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Old 12-18-2001, 12:52 PM   #1
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Default Measuring battery internal resistance

I've been wondering about a method to easy measure packs and maybe single cells internal resistance.

The purpose is 1) to check condition from time to time and thereby control that the battery care is done properly. 2) to match cells.

The method I've been thinking about, is to get 2 different resistors. Fx. 1,2 ohm and 2,4 ohm. Then take a fully charged pack and connect it to one of the resistors. Measure amp and volt. Do the same with the other resistor. Now find the delta voltage and delta amps simply by subtraction. Acoording to Ohms law, then divide the delta volt with the delta amps. Wouldn't that result in the internal resistance?

Of course, those measurements have to be done very fast, so the delta's ain't too much affected by discharging. And the resistors should be strong enough to carry the watt's.

Any comments, please?
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Old 12-20-2001, 12:05 AM   #2
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Default Re: Measuring battery internal resistance

Quote:
Originally posted by Cole Trickle
The method I've been thinking about, is to get 2 different resistors. Fx. 1,2 ohm and 2,4 ohm. Then take a fully charged pack and connect it to one of the resistors. Measure amp and volt. Do the same with the other resistor. Now find the delta voltage and delta amps simply by subtraction. Acoording to Ohms law, then divide the delta volt with the delta amps. Wouldn't that result in the internal resistance?
Uhm... if you do at only when is charged, the reading of the I/R should be quite different than the real I/R.

I think that for measuring consistently the I/R of the cell this measuring should be done at the entire time of discharging the element, because for a given load, at some voltage, you get an intensity, but the intensity decreases when the voltage decreases.

But if you want to do some testing, you should get a very big resistor (we should be talking about a 1 Ohm 50w and 1% tolerance) or better.
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Old 12-20-2001, 02:13 AM   #3
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The best thing thing would be to discharge it with around 30A constant current plus a small ripple of around 1A. Then just measure the averaged voltage and AC voltage as a function of time. Using computers serial or parallel port would help very much.
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