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Cells with different cut off
#1
Hi.

Testing a lot of cells. Half of them go to 2.75 the other half to 3.0

Since im building many packs for a large 20 kWh power wall, I don’t see any reason to let them discharge below 3.0 in my capacity test. Do you agree? Capacity will ofcouse be limited, but that doesn’t matter since I will never be able To use the whole capacity??

Got 1.200 cells like that, and 1.200 of the other kind.
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#2
Capacity test should always be full cycles if you want anything to compare to original numbers.
Note that you arent testing useable capacity but the capacity comparing to new so you can sort them

The useable capacity is easiest tested afterwards when you have built the packs.

Once again you dont test for future capacity but for weading out cells that are out of the scope.
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#3
And, depending on your use..   such as a Solar system battery bank where you're charging/discharging often...  you will probably do as I learned over time - so set the minimum just ahead of the sharp drop in the discharge curve...   typically 3.5-3.6v/cell as the low point.   So 3.0v vs 2.8v is not important as long as cells are evenly distributed among your pack.
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#4
(12-29-2019, 08:18 PM)OffGridInTheCity Wrote: And, depending on your use..   such as a Solar system battery bank where you're charging/discharging often...  you will probably do as I learned over time - so set the minimum just ahead of the sharp drop in the discharge curve...   typically 3.5-3.6v/cell as the low point.   So 3.0v vs 2.8v is not important as long as cells are evenly distributed among your pack.

Yes I was actually planning on doing just that, but mainly because I wanted the cells to last looong. So maybe going to 4.05 and down to 3.2 but you’re correct, I should look at the courves.

(12-29-2019, 04:17 PM)daromer Wrote: Capacity test should always be full cycles if you want anything to compare to original numbers.
Note that you arent testing useable capacity but the capacity comparing to new so you can sort them

The useable capacity is easiest tested afterwards when you have built the packs.

Once again you dont test for future capacity but for weading out cells that are out of the scope.

Hmm okay, I thought I was testing for future capacity, and weeding out cells that got too hot or didn’t
Hold their voltage.

How do you think I should sort them then? Like keeping all cells which are above 80% of new capacity or something like that ?

Thanks
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#5
(12-29-2019, 09:19 PM)M1kkel Wrote:
(12-29-2019, 08:18 PM)OffGridInTheCity Wrote: And, depending on your use..   such as a Solar system battery bank where you're charging/discharging often...  you will probably do as I learned over time - so set the minimum just ahead of the sharp drop in the discharge curve...   typically 3.5-3.6v/cell as the low point.   So 3.0v vs 2.8v is not important as long as cells are evenly distributed among your pack.

Yes I was actually planning on doing just that, but mainly because I wanted the cells to last looong. So maybe going to 4.05 and down to 3.2 but you’re correct, I should look at the courves.

(12-29-2019, 04:17 PM)daromer Wrote: Capacity test should always be full cycles if you want anything to compare to original numbers.
Note that you arent testing useable capacity but the capacity comparing to new so you can sort them

The useable capacity is easiest tested afterwards when you have built the packs.

Once again you dont test for future capacity but for weading out cells that are out of the scope.

Hmm okay, I thought I was testing for future capacity, and weeding out cells that got too hot or didn’t
Hold their voltage.

How do you think I should sort them then? Like keeping all cells which are above 80% of new capacity or something like that ?

Thanks

>I wanted the cells to last looong. 
Its not 'going down to 3.2 (alone) that makes cells last - its also how far up you charge them.   Here's the famous Battery University link that I'm designing toward...   "How to Prolong Lithium-based Batteries" - https://batteryuniversity.com/learn/arti..._batteries    Notice that the lower the top charge voltage the longer the life.   For me, I'm targeting for the 75-45% SOC line in this graph (with my design/settings) and hoping I truely get 1,000(s) of daily charge/discharge...  Smile


>I thought I was testing for future capacity,
I think he's just saying its best to put them into a pack and then test for actual capacity.  Adding up individual cells (before putting them into a pack) is not as accurate.    I build my packs by adding up individual cells...   but I have to agree that after I put them on line, some of the packs are as much as 5% 'off' and I have to add a few more cells to bring them up to.    

>Like keeping all cells which are above 80% of new capacit
Yes - we all have established out own 'threshold'.  Mine is 85% of new - but then I buy pre-tested, used (for the most part) so I know ahead of time they will be in this range.  Doing 'old battery tear-down' from scratch makes it tempting to lower the threshold because of all the work Smile

>How do you think I should sort them then? 
Sort them by 100mah(s)  -   3000 in 1 pile,  2900 in 2nd pile,  2800 in 3rd pile, and so on.  Then distribute them evenly thru the packs of your battery.   Works like a charm.  No problem with 'weaker ones' mixed in with 'stronger ones' as far as functionality or life-span.   Especially when you talking 80% or 85%+ range...  






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#6
Offgrid explained it very well.

You test for current useage span and how they work today.

The mixing of 100s ensures you Will have a Good setup. IF ur picky retest all packs before final usecase and see their total acapacity but it wont help you much.

Used cells age differently and a month later they Will have drifted more.
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#7
Tongue 
(12-30-2019, 09:45 AM)daromer Wrote: Offgrid explained it very well.

You test for current useage span and how they work today.

The mixing of 100s ensures you Will have a Good setup. IF ur picky retest all packs before final usecase and see their total acapacity but it wont help you much.

Used cells age differently and a month later they Will have drifted more.

Great, thanks for sharing your knowledge, both of you. Great forum and source of information. :-)
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