# Circuit for determining the internal resistance of a dry cell

1. Oct 18, 2009

### jackmannn

What sort of circuit would I have to set up to figure out the internal resistance of a dry cell? Also, what would i need to be measuring and doing etc, to find this internal resistance?

Thanks, Jack.

2. Oct 18, 2009

### vk6kro

A battery is considered to be an EMF in series with a resistor.

You can measure the EMF by measuring the open circuit voltage of the battery. There is no voltage drop across a resistor if there is no current flowing through it.

To put a load on the battery and measure the current and voltage, you could use something like the attached diagram.

A voltmeter monitors the output voltage. A variable resistor R is placed in series with the ammeter.
You can monitor the voltage while the heavy load (R) is placed across the battery. A large rheostat would be suitable for R.

Suppose R makes the output voltage drop to half. Then you could say that R is equal to the internal resistance.
You can calculate the value of R by measuring the voltage across it and the current through it.
The voltage does not have to drop to half. If you were testing a rechargeable cell, a NiMH or NiCd cell, you would not do this because these can deliver very large currents. So, maybe a 10% drop would be OK for these.

You just calculate the value of the internal resistance knowing the current flowing and the voltage drop between no load and the measured load.

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