I have no other way to measure IR in my collection of cells aside from the info blurted out by the “Quick Test” function on my Opus. Honestly, I don’t take much stock in it’s accuracy. I have quality cells that measure low resistance (<20), and other cells of the same brand/size/capacity, purchased together that measure significantly higher or lower. I have cells that measure between 250 - 400 ohms that have measured pretty much the same resistance since purchased new, have been cycled several times, but are relatively consistent in capacity despite the variance in IR (again, measured by Opus). All this said, when is it appropriate for the “average consumer”, like me for example, to be concerned about IR in individual cells (with the obvious exception of matching cells run in series)? Is there a recognized threshold to consider cells for recycling, or can they be run until they just won’t hold a charge anymore?
I would replace cells when they feel to be getting weaker. You will know this by owning and using them for several years.
First, just an FYI note… I had to read some of the post, and be puzzled, before I realized that (in this context) IR was Internal Resistance and not InfraRed as my mind first thought the question might be about. Abbreviations and acronyms can be a bugger at times.
Answer to the OP question: about zero importance.
I believe that often the IR value one gets from a cell charger is iffy to meaningless. I buy good cells from good sellers and use them until their performance falls to a level that becomes annoying or detrimental to the task at hand. I don’t have any lights using cells in series because of a past issue with cells that became mismatched after some use. Life is simpler with single or parallel cells. That is me and my preference.
It’s not very usable data. It’s really just a snapshot and a rough indication. The ir can change depending on the state of the cell > charge level > warm temperature > high drain current > low drain current > etc…
I only have single cell lights, but internal resistance is not something I concern myself with. Like @zoulas I would recycle a cell when I felt like it was getting weak.
“Average consumer”?
Not at all. They don’t know what it is, let alone what it means. 
This group? Likely highly variable…like the Opus. 
The ‘charger/slider’ tests can be pretty variable and mostly a pretty rough test. For a lot of people, including me for years, that’s good enough for practical applications.
There are methods and devices that are much more reliable and accurate. Using such devices allows you to more accurately define what you already suspect about cells that aren’t working well anymore-they are getting weak, and the IR is going way up.
Cleaning all contacts, then doing multiple tests, with pressure on the contacts, + throwing out the outliers can get you in the ballpark better. You can usually differentiate “OK” from “So-So” to “Yuck”.
It actually is important, to a point. A battery with high internal resistance (for a li-ion, Nimh are different) means either a) the battery is worn out or faulty, b) its a cheap, low quality battery. Lower internal resistance means the battery can deliver higher current better (ampacity) for longer and run cooler under load.
It depends. Once a certain threshold is reached, the batteries are simply crap/faulty, but even below it can cause issues.
High IR cells will lead to lower brightness in DD drivers and it will lead to lower efficiency (and hence runtime) in buck/boost drivers (what good is it if the driver is 95% efficient, when the battery loses another 10% of the power to its IR?). Only in linear regulated lights it will not matter as long as it is low enough that the highest mode still works. It will just lead to the driver heating less, and the battery more, total being about the same.
I put my worse cells in specialty lights, like UV, IR etc, that will not run high currents or be used for full power over an extended time, and keep my best cells in high discharge rate lights, or lights I plan to use for long time in one go, so I need max efficiency (together with boost drivers).
Well, for the average user, maybe it is not real important. Other factors like capacity are more important. OTH, a cells Internal Resistance (IR) is an important factor in determining a cells State Of Health (SOH).
I have kept records on most of my cells over time. But really this started because I have hundreds of “pulls”. Meaning cells pulled from battery packs that no longer functioned. SO carefully screening cells for SOH becomes more important for understanding where a call can be best used . This is critical if one wants to build new battery packs. (or any application where series cells are used)
So IR may not be important to a casual user who mainly uses cells in single cell lights. But it is a very good tool to use in understanding where a cell is in its lifecycle.
OTH, using DC resistance as is used in all cylindrical cell chargers, is not the preferred way of measuring IR. As you have seen, the measurement is generally unreliable. Indeed it will change significantly with the State of Charge of the cell. In addition to the differences in how the cell sits in the chargers contacts.
Using a 4 wire AC resistance meter is the way to go if you care about the accuracy and repeatability of the numbers.
Most users will never go to the trouble of getting and using such a meter. Plus, it seems that people do get by without this level of testing.
You ask about a level at which you should stop using a cell. There are too many variables to give a good answer for this. Especially when using most commonly available chargers using DC resistance numbers. The size of the cell, the state of charge, contact resistance between the holder and the cell all impact the readings . A rule of thumb I use for 18650 and 21700 cells is a reading over 200mOhms makes me start thinking about retiring the cell. I monitor capacity and temperature during charging as well as performance in the light closely at that point. Any noticeable changes in any of those things will make me discard the cell.
It is interesting to note that many chargers use the IR reading to set charge rates when in auto mode. Others refuse to charge at all at some level of IR. Obviously the manufacturers consider the reading to be important. Regardless of whether the “typical” consumer pays any attention to it.
Food for thought, assuredly, thank you. Most of my cells are under 200 mOhms as measured by Opus. My main concerns revolve around some old (blue generic wrap) Sofirn provided 18350 button tops that I use in my SP40 shorty. They always came in over 250, and still do, but they’ve only been cycled 4 or 5 times each.
Smaller cells always read higher for IR. I just got some Vapcell F16 18350s. Brand new they measured close to 200 mOhms on the S8000 I was using to test them. 250 is probably fine. Just keep an eye on them for changes. For instance, if they started showing 300 and capacity starts falling off, it is likely time to think about replacing them.
One more thing. DC IR changes quite a bit with the SOC (state of charge/voltage) of the cell. When you start recharging the SOC is low (and variable) and the IR is higher. I always test DC IR with fully charged cells. This leads to a bit more standardized measurements.
On a related note-Anyone have any insight into NiMh cells that…occasionally…will show a lowish resistance, but otherwise NOT be a decent cell, when others from the same batch and similar history WILL be showing high resistance and also the associated signs of end of life?
Here is a good article on failure modes of NIMH cells. Seems like there can be lots of causes. That lower than expected IR may be caused by what they refer to as Micro-shorting due to Conductive debris accumulation in separator.
It is a good question and I have seen similar results in the past. I just never looked into it. Glad you prodded me into looking for an answer.
Oh, Excellent! Will read. 