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"Generic" Cells - Are they any good?
#41
(06-23-2019, 11:19 AM)ozz93666 Wrote: I tend to agree , but this is a comparative test .. As long as same procedure was used throughout these are accurate results ...

As far as Cell A is compared to Cell B and C - yes, it's comparative and it's OK, but for checking datasheet's numbers you need absolute results. If the datasheet tells 2.5V cut-off, then you must discharge down to 2.5V. If it tells "10min pause after charge and 30min pause after discharge" - you must follow the instructions to compare your results with manufacturer's ones.

(06-23-2019, 11:19 AM)ozz93666 Wrote: Contact resistance due to cell not being held firmly won't effect capacity readings significantly

I'm talking about the material the contacts are made from, not the power of holding the cell. Compare the voltage drop from the cell to Opus' PCB with the same if the contacts were replaced with pure copper ones.
LiFePO4: QB26650 2500mAh 50A vs A123 ANR26650M1B: https://youtu.be/GOSkte11lRc

Yinlong LTO Lithium Titanate safety tests: https://youtu.be/eAUYbSDEy6I
Toshiba SCiB LTO Lithium Titanate safety tests: https://youtu.be/XsrRDZxEFQE

XTAR DRAGON 4-slot battery tester review: https://youtu.be/S6yVMsIuauE
Miboxer C4-12 4 slot x 3.0A charger review: https://youtu.be/X-ww1YALjvU
Miboxer C2-4000 smart battery tester review: https://youtu.be/jrbJceNXv1g

Samsung 33J for TESLA: https://youtu.be/7FMxgMmPeh4
Samsung 40T - a high drain 35A 21700 cellhttps://youtu.be/lxgKWiQ9580
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#42
Thunderheart, you are correct about the Opus, and we have gone over its shortcomings on the first page of this thread.

We respect your testing as reliable on this board - enough to give you a sticky for your testing thread. You have gone through great pains to make sure that you are testing cells most accurately to their data sheet.

This test, however, is a bit different. Its sole purpose is to track the degradation of 4 cells over a number of cycles. And 3 out of 4 of the cells in this test have no data sheet, and that was part of the inspiration for this test. The full testing method has been described in the Original Post of this thread. I've taken care to eliminate any sort of inaccuracies of the tester, even alternating the slots every 25 cycles.

So, for this test, consistency is more important than absolute capacity numbers.

With regard to the smooth versus jagged line, I believe manufacturers use some smoothing on their degradation curves, as well as testing multiple cells and averaging their plots. If I plotted the average of every 5 cycles (81 data points) instead of the results of 405 cycles (405 data points), it would be a lot smoother. Most of the variances come from the cell resting overnight, or possibly the IR of the cell, but it's never that extreme (besides Cycle 35 where I used an external fan to cool the cells and had like a 3% variance).

ozz93666, I've been testing these cells almost 5 months straight, and I'm only at 405 cycles. If I were to test these cells only 50 times a year, It would take me over 8 years to get to this point. I do think time does play some role, but mostly it's the time cells sit at full charge that makes the most difference. All of the cells in this test are about 8 years old, and I do have some 20 year old Sony's that I've tested that still hold 90%+ of their capacity with an unknown history. Since the cells internals are sealed, I can't see why the electrolyte would break down over time, but everything does have a shelf life.
thunderheart, stevelectric, ozz93666 like this post
Formerly known as Dallski
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#43
(06-23-2019, 08:12 PM)Generic Wrote: This test, however, is a bit different. Its sole purpose is to track the degradation of 4 cells over a number of cycles

In that case everything is OK as all 4 cells are in equal conditions.

Once again, you've done a GREAT job and i can just imagine the time and other resources spent on it!
LiFePO4: QB26650 2500mAh 50A vs A123 ANR26650M1B: https://youtu.be/GOSkte11lRc

Yinlong LTO Lithium Titanate safety tests: https://youtu.be/eAUYbSDEy6I
Toshiba SCiB LTO Lithium Titanate safety tests: https://youtu.be/XsrRDZxEFQE

XTAR DRAGON 4-slot battery tester review: https://youtu.be/S6yVMsIuauE
Miboxer C4-12 4 slot x 3.0A charger review: https://youtu.be/X-ww1YALjvU
Miboxer C2-4000 smart battery tester review: https://youtu.be/jrbJceNXv1g

Samsung 33J for TESLA: https://youtu.be/7FMxgMmPeh4
Samsung 40T - a high drain 35A 21700 cellhttps://youtu.be/lxgKWiQ9580
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#44
(06-23-2019, 01:16 PM)thunderheart Wrote:
(06-23-2019, 11:19 AM)ozz93666 Wrote: I tend to agree , but this is a comparative test .. As long as same procedure was used throughout these are accurate results ...

As far as Cell A is compared to Cell B and C - yes, it's comparative and it's OK, but for checking datasheet's numbers you need absolute results. If the datasheet tells 2.5V cut-off, then you must discharge down to 2.5V. If it tells "10min pause after charge and 30min pause after discharge" - you must follow the instructions to compare your results with manufacturer's ones.

(06-23-2019, 11:19 AM)ozz93666 Wrote: Contact resistance due to cell not being held firmly won't effect capacity readings significantly

I'm talking about the material the contacts are made from, not the power of holding the cell. Compare the voltage drop from the cell to Opus' PCB with the same if the contacts were replaced with pure copper ones.
 This test is all about measuring reduction in capacity over cell life , measuring the % drop , so using opus will not effect things too much , particularly at low discharge currents  ....

My instinct tells me the material the contacts are made from will not have a significant effect ...
Lets do the calculation just to check .... your terminals are 10mm x 0.5mm thick so that's 5mm2 of copper  

Each 1 cm has a resistance of 0.134 mOhm ... 

but that is not where the resistance is , you  connect onto these buss bars with crocodile clips ... Danger High resistance (a few mOhm ) ... much better to solder (Zero mOhm ) , and then thick  wires connecting to your apparatus , probably around 5mm2 ,same as bus bar  but total length looks about 60cm so resistance of wire is 8m Ohm ... 

You have about 10mOhm between battery and measuring device ... How will this effect readings ... it will effect high current discharge much more ...@ 5 A ...   I x  I x r  =  0.25W    So @ 5A  the connectors are disappearing 0.25W over a 1/2 Hr discharge resulting in a 0.125WHr error in capacity measurement ......  at 1A discharge the error will be 5 times lower 0.025WHrs . these losses are totally predictable , perhaps the measuring device has already adjusted readings??

Compared to opus , lets suppose their contact terminals are 10 time more resistive than yours , still that will not have much effect on things because they are very short , connecting wires will be much thinner , but also much shorter ... the killer lies in the weak springs and type of contacts  used to force connection  onto the cell this can cause many 10's mOhm resistance.
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#45
Thanks for your input Thunderheart! Honestly, if I added up all the actual time I spent on this, it wouldn't be more than a few hours. It's just writing down the results every cycle, taking a picture of those results, and resetting the Opus. If all that took a minute, that's 6.75 hours for 405 cycles. Not bad. And its just 1 Opus, so not much material cost either.

oz93666, I know that the Opus overstates the results. The three generic cells are all 2000mAh cells and the Opus was showing almost 2200mAh when the test started. The point is, like you mentioned, that the contact resistance in the cell holders does not change. Whatever that resistance is, does not matter as long as it stays the same throughout the whole test. The only variable I'm testing for here is degradation.
Formerly known as Dallski
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#46
2.5V cut-off does not mean you should normally discharge the cell down to that value. That's the red-alert-emergency-shutdown value. I would never recommend discharging any cell under 3V if you want some life out of them. I always try to keep them above 3.3V as per the original Li-Ion specs.

Yes, some manufactures consider that discharge value when stating capacity, which is not really fair, but this forces you to indeed consider it when testing.
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#47
I did my own amateur comparative testing with my Opus BT-C3100 v2.2 on my haul of dead laptop batteries. Of the 150 cells I pre-tested so far, the LG batteries are consistently worse than others in terms of internal resistance at max charge. (To me it's a quick rough indication of capacity because it's inversely correlated to IR)

Therefore, I'd be much interested if your control test used a Panasonic or Samsung instead Big Grin. Either way, thank you for the rigorous tests! I also get some ASOs in my haul and those are also quite promising in my prelim max-charge IR tests.
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#48
Awesome test Generic! I'm happy to see cells degrade indeed a bit slower then I thought they would.

Do we have some idea as to how much not fully charging/discharging is prolonging there live? And what limits do most people use?
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#49
Thanks for doing these tests and sharing your results.

Back in 2014 I started an individual effort to improve characterization of scavenged cells. I think I was the first english speaker to get a ZKE tester and share info about it, I wrote and shared some code to extract BMS data from laptop packs, and had preliminary results showing that self-discharge rate wasn't a very useful proxy for the cycle-age of cells. I'd hoped to grow a community around it, but I ran out of steam and other priorities took over. I didn't have the impact I'd hoped, but it's great to see that that so many people have taken up the charge on their own.

I hadn't given much attention to "generic" cells, but I suspected that the conventional wisdom that they were junk was probably obsolete even then, and if it wasn't, it soon would be due to ongoing improvements in Chinese manufacturing. You've certainly shown that they should be taken more seriously.

You'd done a good job of refuting those who suggest these tests aren't valid or useful because you are using an Opus, but I'm going to add my two cents, anyway. A "professional" knows that there are multiple dimensions to the utility of any measurement instrument. Off the top of my head, the key considerations are cost, accuracy, precision, and repeatability/consistency. If the cost of an instrument is low enough, two or more can be used. Accuracy can be corrected with a few measurements and simple math, or simply factored out by using relative measurements. Precision can be addressed with larger sample sizes, or, for many purposes, by accounting for uncertainty, and not drawing conclusions unsupported by the precision of the data. Consistency can also be addressed with samples size and uncertainty propagation.

The one problem I see with using the Opus in a test like this is that the effects of systemic sources of error can accumulate over each cycle. For example, the temperature difference between slots means that each cell has different thermal conditions, which is known to impact cell aging.

Also that small differences in the measurement of voltage between channels can impact charge/discharge termination points, and charge/discharge current. This has some impact on capacity measurements, but that is relative. The real issue is that the cell in one slot may be pushed a little further than the other. The difference may only be 0.1% per cycle, but that increases exponentially, so after 50 cycles it grows to 5% (1.001^50-1.0^50).

You've addressed these by switching slots, but doing it more often might be a good idea. I also think that spinning the cell in the slot slightly between cycles could even out the impact of small variations in contact surface/resistance.

The "jaggedness" on the measurement curve are probably influenced at least in part by differences in ambient temperature between measurements. Temperature differences impact both the cell performance, and also the measurements of voltage, current and charge.

As for why these cells are holding up better than expected, one reason is probably just the nominal values from datasheets are close to worst case. They give their industrial customers the information they need to hit the performance and reliability specs demanded for their application. It's bad for profit margins to give away too much, but they can be improved in the future. On the other hand, fail to deliver on specs will kill the business. So, the datasheet values are surely conservative. Over time, though, as the manufacturers get better better at consistency, they'll be able to narrow the gap between the specification and the actual performance of a typical cell.

It's good to have a sense of both the testing system, and the application of its results, because it helps keep perspective about what's good enough (for now, at least).
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#50
(06-26-2019, 04:49 AM)Generic Wrote: The only variable I'm testing for here is degradation.

I was just wondering how stable temperature has been during your testing?
I am doing some degradation testing on a couple of cells too, and I have noticed that temperature seems to make a noticeable difference to the capacity results.
Just from observation, tests overnight (approx 5-10 degrees C) were lower than tests during the day (approx 20-25 degrees C)
An overnight test might show 1950mah while a daytime test might show 2050mah.
I have started recording time and test start temperature along with my tests.
What are temperatures where you are doing your testing? How stable is the temperature? And have temperature ranges changed over the period you have been testing (eg seasons)?
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