LiPo Internal Resistance Readings - please explain

[Romeo Whisky]

I recently acquired a new LiPo charger which has the facility to measure internal resistance of each cell and the battery as a whole.  As is so common in aeromodelling equipment manuals, it tells me how to take the measurement, but says nothing at all about what the readings mean in terms of battery performance. 

 

I tested all my 3S 2200 batteries which are about six years old but fly OK and most gave individual cell IRs as 001.01.001 with a battery reading of 3.  A couple were very slightly higher.

 

Of course I realise that the lower the reading the better, but what is regarded as a high IR reading and what is a normal IR reading?

 

 

 

 

[Martin Harris - Moderator]

If your readings are in the region of a total of 3 mOhms for a 3S battery then you’ve either discovered the holy grail of batteries or your readings are inaccurate.  
 

I would hope to see readings of something around 5 to 8 mOhms per cell from a 2200 in reasonable condition (IRs reduce with capacity) but it also relates directly to the (true) C rating so a high C rating should have a much lower IR than a low one. 
 

Some of my very old small (1300mAh) batteries have individual cell resistances approaching 30 mOhms but are still useable in low demand applications.  Most failing batteries will exhibit imbalance between cells. 

[Allan Bennett]

I once read that an acceptable IR would be in the region of 12,000 divided by the pack's mAh, which tallies (in round figures) with what Martin Harris expects from his 2200mAh packs.

 

Since the battery itself is part of the circuit carrying the power, the higher its IR (a) the more it will heat up when a current flows through it and (b) the more it will try to restrict that current.

[MattyB]

Don't forget IR also varies with SOC (state of charge) and temperature as well. As a result make sure they are at a standard cell voltage (I normally use the storage voltage rather than full, as that way I don't have to charge to compare any two packs) before measuring, and don't bother comparing in very cold or very hot weather when the readings could be significantly different.

 

PS - Remember because of Ohms law you can only every measure IR of a single pack, not multiples in parallel, so remember to disconnect any parallel packs you may have been charging alongside the one to be measured before taking a reading.

 

Edited June 28, 2023 by MattyB

[Romeo Whisky]

Thanks for the responses.  

 

The reason I decided to check the IR of my 8 3S 2200 backs (All Blue Zippys) was because they did not seems to be giving the flight duration I thought they should, and were quite warm-to-hot after landing.   I bench-tested them all using my Tornado wattmeter, on a standard setup I have using a 1000kv motor and 10x5 prop running at full throttle, and monitored the start and run-time pack voltage and sustained Amps and Watts until they had each drawn 650 mAh from the pack during each test which took just over 2 minutes to draw the 650mAh.    I tested the IR both before and after the test runs.  All the packs were slightly warm after the test runs but the IR readings before and after were mostly the same (see above) or increased by only a point or two.

 

I then recharged the batteries using the charge/balance program on my charger, and in every case the recharge input was higher than the 650mAh which the wattmeter had indicated that they had used.  Obviously the higher recharge input can be accounted for to some extent by the balancing process, but if the wattmeter is correct none of the packs has a true capacity anywhere near the 2200 mAh they are rated at.   Can't make up my mind whether to scrap them all, ... but then again packs have got expensive!

[Zflyer]

I must confess i did check some of mine a while back. They were all quite high i.e. double figures. I still use them only showing puffy dont seem to have lost much 8  performance . The science i am sure will eventually catch up with my practical use.

[Philip Lewis 3]

The IR relates to how many amps they can push out which reduces as the IR gets higher, it has little to do with capacity which (as you have noticed) reduces over time to a point where they  are unusable or not worth using due to the short flight time.

[Geoff S]

Just consider a voltage source (in this case a battery) as a perfect one with zero resistance in series with its internal resistance.  The terminal voltage that you can measure off load with a digital voltmeter will be roughly (quite closely) the voltage of the 'perfect' battery but as soon as you draw current there will be a voltage drop across the internal resistance which means the measured terminal voltage will be lower.  Also, there will be heat generated within the battery proportional both to the current draw and the internal resistance - power/ie heat (watts) = current squared (amps) x resistance (ohms).  So a battery with high ir will get warmer than one with a low ir

 

I find 10 milliohms/cell to be a bit borderline but usable in my relatively low power demanding models but perhaps not in high demand models like ducted fans (which I don't have).

[leccyflyer]

2 hours ago, Zflyer said:

I must confess i did check some of mine a while back. They were all quite high i.e. double figures. I still use them only showing puffy dont seem to have lost much 8  performance . The science i am sure will eventually catch up with my practical use.

LIkewise. I found that my newer packs in the 2200-5200mah range all had measured IR in single digits, the larger ones typically <5milliohm/cell. Some of my older 2200-3300mah packs were in double figures 10-20 milliohms/cell, but still did fine in use. Some of my smaller 450-1300mah packs, some of which are getting on a bit, had much higher readings than 10milliohm/cell - many of them up to 80 milliohms/cell, to a degree independent of the age of the pack -ie even newer, little used packs were in the several tens of milliohm/cell range. The highest I measured at the time was a 2s1p 500mah Tipple pack - so that shows how old that is - which was 169 milliohms/cell and has since been retired.

[RedBaron]

There is no standard answer to the question what is a good IR?, it depends on the number of cells, the capacity, how you have used the batts, how you have charged the batts, the make etc.

Generally large capacity batts have lower IR per cell than small capacity batts.

 

What you have to do is measure the IR for every cell of every battery that you use and record the information. If you have not already done it label each of your batts.

After a period of time like two months repeat the exercise at exactly the same temperature as your original readings. A small change in ambient temp will make a noticeable difference to the resistance measured.

What you are looking for in this information about your batteries is for changes in the IR of cells. Usually if done accurately IR becomes higher with age as the the chemistry of the "ode"s deteriorate.

If all the cells of a particular battery have drifted about the same amount then it is just getting older. If one of the cells has drifted more that the other cells then you are headed down hill, test this battery more often.

If you have any batts that are more or less puffy it is a high probability that one cell may well be much worse than the others. Unmatched restistance + puffy = bin time.

 

Your data will be of no use to anyone else and vice-versa. It is all about changes in the IR of your set of batteries.