FYI: the internal resistance of lipos, and the inevitable consequences...

[redbird300]

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.

[B2TheEYo]

Awesome write up Raf.

[psilo]

My Cellpro 10S also measures IR. This is where quality comes in too. The turnigy packs, brand new from HK came with individual IR of 3-5 on each cell, however 1 pack came with a weak cell and the ir on that cell is 12 from new!. The KongPower I have was 0-2 per cell from new.

I love this feature on the Cellpro as I always record the IR when I charge and very quickly you can see when a cell or pack is failing. Never really understood how it measured it though, nicely explained Raf

[Ber60]

As always a very good guide, good work Raf

[B2TheEYo]

Originally Posted by psilo

My Cellpro 10S also measures IR. This is where quality comes in too. The turnigy packs, brand new from HK came with individual IR of 3-5 on each cell, however 1 pack came with a weak cell and the ir on that cell is 12 from new!. The KongPower I have was 0-2 per cell from new.

I love this feature on the Cellpro as I always record the IR when I charge and very quickly you can see when a cell or pack is failing. Never really understood how it measured it though, nicely explained Raf

It's a feature in everyone's best interest of having - sucks it only on the more expensive of chargers. I love my iMax B6, works like a charm but doesn't have this feature that I'm aware of.

[chansen1953]

hi guys.. excellent report there redbird . I also have an imax B6.. but I do have a good quality multi-meter-- so I am thinking i can meaure some shit !!

I just tossed a 3S 300mah 35C pack which was working fine-- it just puffed up very badly a few months back.. I had been charging it carefully, and using it only for testing and bench work.. and it worked fine.. even carried a full charge etc.. just was sooo tight I thought it might burst..
It also never got hot at any time.. maybe got discharged too low one day maybe.

so I decided to give it one more chance-- rather than risk a sudden fire in my car/house !!!

I put it in the freezer for a day.. just ot see if it deflated at all.. i also discharged it to storage level before I did this .. guess what -- it looked exactly the same after the freezer.
so , I clipped off the fittings and chucked it into the garbage bin.

it is about -2 degrees average here in Orange overnight and max of 10 degress daily -- so I don't think much danger til lthe big munchers at the dump get hold of it !!

[CyprusDave]

There is a way to safely dispose of LiPo batteries, and as they don't contain any heavy metals, they are OK to dispose of with normal waste. First, discharge it as low as possible. Clipping on a car brake light bulb, or headlight bulb for higher capacity packs, is good for this, as it doesn't take too long, and you can tell the pack is discharged when the light goes out Then puncture the outer cover of each cell, using a plastic blade. Then drop the battery in a bucket of salt water for a couple of days. After that, it can safely be thrown away.

Haven't seen this posted for a while, so probably worth repeating.

BTY, best to do all this outdoors, away from anything combustible, just to be safe

[Bugster]

Hi guys - just catching up with events after a busy 6 weeks mostly away from helis doing what seemed important at the time.

I reckon this thread and a couple of others should be stickied. But as you may know I don't just sticky posts at random because it may seem like favouritism so I need a few votes of approval from you regulars if you wouldn't mind please

[dayshaddow]

hay there stranger long time no see
this one gets my vote..time to add some glue sir

[Tomstoy2]

Hey Buggs, I agree that we can't let great information like this go to waste dying in the back threads. Best to keep it up front where it can be easily found.

After all, this is the Belt forum, an intro heli, and this is knowledge every beginner should have easy availability to.

[sutty]

Gets my vote too.

[pril250]

Still doing well in my books

[chansen1953]

+1 here.. need ot keep this info on hand.. I had forgotten about this post.. verey handy !

[Skiddz]

Great write up, but we thought it'd get "lost" in the Belt CP forum so it's been moved to the LiIon, LiPo, NiCd & NiMh General forum and stickied.

[redbird300]

Originally Posted by Skiddz

Great write up, but we thought it'd get "lost" in the Belt CP forum so it's been moved to the LiIon, LiPo, NiCd & NiMh General forum and stickied.

Thank you very much Kevin, and it that sounds very logical.
I'll pay more attention in the future to post in the appropriate forum from the beginning. :o

[monocleman]

Originally Posted by redbird300

"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."

How does this relate to the discharge rating in "C" of a battery? I like the explanation of why the voltage drops across the battery when connected to a load, but I wonder what happens when large amp draws are used. I suppose the internal resistance R = Vdrop / I should stay the same up to the max discharge rate? And then what?

[redbird300]

Originally Posted by monocleman

I wonder what happens when large amp draws are used. I suppose the internal resistance R = Vdrop / I should stay the same up to the max discharge rate? And then what?

If you would exceed the manufacturer’s maximum discharge rating with a very low rated resistor, it would be almost like shortening the cells. The current wouldn’t be able to go much higher anymore. But the resistance is a fixed low value in this example, and to respect R=U/I, the voltage would need to drop heavily. Which it will do, because you are destroying the cells in fact. Making for a (permanent) high IR value. And a damaged pack.

[GB ian]

Does anyone know where I might be able to find a tool to measure IR...other than buying a new charger?

[vanillagorilla]

Does the battery have to be at a certain voltage/capacity to check the IR?

I have brand new (two flights) zippy packs at 8, 8, 8, 10, 7, 6 and a two-flight hyperion pack at 6,6,6,7,6,6

Suprizingly the 3S 2200 pack I thought was going bad is at 5,8,2 while my others I assumed as good were 12,12,8 and 12,11,9....but the 5,8,2 pack is the only one with a full charge.....?

[GB ian]

Would it be possible for someone to post a step by step method to measure IR on each cell?

Assuming you don't aready own a charger that measures it and that you do own a multimeter.

If i read correctly the procedure is ;;

measure voltage across cell.

Apply small load and remeasure voltage under load, as well as measuring current.

What I need is the formula for calculate IR from Open Circuit Voltage (OCV) and the Load Voltage (LV) and Load Current (LC)

Is it (OCV-LV)*LC= IR...?