Internal resistance testing is a relatively simple way of measuring the health of your LiPo batteries. Internal resistance is the electrical resistance that occurs inside the battery cells themselves as the battery creates electricity. The higher the internal resistance the less current can flow from the battery to your ESC and motor.

Older, abused, puffed, crashed, and lower quality batteries have a higher internal resistance and therefore cannot maintain voltage to your ESC and motor when you apply power. Because the battery itself creates current, you cannot simply hook up an ohmmeter and measure resistance. Instead you must measure the voltage, then apply a known electrical load (halogen light bulbs in this demonstration), re-measure the voltage and also measure the amperage. The internal resistance is the voltage drop divided by the amperage.

Measuring this occasionally will give you a good idea of the "health" of your battery packs. If you have a battery that doesn't give the "juice" or "oomph" like it used to, this is a good way to actually quantify the loss in power and determine whether the battery should be reassigned to bench work or the garbage.

The video will demonstrate how to actually perform the test. Once set up it takes about 30 seconds per battery and you can do all the testing and another 30 seconds to do the calculations. I like to use a 100 watt load, but this is up to your discretion.

Below is a link to a shared spreadsheet you may download to your computer and fill with your own values (some sample values are entered by default). This spreadsheet will do all the calculations for you, including compensating for different numbers of cells, different battery pack capacities, different C-discharge test variables. Enter your specs and measurements on the left from "DATE" to "AMPS" and it will calculate the rest. This will give you pretty close to an apples-to-apples comparison of internal resistance for all your batteries.

**https://docs.google.com/spreadsheet/ccc?key=0AtSZzrsraF1-dDZLVTJNejJxaWNRdnlZTHFXc0hyQ1E **

This should not be considered a highly precise measurement for purposes of publishing or comparing specifications. But it is a great way to keep tabs on the status of your batteries.

Please contact me if I can be any help.

Congrats.

5 stars

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I have a question though: I built the same "tool" with two 50W 12V bulbs and I am using a simple HK wattmeter to display the data.

The results are not so comforting, meaning that if I connect 2200mAh 3S batteries, the internal resistance for the whole pack is around 20 milliohms, but if I connect some 1300 mAh 3S batteries, the milliohms raise to 120 and of I dare to connect some 850 mAh 3S batteries, the milliohms are almost 200.

Now, the batteries are all new, with no more than 2 cycles each, is this normal?

The initial voltage is always around 12.6V but the drop varies from .5V (2200mAh batts) to almost 2V (850mAh batts). The Amp draw is consistent at around 8,2A with every battery.

Being the batteries new I suspect there is something wrong with the measurement. Can you please advise.

Many thanks

Gio

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Thanks again for a great article

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This is brilliant and im sure Flitetest will agree when i say..

Articles like this and the knowledge they spread is the very reason flitetest was created.

GReat work !! :)

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Using- Nanotech 2650mah, 3cell Lipo, Turnigy 130watt meter.

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2650mah- 10 milliohms (21)

1600mah-16 milliohms (38)

950mah- 12 milliohms (41)

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There are many limitations to this method and it's best for tracking the health of any given battery and not as good at comparing batteries of different size, manufacture, age, etc. or with the results of a different tester. IR measurement will vary when done with different loads, states of charge, temperature. The Turnigy charger probably uses a lower test load. But for tracking decline in health (increasing IR) it's still probably a good tool, maybe even better than the lightbulb load test.

Your results don't seem too far out. Apples to apples, smaller batteries will have a higher resistance so that part makes sense. The figures in parentheses are actually pretty linear related to the battery size. Your 1600mAh pack may be in a little worse shape so that when you subject it to the higher load of the bulb the voltage really drops off more, so the IR measurement jumps up above the 2650 pack. That's the limitation of using different methods of measure.

I'd recommend testing the IR with one chosen method on all new batteries and then again with the same method every few months. Then you'll see when batteries start to take a dive.

Another way to look at this is to periodically subject each battery to a pretty large load, like 10C, and measure just the voltage drop, not even worrying about the IR. Carefully using a motor and prop is the most accessible way, but you can hook up a bunch of lightbulbs in parallel.

For your 2650 a 10C load is 2.65x10 or 26.5 amps. If you run it up to that and your voltage drops more than 2 volts then it's a bad sign. Ideally it should not drop more than 1v. Remembering that watts = volts x amps, you may be able to pull a lot of amps out of a battery but if the voltage drops 20%, so does the wattage output. And that voltage loss comes from one place, and that's internal resistance, and it's manifested by heat production in the battery, which we know is the enemy of battery longevity.

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