but they sure are better than alkaline batteries in a photoflash camera because they can dump serious current for short periods.

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3. ### crutschow Expert

Mar 14, 2008
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A common way to measure the battery internal impedance is to measure the output voltage both open circuit and with a known load. From that you can determine the internal resistance with a little algebra for two resistors in series (one of which has the known value). You use a resistor value low enough to give reasonable measurement error (probably at least 10 times the anticipated battery internal resistance), but high enough to avoid seriously overloading the battery.

Aug 27, 2009
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5. ### Bernard Expert

Aug 7, 2008
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I screwed up strantor's figures a bit by not including battery box which has 4 spring contacts worth .25Ω's each; not a fixed figure because as battery cells are added some spring turns are shorted giving a smaller figure.

Last edited: Apr 30, 2012