# Some question about testing the internal resistance of a battery

Discussion in 'The Projects Forum' started by HTe, May 14, 2013.

1. ### HTe Thread Starter New Member

May 14, 2013
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I am going to use the voltmeter-ammeter method for finding the internal resistance of a battery.Why is it necessary to turn off the switch after each reading is taken?And what assumptions might i have made in this experiment.
PS:I am using battery bought from the supermarket, and typical school- laboratory moving-coil voltmeter and ammeter.

2. ### GopherT AAC Fanatic!

Nov 23, 2012
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Normally, when a test is done, you measure the open circuit voltage of the battery (switch to circuit turned off. This becomes your battery voltage.

Then add a load of known resistance. Measure the battery terminal voltages again (it will also be voltage drop across the resistor). From the voltage across the resistor and the resistor value, you can calculate the current flow using ohms law.

Now, subtract the closed circuit voltage (with load) from the open circuit voltages of the battery. Divide your voltage difference by the current flow you calculated above to get internal resistance.

Very easy and no ammeter needed.

3. ### MrChips Moderator

Oct 2, 2009
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3,461
What is the voltmeter-ammeter method for fining the internal resistance of a battery?