Measuring Internal Resistance of an AAA
Measuring Internal Resistance of an AAA
(OP)
I'm a newbie at circuits and I'm still learning. I was wondering how I can effectively test the internal resistance of several different AAA batteries? Can someone explain to me exactly what internal resistance of a battery is? I've looked on the forum for other posts but didn't really find anything useful. Thanks!





RE: Measuring Internal Resistance of an AAA
Real world voltage sources don't behave like that. If you draw an increasing amount of current from a voltage source, the voltage will fall. The relationship between the current drawn from the source and the volt-drop caused by it can be represented by an ideal voltage source in series with a resistance. It isn't possible to measure this resistance because it doesn't actually exist, it is just a convenient way of modelling the real-world properties of a cell, battery, power supply, amplifier, etc.
In your example, your AAA cell will be (roughly) a 1.5V DC ideal voltage source in series with a resistance of a few hundred milliohms or maybe an ohm, depending on the battery chemistry.
The battery manufacturers should have some data on their websites. Of the manufacturers of small cells, I know Duracell have some reasonable data on their site. I've never really had cause to look at others because I usually deal with much bigger cells.
The following links might help:
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RE: Measuring Internal Resistance of an AAA
A very simple measurement can be done by loading the cell with a resistor R1 and measure the viltage U1. Then use another resistor R2 and measure U2. Chose the resistors so that the resulting current is about half the normal current in operation and about twice that current. That will give you a representative value.
Then solve the two equations U1 = E - Ri*U1/R1 and U2 = E - Ri*U2/R2, where E is cell EMF and Ri is cell internal resistance, for Ri.
If you are going to use the cells in a more dynamic situation, it may be better to measure the impedance, Zi. You do that by injecting an AC current (use a blocking capacitor for DC) and measure the resulting AC voltage and current. Calculate Zi from U and I. Select frequency to suit the application. Remember that a high HF impedance usually can be improved by putting a capacitor parallel to the cell.
Gunnar Englund
www.gke.org
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RE: Measuring Internal Resistance of an AAA