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Testing cells by spec sheets.
#1
I have arrived at a dilemma and need some input and maybe come to some conclusion with your help.
As I am adding the Standard Charging , Standard Discharging Amps or for those who prefer C, Max V and Cutoff V into my cell database I am finding some variation on those parameters. By the way Daromer you are right as I have delved into these spec sheets I have found although sometimes hidden the recommended IR (there I said it but no more in this post) of the cell.
So here is what I have found. I'm sure that all of you know this but nevertheless it is relevant to this discussion.
We are dealing here with 18650 cells that in general have a max V of 4.2V and a 3V cutoff.
So lucky us we have chargers/testers that charge the batteries to ~4.2V and then most run a discharge cycle to ~2.8V and then give us a mAh result.
For me if it gives me a result of at least 80% of capacity I am happy knowing that it might be a little inflated because of the discharge to 2.8V instead of 3V but at the bottom end if there is 50mAhs left I would be surprised.
So it just so happens that I tested a set of 4 ICR18650-28A cells (actually 40) on my Opus and LiitoKala and the results came back less than stellar.
I was curious as other factors (which I won't mention) :P  Indicated that this cell should actually be within a reasonable margin of "good".
Looking at the spec sheet the Max V on this cell is 4.3V and the cutoff is 2.75V. 1400mA or 0.5C standard charge and 560mA or 0.2C standard discharge.
Well those settings are not within my "normal" charger/tester parameters. Out comes the SKYRC MC3000. Program it to all the parameters and well the results are different. Enough so as to qualify a cell good or not good for me.
Results are as follows
Initial results
 

SKYRC results


The question then begs to be answered are we wrong in testing all cells "generically" with just the built in given parameters and because of that do we get falsely rated cells showing to be underperformers?
I also considered that hey we are running a powerwall more than likely at ~4V and discharge to ~3.3V so what does that matter we dont get close to the Max V and or Cutoff V. Will the cell operate just fine within those parameters? I do believe so.
But with my strict criteria of 2200mAh and 80% capacity those cells would never make it but yet when tested at the "Proper criteria" would be very happy in my powerwall.
So you see my dilemma here. Or am I over complicating a simple solution here?

Wolf
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 5/19/2019
Total Number of Cells                          5335
Cells  >80% of Capacity                      3744
Cells <80% of Capacity                       1546
Cells ≥2200mAh & ≥ 80% & ≤75mΩ    2593
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#2
Stick to One way of testing. We talk about thousands of cells. As Long as your min criteria is on a decent level i doubt that 100* extra time Will save you any capacity over time. And those saved hours you can spend om something else.


Yeap ir can be found Wink
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#3
(02-08-2019, 02:12 PM)daromer Wrote: Stick to One way of testing. We talk about thousands of cells. As Long as your min criteria is on a decent level i doubt that 100* extra time Will save you any capacity over time. And those saved hours you can spend om something else.


Yeap ir can be found Wink

Well yes I do understand that except that I am going through a rash of samsung and sanyo high mAh cells right now and looking back at the ones I have recorded there are a lot of them not thousands but at least several hundreds that have as high as 4.35V max and 2.75 cutoff  that would appreciate a ~5% boost in value and considering for me 200 Cells would be 1 link in my 14s200p.

Now looking at these high mAh cells with a closer eye I do also see that when they fall they fall hard. Do you think it might be something to do with the manufacturer stretching the MaxV to 4.35V and the cutoff to 2.5V?
It sure looks like they abuse the cell chemistry to the max to get the most bang for the buck so to speak and not necessarily for longevity.

Wolf
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 5/19/2019
Total Number of Cells                          5335
Cells  >80% of Capacity                      3744
Cells <80% of Capacity                       1546
Cells ≥2200mAh & ≥ 80% & ≤75mΩ    2593
For Info Google Drive
Not your average Wolf       
            Cool



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#4
The normal low cut off is 2.5-2.8. Its rare that you have anything else.

I doubt you gain anything. I would for sure not change the testing. If i would do anything if you think they fail on the test due to 100mAh to low result (I sort in 100mAh buckets to get rid of the errors...) you can just add 5% if thats what you think they are lower due to not tested to 4.35%


Thing is if you do test them differently you NEED to do a 2nd test to know the capacity. For me im ONLY interested in the capacity in between 2 voltage spans and thats used for the build.

So for instance all normal 4.2V cells they need to be 2Ah or up
And all 4.35v they need to be 1.9Ah or up

Above assumes the capacity when new is the same and thus above would make sure you dont miss anything due to the test.

You know me. I would not spend more time than needed during a test phase since 10 seconds per cell or even 3 minutes extra time would in the end result in days for me Wink
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#5
Last time i proposed using only the "real" Voltage swing (like 3.3 -4.0) i got a bloody nose from Daromer. Big Grin 

So, i second his opinion of taking some general value like the Opus and live with the slight deviations.
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#6
Sorry for that Cherry :=)

There is of course a use testing only a general swing in the middle but then you need to understand what the value you get is compared towards Tongue
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#7
(02-08-2019, 01:58 PM)Wolf (my emphasis) Wrote: [...]
I also considered that hey we are running a powerwall more than likely at ~4V and discharge to ~3.3V so what does that matter we dont get close to the Max V and or Cutoff V. Will the cell operate just fine within those parameters? I do believe so.
But with my strict criteria of 2200mAh and 80% capacity those cells would never make it but yet when tested at the "Proper criteria" would be very happy in my powerwall.
So you see my dilemma here. Or am I over complicating a simple solution here?

wow, what a great thread... : )  this is indeed a dilemma.  i think this problem you have described is very much a style thing, and one of the many things we all do differently.  perhaps there is no 'best practice' here..  ultimately all testing should be done the same if possible on all cells, so that the results are comparable (and therefor meaningful).

another thing to remember is that all cells age, and they do it  differently each cell.  so no matter how accurate our measurements are, they will always be dated, sooner or later.

my solution is very simple -- i dont test for IR (much) and i dont compare the results of capacity tests with specs for capacity on cell when it is new.  sort of like sticking my head in the sand, but hey, i sleep very well at night, still have my hair, and the batteries i make are very stable, so ...
---    ----    ----    ----    ----    ----    ---
nothing to see here ... move along
---------------------------------------------
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#8
It seems there's two different criteria:
a) testing the cell against manufacturers spec (to understand its new vs now capacity left % & done to the manufacturers voltages).
b) testing the cells to the voltages you'll be using in your system (so you know the mAhr storage you'll actually be getting out of it, but it's a reduced number vs the manufacturers spec).

You can estimate a from b but it's not a linear thing & will vary with cells types (but hey, everything does anyway!).
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#9
Its better to estimate b from a... The other way around just fool you of the SOH. Whats the most important factor in a battery bank you build?

To know the state of health and be able to tell how Long it could last?
Or
Be able to know they current capacity betweeb 2 voltage intervalls and No idea its true soh?

The later can easily be tested when in use later on
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#10
This is almost like an Android vs. Apple debate. There is no winner.

I would say that most on this forum use whatever their tester tests for, and use that number to build a balanced series of cells. I'd mostly agree with that way, too.

I can see how SOH can be important because 2Ah from a 3Ah cell is very different than 2Ah from a 2Ah cell. But you can never win with SOH as it adds more complexity to the pack building without any real advantage. Like are you going to only use > 2Ah cells that have at least 80% capacity left? With used batteries, you will probably be rejecting more than half your cells that way unless you only deal with new old stock packs. And unless you will use your powerwall at the same rate of discharge as your tester, you are probably rejecting more cells that could be useful. Most powerwalls probably use around 0.2A per cell (just a guess), so unless you test at 0.2A, you are not really getting the whole picture. Of course, who wants to spend 13 hours discharging one 2.6Ah cell? But that's what it would take to get the most useful data. Because a tired cell that has like 50% of its life left at 1A discharge, might have a lot more capacity at a 0.2A discharge. But even that isn't great, because tired cells with higher IR's usually have a big voltage drop under load, and most of the capacity would probably come in at <3.5V. And if the good cells in a pack are mostly discharged by the time they get to that 3.5V, the tired cells will be disproportionately drawing more current. So maybe SOH sorting is important after all? I don't know. All I know is that the people on this forum with the biggest packs in use the longest time swear by absolute numbers obtained by consistent testing. As long as you test every cell the same and balance your pack based on those numbers, you should be fine.
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