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Old 08-02-2010, 01:11 PM   #1 (permalink)
redbird300
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Lightbulb FYI: the internal resistance of lipos, and the inevitable consequences...

A few words this time about the internal resistance of our beloved lipos.

As many know, internal resistance is a dynamic thing, and the resistance will become more important with an increasing number of discharge/charge cycles. Meaning the cells get a harder time delivering their power, up to the point that the pack becomes useles.

Today for example, I had two rather old and very much abused KongPower 3S/2200mAh/25C packs that became unusable. I did notice the decrease in power since a long time, but today they fainted away after 2 minutes, and I even had to land the heli, because I felt that I could not keep it in the air much longer. I only used these for simple practice anymore in fact.

All this made me decide to do some measurements, and to compare with new and used lipos, to get an idea what internal resistance value is becoming dramatic. It can also allow to predict to some extent when batteries will be starting to get weaker, by doing regular measurements (like every 10 flights) and looking at the increase rate of the resistance.

But how does one measure this internal resistance ?

-) Well, there are some devices as found in specialised hobbyshops, but these can be rather expensive.
-) And you can also do it yourself, making measurements without and with a load using an accurate multimeter and making some calculations. Nerdy and complicated, lol.
-) Some lipo chargers have a program though to measure this. That's exactly what I did using a iCharger106B+, which has proven to be fairly accurate to me. This charger has the added advantage of showing the individual internal resistance of each cell, not only of the whole pack as such.

The theory behind it (in short) is this, and I quote:
"To determine the internal resistance, first it is necessary to measure the open circuit voltage of the cell. Then a load should be connected across the cell causing a current to flow. This will reduce the cell voltage due to the IR voltage drop across the cell which corresponds to the cell's internal resistance. The cell voltage should then be measured again when the current is flowing. The resistance is calculated by ohms law from the voltage difference between the two measurements and the current which is flowing through the cell."
Read more about it here: http://www.mpoweruk.com/testing.htm

Remember that the next values are based on a limited number of samples, about a dozen or so, but they should give a good idea anyway, and I came to these conclusions:

a) Packs that are as good as new: between 0 and 5 milliOhm/cell
b) Packs that have been used dozens of times, but still feeling fairly good: between 5 and 10 milliOhm/cell
c) Packs that have been used even more, and start to feel weak: between 10 and 20 milliOhm/cell
d) Packs that almost won't hold your heli in the air anymore: over 20-25 milliOhm/cell

Remember that the cells are wired in series, and the resistance values of the cells need to be added up to know the total resistance of the pack.
For example: a 3S pack with IR cell 1 = 4, IR cell 2 = 6 and IR cell 3 = 5 will have a total IR value = 4 + 6 + 5 = 15 milliOhm

Some measurement examples:
A very old, completely used up 3S pack, barely holding a heli in the air:



A 3S pack with dozens of flights, but still in very good condition, feeling about as good as new:



An almost brand new 6S pack: (yes, the last 3 numbers are "real zeros" in this case, not undefined values as the previous pics)



I hope someone has some use for this, enjoy.
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Last edited by redbird300; 08-02-2010 at 04:17 PM..
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