LiPo Battery Internal Resistance Testing

by ExAir | July 14, 2012 | (49) Posted in Tips

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.

COMMENTS

Gersio on July 17, 2012
Outstanding material, very easy to understand and apply at home.
Congrats.
5 stars
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bgotro47 on September 30, 2016
What watt meter are you using. I have one that looks identical to the one you are using but if I only connect the battery to the power source side nothing happens. I have to also plug the battery into the balance adapter.
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motomad111 on October 16, 2012
being new to elecrics at what point is the battery no good anymore. these are just a bunch of numbers that dont mean a whole lot until i crash my plane and probably blame it on my flying skills... is 50 milliohms still good? Great article just need a little more info/experience
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FliteTest on July 16, 2012
Awesome! We need more articles like this!!
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michas on July 17, 2012
This is great! You got 5 Stars from me!
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Gionir on August 15, 2012
Great article!
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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Gionir on August 15, 2012
Apologies, I tried to do the math again using your spreadsheet, which normalises the internal resistance taking into consideration the C figure. All good, the batteries are excellent!
Thanks again for a great article
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ExAir on August 15, 2012
Yeah, sorry the original video didn't go into any detail on that. But the updated spreadsheet calculates it as you found. Glad it's working out for you!
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Benkelm on August 6, 2012
That is really what we need - thanks for sharing with us your knowledge.
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takilara on July 18, 2012
Is there a similar way to test other cellcount batteries with extending this setup? Could one use for instance use lamps in series to test 6 cell? (would probably need 500 to 1000 watts to properly test 6 cell, so lots of bulbs i guess:()
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colorex on July 14, 2012
Great guide! I need a wattmeter now :)
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SuperCub92 on July 17, 2012
Definite 5star rating for this!!
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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Jake Wells on October 4, 2012
Thank you!
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EP51-C on May 31, 2014
Came accross this while surfing the web. This is a great video and article on LiPo IR. I went ahead and built a Test Bed with an On/Off switch to disable the load, this is so you do not have to keep disconnecting the lamps from the Balancer/Watt Meter after 10sec. I have 1 question, "What is the Rule of Thumb as to the high point in total IR milliohms for determining whether a Lipo pack should be placed in Non-Flying Status?
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ExAir on December 18, 2015
You're on the right track. That will give roughly 8 amps draw, which should give you a voltage drop large enough to work into your calculations.

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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Jackthelad on December 17, 2015
Now with a 12v/100w lamp. I tested three used 3cell Nanotech Lipos. I also have a Turnigy Accucell charger which measures resistance. Their figures in brackets are somewhat suspect (?)
2650mah- 10 milliohms (21)
1600mah-16 milliohms (38)
950mah- 12 milliohms (41)
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HilldaFlyer on September 17, 2015
Excellent information - as always. Keep up the great work.
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HilldaFlyer on September 17, 2015
So - do you do any 4S cell testing? If so, what bulbs or what do you do for load?
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ExAir on December 15, 2015
Yes a 240v bulb will have 20x the resistance of a 12v bulb at the same wattage. So the load is 1/20th. The 12v battery can only trickle a little amperage through that so you won't see enough of a voltage drop to make a good measurement. You could add more bulbs like that in parallel but I propose it's easier to just get 12v bulbs. The idea is to let the battery pump a fairly high amount of amperage through the load (bulb) against low resistance so the ratio of resistance in the battery (internal resistance) is relatively high compared to the bulb. This will result in a larger voltage drop and make it easier to measure.
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Jackthelad on December 14, 2015
I used a 240 volt 100 watt globe as the load. The voltage drop and current drawn are minute- .01 and .001amps. I am doing something wrong but just can't see what. It looks as if the load is too much?
Using- Nanotech 2650mah, 3cell Lipo, Turnigy 130watt meter.
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mango_fpv on November 16, 2016
this is excellent! Thanks for doing this...
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PerryStephen on November 9, 2017
Question: What load is best to use for testing 5,000 mAh 6S Packs? Do I use the same 50 watt bulbs, but wire them in series to get to 24 volts? Or do I need something else? Also, what about 4S and 2S packs? What load do I use for those?
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LiPo Battery Internal Resistance Testing