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Cell capacity decreasing over time?
#1
Hi all,

I started collecting cells in Mai 2018 after I understood the recycling of old Laptop Batteries.

Now I started to build my packs and I realized that the cells I tested in the early days have lost a lot of their capacity.
For example, a cell initially tested about ~1500mAh between 4.0 V to 3.5 V now has only ~1150mAh.

Is it because of aging, or what can be the reason?
When I stored them they had about 3.5 to 3.7 Volt, which they still nearly have, so they did not discharge by themself.

Does it mean, I should not let them rest for such a long period?
I don't expect them to lose their capacitiy when they are in use, especially only between 3.5 and 4.0 Volt, or do they?


regards

cadric
1. Grid-Tie System 09.2017 16,5kWp:  55 x 300Wp BenQ, SMA Tripower 15000-TL30 (18-east/17-south/20-south)
2. Offgrid System 09.2019 7,5 kWp: 25 x 300Wp Solarworld (5x( 3S + 2S )), 2x PIP-5048MS, Batrium WM4, 1st Battery Bank: 840 cells in 14S60P with 80Ah cap.
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#2
It seems unlikely to me that they would lose that much capacity just sitting.
Have you considered other variables?
Are you using the same charger, capacity tester, same conditions?
Is the tester ok? I have seen some posts about testers reading the wrong voltage.
What does a test from 4.2v down to 3v show?
I have noticed a drop in capacity when testing cells at colder temperatures compared to warmer temperatures.
thunderheart likes this post
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#3
As Oz told, it's very important to test in the same conditions (tester, holder, temperature, etc...). Whatever, capacity drop from 1500 to 1150Ah in 4.0-3.5V range seems too big to me too.
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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#4
Hi,

and thanks for your response.

For Charging and Testing I use two Sky RC 3000.

Ok, the first tests with this cells I did in November 2018, the shed was not warm at that time if I can recall correctly,
so maybe it is because of the difference in temperature?!
But that would mean, it was colder on the first check but the capcity was much higher, which also does not make sense.
So next step is to check with cells I did last year in June/July to see if there is any difference.

regards

cadric.
1. Grid-Tie System 09.2017 16,5kWp:  55 x 300Wp BenQ, SMA Tripower 15000-TL30 (18-east/17-south/20-south)
2. Offgrid System 09.2019 7,5 kWp: 25 x 300Wp Solarworld (5x( 3S + 2S )), 2x PIP-5048MS, Batrium WM4, 1st Battery Bank: 840 cells in 14S60P with 80Ah cap.
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#5
Could you have been using different rate of discharge during previous testing compared to now?
Are both skyrc devices giving the same results as each other?
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#6
(07-22-2019, 10:36 AM)Oz18650 Wrote: Could you have been using different rate of discharge during previous testing compared to now?
Are both skyrc devices giving the same results as each other?

no, always charging and discharging with 500mA only.
yes, they both are in the same range, with a little difference but not to mention.
1. Grid-Tie System 09.2017 16,5kWp:  55 x 300Wp BenQ, SMA Tripower 15000-TL30 (18-east/17-south/20-south)
2. Offgrid System 09.2019 7,5 kWp: 25 x 300Wp Solarworld (5x( 3S + 2S )), 2x PIP-5048MS, Batrium WM4, 1st Battery Bank: 840 cells in 14S60P with 80Ah cap.
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#7
For storage is temperature very important.
Also the voltage, the optimum voltage storage would be 3.4v.
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#8
Is this happening with a particular brand or model of cell?  I experienced the same thing with sony green cells but others using sonys didnt have the issues I experienced.  
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#9
(08-02-2019, 02:06 AM)jm1 Wrote:
Is this happening with a particular brand or model of cell?  I experienced the same thing with sony green cells but others using sonys didnt have the issues I experienced.  

NO, cells are mixed, no special brand.
1. Grid-Tie System 09.2017 16,5kWp:  55 x 300Wp BenQ, SMA Tripower 15000-TL30 (18-east/17-south/20-south)
2. Offgrid System 09.2019 7,5 kWp: 25 x 300Wp Solarworld (5x( 3S + 2S )), 2x PIP-5048MS, Batrium WM4, 1st Battery Bank: 840 cells in 14S60P with 80Ah cap.
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#10
(08-02-2019, 09:33 AM)cadric Wrote: NO, cells are mixed, no special brand.

You say that the cells have originally tested at ~1500mAh between 4.0V to 3.5V.
First of what was the original mAh rating of the cell? 
Second what was the IR of the cell? (Proper IR measurements can only be done with a 4 wire kelvin tester such as a YR1030)
If you are testing a 2800mAh cell and you are getting 1500mAh that cell has degraded to 53.57% of its original capacity.
I would also venture to guess that the IR of that cell is in the stratosphere so the loss of mAhs on the second test after sitting would not be a surprise to me.

Wolf
Ramos1970 likes this post
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 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
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