Measuring Internal Resistance of an AAA

[creature0077]

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!

 

[ScottyUK]

Are you familiar with the concept of an ideal voltage source? An ideal voltage source will maintain its output voltage regardless of the amount of current drawn from the source.

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:

http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Source/Source1.html

http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Source/Source2.html

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Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

Battery testers do resistance measurement, sort of. They do it by loading the cell/battery and measuring the voltage. The internal resistance varies with temperature, discharge current and cell age. So you do never get THE resistance, rather a range of typical resistance values.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[creature0077]

ScottyUK, your info helped and the sites helped understand what I should be tying out. skogsqurra, I'll give your suggestion about measuring internal resistance with different resistors. You totally lost me when you started to talk about the impedance. I have no idea what any of that stuff is.