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Thread: IR threshold between cells that should not be paired anymore?

  1. #1

    Default IR threshold between cells that should not be paired anymore?

    Is there a limit between a batteries internal resistance compared to another that would warrant them not being paired anymore in parallel or series in a flashlight?

    I have a neat little tester, the SM8124A, that shows the IR difference between some batteries I purchased in groups of two or four. With one pair of Protected 26650's the IR for the first battery was 30 and for the second it was 37 . Seems close enough but is it still ok to use them together in a flashlight since it is still a difference of more than 20%?
    Last edited by klrman; 07-01-2018 at 01:12 AM.

  2. #2
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    Default Re: IR threshold between cells that should not be paired anymore?

    Being protected really helps you out here, so I definitively wouldn’t worry too much. If you’re worried, just doublecheck that the protection includes both overcurrent and undervoltage.

    A little IR difference itself isn’t what’d ruin a day, but rather things like overdischarging one of the cells, and the protection helps you out with that.

    Still good to keep an eye on IR, it you see one at 30 and the other at 200, it’s probably time to think about replacements.

    If the cells are in parallel, it matters a bit more, but overcurrent protection should still have your back.

  3. #3

    Default Re: IR threshold between cells that should not be paired anymore?

    Helpful info, thanks. All the batteries have overcurrent and undervoltage protection. Just a little surprised they came in pairs with one having much more resistance than the other battery as I thought coming pairs mean they should have been evenly matched.

    BTW, nice to have this forum have no lag anymore like the past. Pages loads instantly now.

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    peter yetman's Avatar
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    Default Re: IR threshold between cells that should not be paired anymore?

    I was also worried about IR for pairs of cells in series a while back, until I realised you need to look at the capacity more. THe IR is a good indication of the cell's age, but capacity is much more important if you are matching cells.
    P

  5. #5

    Default Re: IR threshold between cells that should not be paired anymore?

    Good to know, thanks Peter. Never tested these two cells for capacity so that will be my project today.

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    peter yetman's Avatar
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    Default Re: IR threshold between cells that should not be paired anymore?

    Can't find the thread, but it was HKJ that put me right.
    If I come across it I'll let you know.
    P

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    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by klrman View Post
    Helpful info, thanks. All the batteries have overcurrent and undervoltage protection. Just a little surprised they came in pairs with one having much more resistance than the other battery as I thought coming pairs mean they should have been evenly matched.
    It’s a lot when you view it as percent, but not really if you view it as an absolute number. Also, there’s a certain margin of error and noise here. A few microohms can be the result of something unrelated as well, such as how well each of the cells connected electrically to your tester that day, or even minute temperature variations. Not saying any of those were a factor here, just trying to put a scale to how small the difference is when not seen as percentage.

  8. #8

    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by peter yetman View Post
    Can't find the thread, but it was HKJ that put me right.
    If I come across it I'll let you know.
    P
    That's ok as I remember reading one of HKJ's tests and he said +-5% of total capacity is the tolerance for using in series.

  9. #9

    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by terjee View Post
    It’s a lot when you view it as percent, but not really if you view it as an absolute number. Also, there’s a certain margin of error and noise here. A few microohms can be the result of something unrelated as well, such as how well each of the cells connected electrically to your tester that day, or even minute temperature variations. Not saying any of those were a factor here, just trying to put a scale to how small the difference is when not seen as percentage.
    Now that you mentioned it, I tested the ir a few more times with my SM8124A tester and results were still the same. This 4 wire tester seems very consistent, but I see what you mean in the overall sense that the difference is small. Still glad I started this thread as I was not 100% sure and thought it couldn't hurt to ask just in case. Thanks for help.

  10. #10
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    Default Re: IR threshold between cells that should not be paired anymore?

    Just as I mentioned in the other thread, AC (1kHz) impedance is not the correct test to be using here, since there is not necessarily any correlation between the AC impedance and low frequency DC impedance (e.g. they might be close on AC impedance but much further off on low-frequency DC - which is what matters for most applications here).

  11. #11

    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by Gauss163 View Post
    Just as I mentioned in the other thread, AC (1kHz) impedance is not the correct test to be using here, since there is not necessarily any correlation between the AC impedance and low frequency DC impedance (e.g. they might be close on AC impedance but much further off on low-frequency DC - which is what matters for most applications here).
    After reading the thread a little, it seems there is for and against for both types of measurements but that the AC(1kHz) is still a decent way to test.
    Last edited by klrman; 07-01-2018 at 04:09 PM.

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    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by klrman View Post
    After reading the thread a little, it seems there is for and against for both types of measurements but that the AC(1kHz) is still a decent way to test.
    It seems that you did not read closely enough. Why do you believe that "AC(1kHz) is still a decent way to test"?

  13. #13

    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by Gauss163 View Post
    It seems that you did not read closely enough. Why do you believe that "AC(1kHz) is still a decent way to test"?
    Sorry about that. I thought when you did not reply to Jasonck's query that maybe it was a stalemate as to which is the better or more reliable way of testing. Give me time and I will go through it all link per link to understand better!

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    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by klrman View Post
    Sorry about that. I thought when you did not reply to Jasonck's query that maybe it was a stalemate as to which is the better or more reliable way of testing. Give me time and I will go through it all link per link to understand better!
    I'm not sure what you refer to. In any case, to be much more explicit I quote below from a paper published this year in Nature's Scientific Reports on precisely this subject (my emphasis)

    Quote Originally Posted by Anup Barai et al.
    The (AC) 1 kHz resistance test, although useful for a quick check in a manufacturing environment e.g. quality check, not much value is derived from it. Furthermore, whether the measured 1 kHz resistance value lies in the inductive or conductive region is highly dependent on the battery sample, therefore the single frequency (e.g. 1 kHz) needs to be chosen depending on the sample when employed for quality check. [...]

    Measuring battery resistance with a 1 kHz AC signal (or similar single frequency signal), is common practice in industry, especially for measuring lead-acid battery resistance. It is a relatively fast (in the order of seconds), low power consuming and low cost technique, using handheld equipment. Usually a low current sinusoidal signal of 1 kHz is applied to the battery and the voltage response is measured. Although this technique is time-efficient, a single value of resistance is not sufficient to characterise the battery’s performance. This is because charge transfer through multilayer surface films and kinetic and diffusional processes in the solid and liquid phases of the battery lead to a frequency dependent resistance
    If you peruse that paper (or related papers) you will learn that such AC impedance tests only measure one part of the impedance (mostly Ohmic). In particular they omit the very important charge transfer resistance. This is the component that typically grows most as the cell ages so it is what you want to be tracking for battery health. Since AC impedance tests completely miss that, they are of little value for tracking battery health.
    Last edited by Gauss163; 07-01-2018 at 05:12 PM.

  15. #15

    Default Re: IR threshold between cells that should not be paired anymore?

    Interesting info Gauss, thanks very much for the help. Is there any reasonably priced meter that could accurately test for aging cell health then that you think would be good enough to do the job?

  16. #16
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    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by klrman View Post
    Interesting info Gauss, thanks very much for the help. Is there any reasonably priced meter that could accurately test for aging cell health then that you think would be good enough to do the job?
    Any charger that competently implements an IR test using a DC pulsed load method should serve well for most hobbyist applications. This includes some popular analyzing chargers, e.g. Opus BT-C3100, SkyRC MC3000, etc. Check reviews to see if the charger yields repeatable accurate results.

  17. #17

    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by Gauss163 View Post
    Any charger that competently implements an IR test using a DC pulsed load method should serve well for most hobbyist applications. This includes some popular analyzing chargers, e.g. Opus BT-C3100, SkyRC MC3000, etc. Check reviews to see if the charger yields repeatable accurate results.
    Thanks. I have the latest MC3000 but got frustrated with the changing IR values every time I rotated a cell or switched bays, but I did read yesterday that HKJ mentioned to generally take the lowest reading as the most accurate, so I will go with that more or less and use my SM8124A as a tool to detect if I receive any fake cells from sellers.
    Last edited by klrman; 07-03-2018 at 02:28 AM.

  18. #18
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    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by klrman View Post
    Thanks. I have the latest MC3000 but got frustrated with the changing IR values every time I rotated a cell or switched bays, but I did read yesterday that HKJ mentioned to generally take the lowest reading as the most accurate, so I will go with that more or less and use my SM8124A as a tool to detect if I receive any fake cells from sellers.
    What sort of variations are you seeing? Iirc reviews of prior versions reported highly consistent IR values. If yours are not then maybe your unit is faulty, or maybe QC has gone downhill in later versions.

  19. #19

    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by Gauss163 View Post
    What sort of variations are you seeing? Iirc reviews of prior versions reported highly consistent IR values. If yours are not then maybe your unit is faulty, or maybe QC has gone downhill in later versions.
    Just by rotating a protected 18650 cell, numbers can jump from 48 to 130 in the same slot as well as the same type of variations between slots with the same cells. I have noticed that my protected 26650 readings in the MC3000 seem to be much more stable but never figured out exactly why.

    On another note, for the protected 26650 batteries I use in series in one of my flashlights, the MC3000 reads them both at 48 for ir and the SM8124B reads them both at 31ir. After you guys helped me out to understand that both are using different methods of measuring resistance, it makes much more sense now.

  20. #20
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    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by klrman View Post
    Just by rotating a protected 18650 cell, numbers can jump from 48 to 130 in the same slot as well as the same type of variations between slots with the same cells. I have noticed that my protected 26650 readings in the MC3000 seem to be much more stable but never figured out exactly why [...]
    That's not normal. Typically you should see only a few milliohms variation whether rotated in-slot or between slots. The MC3000 should serve better for tracking health because - unlike the AC 1kHz test - it will incorporate some of the resistance components with longer time constants (but not all of them since the test duration is too short).

    It would be interesting to see how the AC test compares to the MC3K pulse test on a wide variety of cells. For example, what's the largest difference between the two that you've encountered?
    Last edited by Gauss163; 07-03-2018 at 02:51 PM.

  21. #21

    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by Gauss163 View Post
    That's not normal. Typically you should see only a few milliohms variation whether rotated in-slot or between slots. The MC3000 should serve better for tracking health because - unlike the AC 1kHz test - it will incorporate some of the resistance components with longer time constants (but not all of them since the test duration is too short).

    It would be interesting to see how the AC test compares to the MC3K pulse test on a wide variety of cells. For example, what's the largest difference between the two that you've encountered?
    When I get back home tonight, I will do an extensive test with the MC3 and the SM8124A on all my cells, which is not that much really, just 21 at the moment and another 12 on the way and will post back here the largest difference between the two readings. Going to test all my 26650's as well and see if everyone has a stable reading in the mc3 and what's going on with the 18650's why they don't.

  22. #22

    Default Re: IR threshold between cells that should not be paired anymore?

    Tested all my cells and after taking the lowest readings of any cell in the MC3000, which consisted of rotating the batteries in their bays and swapping them in other bays, the results have been always consistent. All batteries have shown IR readings to be 60 to 62 % higher in the MC3000 vs the SM8124A tester. As I get more cells and as they age, I will continue to do this test to see if it still remains the same. If so, then I could just add 60% to the sm8124a figures to get a fairly accurate reading of how the batteries are wearing, or at lest it could be useful for cross checking to identify if there is a problem with either the meter or the charger.
    Last edited by klrman; 07-05-2018 at 03:09 AM.

  23. #23
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    Default Re: IR threshold between cells that should not be paired anymore?

    Quote Originally Posted by klrman View Post
    Tested all my cells and after taking the lowest readings of any cell in the MC3000, which consisted of rotating the batteries in their bays and swapping them in other bays, the results have been always consistent. All batteries have shown IR readings to be 60 to 62 % higher in the MC3000 vs the SM8124A tester [...]
    Most likely as the cells age that 60% excess will increase since - as I explained above - the AC test typically does not include the IR components that increase the most during aging (e.g. charge transfer resistance), but the MC3000 does include (some of) these. I've seen reports of much higher differences between these types of IR tests. You might try some old/unhealthy cells to see such.

    To get consistent IR measurements in the MC3000 (or similar cylindrical cell chargers) it helps to verify that the batteries aren't loose (with high contact resistance) by ensuring that they don't rotate if you run a finger across them with very slight touch (if they do rotate then adjust them to get a tighter fit).
    Last edited by Gauss163; 07-05-2018 at 08:32 AM.

  24. #24

    Default Re: IR threshold between cells that should not be paired anymore?

    Thanks for the info. Yes, the cells were tight and hard to rotate. Unfortunately, I only have newer cells at the most a year since I purchased them. Not a great reference but as time goes by I can record what is going on and post back here. Will ask some of my friends if they have any real old cells that I could test as well to compare. That would be a good start I think. Again, thanks for all the help Gauss, I learned a lot in the last week or so, very helpful.

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    Default Re: IR threshold between cells that should not be paired anymore?

    ^^^ A good source of old/unhealthy cells are older laptop batteries (there are threads here about how to safely harvest cells from laptop batteries).

    It would be interesting to collect further data showing the differences between the AC and DC IR tests on common testers. Currently there is little data available - probably because few people have access to both. Such data would prove useful for comparative analysis when making purchasing decisions. Iirc HKJ has some of the AC testers so maybe he'll post some data in the near future.

  26. #26

    Default Re: IR threshold between cells that should not be paired anymore?

    Good idea. Just called a local computer shop and he said he has a box of old laptop pulls, so I'm going to pick a couple and see what readings we can get out of them.

    Should I attempt to revive and charge then first before testing for final IR with the mc3000 and sm8124a?
    Last edited by klrman; 07-05-2018 at 05:51 PM.

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