# 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.

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3. ### MrChips Moderator

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

You need to disconnect the load to allow the battery to recover.

4. ### nsaspook AAC Fanatic!

Aug 27, 2009
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This is one method. http://data.energizer.com/PDFs/BatteryIR.pdf

5. ### Jack_K Senior Member

May 13, 2009
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I use LiPo batteries in my RC airplanes. The batteries are 3, 4, 5, and 6 cells with mAh ratings of 1500 to 6000 (1.5 to 6 Ah). The batteries have labels indicating the discharge rate in C. They vary from 20C to 60C.

I want to measure the internal resistance of the batteries. I'll be using a Volt/Amp/Watt meter in series between the battery pack and the load resistor. I'll first read the no load voltage, then apply the load, and read the voltage and current. To calculate IR, I subtract loaded voltage from unloaded voltage and divide that by the amps. I can estimate the individual IRs by dividing the total IR by the number of cells. OK so far.

I used a 1 ohm 250 watt resistor on some 3 cell packs (12.6 volts fully charged). I waited about 10 seconds for the voltage to settle before taking the readings. The IR calculations are way too high. I assume I should use a much higher resistance?

My question is how much of a load to put on the battery and how long to leave it connected before taking the reading?
Jack

Last edited: Aug 28, 2018
6. ### oz93666 Senior Member

Sep 7, 2010
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I would say the resistance is too big ... under this test you only draw about 10A (the ammeter, unless a clamp type, will have a significant resistance) ... this all comes down to what ammeter you have , clamp types can measure very high and are not in the circuit . the higher the current the more accurate should be your measurements . I would try to measure at the current levels you are using when the battery is in use.

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7. ### ArakelTheDragon Well-Known Member

Nov 18, 2016
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If you use a load resistance it is easy. Otherwise you need to check the battery processes and how they change.

8. ### nsaspook AAC Fanatic!

Aug 27, 2009
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https://www.eetimes.com/author.asp?section_id=36&doc_id=1320427

https://batteryuniversity.com/index.php/learn/article/how_to_measure_internal_resistance

9. ### ArakelTheDragon Well-Known Member

Nov 18, 2016
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The internal resistance is high when the battery is charged and low when the battery is discharged. The idea is that when the battery is discharged (less than 11.2V for a car battery) the resistance drops, it can even drop to less than 1Ohm. Then the battery drains a lot of current when charging. From there we have Ohm's law and like this we can see how the resistance changes and is it linear or not.

10. ### marcf Member

Dec 29, 2014
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I am sure that you mean this other way around. The lower the internal resistance of the battery, the more able it is to deliver current to a load. A high internal resistance is indication of a bad battery, or one needing a recharge.

Also this is the reason that you need to limit the current when charging a battery.

A'perfect' battery would have an internal resistance of 0 Ohms and would be able to deliver an infinite amount of power (Voltage * infinite Current) to a 0 ohm load.

Also, with the exception of SLA batteries, it seems the better batteries are made, their internal resistance stays very constant even as they are reaching the end of their capacity, so I am wondering how useful other than confirming that the battery meets the manufactures specs, measuring it is.

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