Easy internal resistance (IR) measurement? - RC Groups
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Oct 12, 2005, 01:03 AM
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Easy internal resistance (IR) measurement?

I would like to know how internal resistance is measured on a cell. Is it necessary to place a load on the cell, and if so, how is it known what load to put on it? I have some cells that are not holding their charge for more than an hour or so, and want to measure the IR of them, to see if this is the reason for failing. I have hooked them up to my DVM, with varying results. On a 200ohm scale, they are reading 190's and on a 2000ohm scale, they read 1800's. This has me confused, and I've googled for how to measure with all results saying a load is placed on a cell then IR is measured by voltage drop, or something along that line. Thanks for any input provided.

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Oct 12, 2005, 03:42 AM
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The internal resistance is very low, usually just 10-20mΩ for a 2000mAh battery. The higher capacity the lower resistance it is. And the resistance is change every time if you diacharge with great power(more than 8C). So you must use a professional instrument. I saw some test report here contained the resistance, but I lost it. Maybe someone could tell you.
Oct 12, 2005, 04:35 AM
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There is no simple way. A DVM will tell you nothing, the voltage across the battery terminals confuses (and can damage) the resistance reading. You basically need to switch between 2 different accurately calibrated loads and measure the difference in cell voltages at the 2 different currents. It doesn't much matter what the loads are but it's sensible to make them something like the load you intend to use. There's no real point measuring at 10mA if you're going to be using 40A and resistance does change with load.

Oct 12, 2005, 08:01 AM
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Gene Bond's Avatar
The simplest way:

Put a load on a pack. Measure voltage (V1) and current (I). Disconnect the load, measure voltage (V2).

(V2-V1) / I = Resistance

The resistance changes with load and discharge point, so you may want to try different loads and different times in the discharge.
Oct 12, 2005, 08:24 AM
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I believe the open circuit voltage (V2) may not be the best choice as your second measured voltage/current pair. I would take two readings (i.e. (V1, I1),(V2, I2) close to your operating conditions (i.e. 20amps, 15amps) and use those pairs in the calculation:

Resistance = (V2-V1) / (I1-I2)

[this equation assumes the current I1 is higher than I2 to give a positive value of calculated resistance.]

I'm not implying that there is anything non-linear about the resistance, only that battery voltage under load drops over time and your two measurements may be taken at different locations on the steep part of this decline.


P.S. You can use many of the graphs in the Batteriers and Charger's forum to calculate IR. When multiple current levels are shown in one graph, you can 'measure' voltage differences between current curves at one point of charge (mahrs).
Otherwise, one of the many Whatmeters would be a good choice for obtaining your own measurements.
Last edited by Al Offt; Oct 12, 2005 at 08:35 AM.
Oct 12, 2005, 12:12 PM
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Oct 12, 2005, 03:48 PM
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The Duratrax ICE charger displays the battery's internal resistance. Check out the manual. I have one on order.
Oct 12, 2005, 07:10 PM
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Thanks for the replies guys, I plan on an emeter, when time comes...no funds right now. I will invest in some better packs before I get that. I'm using a cheap 2400 sub-c pack that loses voltage over time, and was just wondering if the internal resistance had gotten high, causing the voltage/capacity drop with time. I never measured the cells, and thought over 50mohm sounded high, but wanted to make sure I wasn't measuring it wrong, as I was. I know these cells are junk, so I will be better off using my hobby $ on packs, then on some type of meter (emeter or wattmeter) I like the emeter for ability to save things, and pc interface.


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