In regards to my post in in this topic, I promised are more detailed/extensive reaction, as not wanting to spam the other topic.
Those numbers are not exactly accurate and in my experience with my Opus BT-C3100 it is often also not very consistent between several measurements and/or slots.
I did a quick/short test with my Opus BT-C3100 tonight, for my JP40. I cleaned all contacts of the charger and battery with Iso, so that it not would be an influence. I did 5 “Quick Tests” with the cell, for each of the 4 slots. Every time disconnecting the battery for several seconds, before doing a next test.
Here are the results in mOhms:
| Slot 1 |
Slot 2 |
Slot 3 |
Slot 4 |
| 39 |
32 |
34 |
30 |
| 39 |
32 |
34 |
36 |
| 41 |
32 |
33 |
35 |
| 40 |
32 |
35 |
29 |
| 36 |
32 |
35 |
33 |
Since I was so surprised by the consistency of ‘slot 2’ (I literally never had 5 identical results in a row before!), I decided to test a 40T5 in slot 2, which was (almost) charged to the same voltage, to see if that slot really test so consistenly! (Also cleaned the contact points of the 40T with Iso.)
This gave the following results:
| Slot 2 with 40T5 |
| 63 |
| 49 |
| 78 |
| 67 |
| 59 |
So it was wildly inconsistent with the 40T5. I have no clue what caused that, other than that the Opus may favor the (way) lower IR of the JP40 over that of the 40T.
I also did 3 quick tests on my Fnirsi HRM-10 Battery Internal Resistence tester, just to check the consistency and the ‘actual’ difference between the JP40 and the 40T5. There are some fluctuations between the measurements, but remember that we are talking about way more accurate and lower numbers, going down to sub-mOhms, and therefore minute differences.
Results:
JP40: 2,93 mOhm, 2,90 mOhm, 2,93 mOhm
40T5: 11,05 mOhm, 11,03 mOhm, 10,95 mOhm
Quite consistent results, with minor differences, which could even be down to placement into the somewhat finicky battery holder tester.
The difference/delta between the JP40 and 40T5 is to be expected and also consistent, in contrary to the readings from the Opus.
Most “cheap” dedicated battery testers, including my Fnirsi and also the one that Austin used in that photo, measure ACIR/AC IR, which means there is no load put onto the battery and it just measures it’s (‘at rest’) impedance. Most of the affordable testers are quite accurate ‘out of the box’ now a days. These testers can measure way lower/smaller resistances than for example most digital multi meters. The other advantage is that they use a 4-wire measurement (so 2 different contact points and individual measurements), which gives a more consistent result and less chance on ‘faulty readings’, than a 2 wire measurement, like what the Opus uses.
I do assume that the Opus also does an ACIR measurement, given how quick it does the test. The other test method, I will explain more about that later, is the DCIR test measurement, gives a bit higher value than the ACIR measurements, but nowhere near the amount to explain the high numbers of the Opus.
What I think is that the Opus, has higher internal resistance, influencing the results and that the fluctuations between readings could (maybe) partially be explained by the ‘2-wire-method’ which has a higher chance of faulty readings.
It would/could still be usefull if it gave consistent readings (almost) every time, with every differnt cell. That way you can at least extrapolate which cell has lower IR than others, between different types or between batches of the same cells, to see which are better than others.
If you have a new battery and you want to know if your test results match the factory spec (and so if they are accurate), you can always compare them to the (official) data sheet.
As an example below a picture of the (official?) data sheet of the Liitokala 4695E I recently purchased:
As you can see it mentions the AC(I)R value to be equal or below 1.8 mOhm at “BOL status” (= Beginning Of Life) and the DC(I)R equal or below 4.0mOhm.
I measured my 4695E ACIR and with 1,72 mOhm it was indeed within spec!
Now onto DCIR measurements:
Since this is a bit more complicated than ACIR measurements, and my knowledge is still very limited on this subject, I will give you some general explanation, to prevent me from getting certain details wrong. I will link you afterwards to a really interesting topic I found here on BLF a couple of days ago, with a lot of information about DCIR measurements. (I believe I also found an interesting topic on EEVblog(.)com, but I will try to find that one some other time.)
DCIR measurements are internal resistance tests when the batteries are under load, which means it gives a way better indication of how the battery will perform when in (high drain) use. Equipment for properly measuring DCIR is quite a bit more expensive (and extensive) than the “cheap” ACIR tester. It also requires a lot more indept knowledge and skill to perform (and interpret) these measurements.
There is an IEC standard for how to measure DCIR, although a lot of people testing battery prefer their own process/standard, for several reasons.
In this topic (Measuring impedance) you can find tons of information about measuring impedance, including a LOT of information on DCIR testing, including some very interesting posts by @docware, @Pajda_cz and also @HKJ.
To the experts on this subject who maybe read this post: I hope I didn’t make too much mistakes. My knowledge is still pretty fresh and limited, so therefore I kept it more of a ‘general explanation’, to avoid incorrect detailed information. 
