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

[GB ian]

actaully how about V=IR so

(v1-v2)/I=R


Schoolboy physics is a bit rusty

[redbird300]

Originally Posted by GB ian

actaully how about V=IR so

(v1-v2)/I=R


Schoolboy physics is a bit rusty

This one is right.

Or, if you want to use the abbreviations from your previous post: IR = (OCV-LV)/LC

Succes with it.

[GB ian]

Thanks RedBird.

I am going to make an IR tester rather than buying a new charger.

I hope Radioshack has some inexpensive voltmeters I can solder in line and some sort of current measuring device.

I would like to rig up something that uses a load such as a small bulb.

I'll post if I get this working.

[alexf1852]

There's a guy in rcgroups that made and is selling IR testers. Supposedly, they are pretty accurate, more so than chargers.

[GB ian]

alexf ..I'll try to find it thanks.

[redbird300]

The voltage differences will be rather small, so you'll need a voltmeter with good accuracy and plenty of resolution, like at least a 3 3/4 LCD type display, not sure if a cheap one will give you satisfying results.

[M52 POWER!]

I have a nice Fluke DMM, how can I do this?

[alexf1852]

Using a meter alone will not get you accurate measurements. You have to take measurements under load. I have not looked into all the specifics. You can probably get more detail info on rcgroups.com

[redbird300]

It's all explained in the very first post of this thread:


"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

[joeblades]

Excelent thread. I have noticed a dramatic drop in flight time. Yesturday I told myself I was going to test my Lipo's "3S"s". but was unsure if I was approaching the issue correctly. It seems I was on the right track and it's great to have the conformation.

Now if I could just get my DMM probes to fit in those tiny JST conectors. LOL

FYI I have a Radio Shack DMM # 22-178, been using it for about 17 or 18 years so they are good meters, or at leaste they were. It has the same capabilities as a high dolor Fluke, but about 1/3 the cost. I think I paid 70 bucks for it.

[MaSt0RaS]

hey guys with my Hyperion DUo chargers i saw that my 6s lipos VOltz have 10-16IR...

Thats a good number...

TOday i charged the same lipos on 2C on the field and i saw that the Ir was 5-7....

ANy ideas why??!

[TJinGuy]

Packs were likely warmer. IR is dependent on temp.

[scoio]

Originally Posted by 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.

Awesome post!

I just got my new Icharger 208b and finally have a chance to try out the IR of my lipos.
I found 2 x 6S that have to be disposed
Curious thing a have 3x 6S Flightpower and the IR of all 6 S is 35.....guess they are garbage but wondering if those high numbers make sense.

Thanks

Chris

[aramb]

Is it best to check internal resistance when the pack is fully charged or after it is down to about 20%? When I measure the IR before charging, I get 6 ohms on each cell. After charging it drops to 4 ohms per cell. Which figure should I be concerned with?

[TJinGuy]

There are many factors that change cell IR, temp, charge state, age, condition, etc. The only way to get useful results is to keep as many things constant as possible.

[Flatearthman]

very good info C.D. i use a plastic fork to puncture them

[isogloss3d]

Here's a video with great I.R. explanations and real world examples with simple math. I only halfway understood IR for a couple year and not enough to have a working knowledge of it. It wasn't until watching this vid a couple times that it began to click.

Oh and you can ignore the fancy gadget being demonstrated and still get something out of it.

Product Spotlight: ESR Meter (5 min 28 sec)

[mxman311]

This program is really hard for me to figure out. Does anybody have a link to a quick user guide or have some advice. I have 6 3 cell batts all different c rating but same mah and 6 1 cell mcpx batts. I would like to have a profile or something like that to just click on when I charge a batt to monitor its health. How do I do this?

[mxman311]

Originally Posted by 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.

I have a couple 1 cell mcpx batts and they are all at around 80 to 120 ir. They still fly and pull 3d but do feel a little weak. Your figures seem really low to me but this is my first time having a nice charger icharger 206b and I am still very new to this hobby so I really have no idea what im talking about. I was under the impression that my 3cell 2200 packs were gonna fly good till at least 50 ir based on my experience with the 1cell mcpx batts?? Why would my 1 cell packs fly pretty solid with 80 to 100 ir? My 3 cell batts are all very new and are around 0 to 8 ir per cell.

[desertstalker]

Originally Posted by mxman311

I have a couple 1 cell mcpx batts and they are all at around 80 to 120 ir. They still fly and pull 3d but do feel a little weak. Your figures seem really low to me but this is my first time having a nice charger icharger 206b and I am still very new to this hobby so I really have no idea what im talking about. I was under the impression that my 3cell 2200 packs were gonna fly good till at least 50 ir based on my experience with the 1cell mcpx batts?? Why would my 1 cell packs fly pretty solid with 80 to 100 ir? My 3 cell batts are all very new and are around 0 to 8 ir per cell.

The relationship between IR and C rating is cell size dependent, for the same C rating a cell with twice the capacity should have half the IR.

If you were to only pull 2A or so from your 2200s they would be fine to an IR of 50, less so if you want 20A from them (or more).