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Dec 04, 2015, 02:10 PM
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Thread OP

What are LiPo Internal Resistance "normal" values?


At what point should I be concerned about differences in internal resistances between cells?

I've searched around and here's all that I could find on "ideal" internal resistance measurements.

http://www.rogershobbycenter.com/lipoguide/#lipo_IR
Quote:
Based on my online research, combined with my own experience and findings, I would say, as a general rule, a per cell rating of between 0-6 mΩ is as good as it gets. Between 7 and 12 mΩ is reasonable. 12 to 20 mΩ is where you start to see the signs of aging on a battery, and beyond 20mΩ per cell, you'll want to start thinking about retiring the battery pack.
Here's why I'm asking:
I'm new to the hobby and I have a new Hobbyzone F4U Corsair arriving in a few days. It's my 2nd plane. The first is a Sport Cub S.

The Cub takes1s batteries, which are simple and easy (stick one in the USB charger and when the light turns off, it's ready)

The F4U takes 3s batteries, so I had to buy equipment and learn up on multiple-cell LiPo batteries.

I'm using these batteries (Floureon 3S 25C 11.1V 2200mAh)
and this charger (SKYRC iMAX B6AC V2)

Today, I set up the charger and started learning how to use it. The batteries were about 3.8v per cell, but the internal resistance has me a bit concerned.

Battery-1 has the following per-cell readings:
7 - 5 - 6 mΩ
3.80 - 3.82 - 3.86 Volts

Battery-2 has the following per-cell readings:
12 - 6 - 6 mΩ
3.86 - 3.86 - 3.80 Volts

The first battery seems fine, but 12 mΩ reading on Battery-2, Cell-1 seems off both because it's high (according to the above post) and because it's twice as high as the other two cells.

Is this normal, or did I possibly receive a defective (or sub-par) battery? I could be over-thinking this, but as I'm just learning all of this stuff, I figured it would be best to ask. They were cheap batteries, so if it's just the fact that they're lower quality because they're cheap, I can live with that.

I have an electrical background, so if you need to be tech-y with your answer, that's fine. I know how voltage, current, resistance, etc. all work.

Thanks in advance for any help.
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Dec 04, 2015, 02:21 PM
Space Coast USA
hoppy's Avatar
Clean the contacts and try again.

Be sure and balance charge your packs. I'd run them for a couple of cycles and see if it stays constant.
Dec 04, 2015, 02:24 PM
EDF all the way!
bruff's Avatar
Also cycle them about 5 times to break them in.
bob
Dec 04, 2015, 02:50 PM
Registered User
The chances are that you have a pack with a weak cell. 6m.ohms is ok for a 2200mAh cell , but 12 m.ohms is poor and very mismatched. When you use the lack it will not perform as well as the other pack, but more importantly the weak cell will dissipate twice as much heat as the other two cells (I2R). Rhis means that it will run hotter, age more quickly and die earlier.
A max reasonable variation in IR on a pack is 15 - 20%.

Try looking at the Lipotool and putting values into it. See it at

http://www.jj604.com/LiPoTool/


It is based on limiting the initial heat dissipation to forecast a safe current.

Wayne
Dec 04, 2015, 02:53 PM
Registered User
Thread OP
Quote:
Originally Posted by bruff
Also cycle them about 5 times to break them in.
bob
Should I just use the charge and discharge functions on the charger to accomplish this?
Dec 04, 2015, 03:40 PM
Registered User
Thread OP
I have it on the charger now. It's set to Balance charge, but the voltages are diverging instead of converging.

Right now individual cell voltages are:
4.07- 4.13 - 4.21

I kind-of expected them to all merge to the same voltage as they came up to 4.2



EDIT: Ok... Cell 3 went up to 4.22 and is now stable as the other continue to climb (currently at 4.11 and 4.13), so I'm a bit less worried now. I just need to cycle the batteries a few times to see about the resistance readings.
Last edited by Skoot; Dec 04, 2015 at 03:50 PM.
Dec 04, 2015, 04:25 PM
Space Coast USA
hoppy's Avatar
Stay with the packs - Just don't go off and leave them assuming they will be fine. Or put the charging station outside.
Dec 04, 2015, 09:05 PM
Registered User
ggcrandall1's Avatar
What charging current are you using? A 1C charge would be 2.2A.

I think the balancing current on that charger is pretty low so it may take a while for the pack to balance. If the charger shuts off before the cells are balanced you might try charging at a lower current. This could help to let the balancing catch up.

Keep an eye on that cell that's reading 4.22V. And I would verify the cell readings with a DVM.

Glen
Dec 05, 2015, 08:57 AM
Registered User
Thread OP
I'll never leave a charging battery in the house unattended, so no worries there. It's in a lipo bag too.

None of the cells went above 4.22
The cells that reached 4.22 first just sat there while the others SLOWLY came up to match.

I then discharged them to storage voltage (3.8v)

Afterwards, I checked the internal resistance and that 1 odd cell was more in line with the others.

I charged / discharged at 1amp just to be on the safe side. Next time I'll bump it up to 2.2 (1C)

Thanks again for helping this noob out.
Dec 05, 2015, 09:07 AM
Space Coast USA
hoppy's Avatar
Let us know what the IR's are after a couple of cycles.
Dec 05, 2015, 06:09 PM
Registered User
Race Miata's Avatar
There's also a very likely possibility that the IR readings from the charger itself is not accurate. All cheap chargers up to but not including HK Reaktor chargers I have experience with always read high IR on cell #1.

Also note that cell IR is pretty much inversely proportional to cell capacity. Healthy 2200mAh cell should have IR around 3-8mOhm depends on their C-rating. A 5000mAh cell should have about 1-3mOhm. Measuring condition espeically pack temp makes a huge difference so don't worry too much about the absolute values of your IR numbers.

Hard to say if it's a problem with your packs for now. Just cycle them a few more times perhaps fly gentle a bit for now. If your balance charging sessions continue to take unusually long and things don't improve then we'll see from there.
Last edited by Race Miata; Aug 12, 2016 at 10:47 PM. Reason: typo: do->don't, directly->inversely
Dec 06, 2015, 06:49 AM
Youtube channel : solentlifeuk
solentlife's Avatar
I will be the outsider ... and say ... I have packs with very high numbers that fly low power demand stuff beautifully and have done for many months ... years in fact.

I have mid range iR packs with numbers in the 30 ... 40 range that fly all sorts of sunday stuff I have ... no problems.

I have low iR packs ... 5 .. 7 ... 9 etc. that fly my high power demand stuff like EDF's / high speed pushers etc.

What has become evident to me over significant time ... if a pack works in a model ... its fine. It if puffs - its being pushed too hard and needs to be retired to lesser demand duty.... if it fails to fly a low power job ... its either retired to bench use or binned.

Do I read iR figures anymore on packs ? No.

Can I read iR figs on packs ? Yes by two methods, a) by iR meter designed specifically for the job I purchased few years back, b) chargers that have the facility.

Has it made any difference to my use of packs reading or not reading ? None whatsoever.

I used to measure new packs and make a note of the readings ... checking again after a few cycles ... usually very little change. I would then check periodically and refer to initial. My biggest observation was that I noted flight change before the readings usually. So I stopped.

Others will argue against what I write here ... no problem. Not everyone follows same roads ...

Nigel
Latest blog entry: Online posting behaviour
Aug 12, 2016, 09:25 PM
tic
tic
thunderscreech
tic's Avatar
I would like to argue against what you wrote here Nigel.. Thank-You
Aug 13, 2016, 12:48 AM
Youtube channel : solentlifeuk
solentlife's Avatar
Quote:
Originally Posted by tic
I would like to argue against what you wrote here Nigel.. Thank-You
Your choice and we are all entitled to opinion.

My intent there was to emphasize that model performance is the biggest identifier of packs suitability in use.

Nigel
Latest blog entry: Online posting behaviour
Aug 13, 2016, 11:40 AM
tic
tic
thunderscreech
tic's Avatar
I have no intent and don't want to emphasize anything. I just want to argue.. .... I do agree that a pack that suffers from high IR can still be very useful in a model who's drive system is not demanding.


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