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Behaviour of cells in parallel take 2
#1
Another experiment has taken place at the Wolf's PowIRwall location.
Previous posts on this subject are at https://secondlifestorage.com/showthread.php?tid=7846
I dragged out my 4p testing rig and decided to test 4 ICR18650-28A cells that had a reasonable IR at ≈ 50mΩ but their mAh testing was in the low 2000mAh with the SKYRC tester.

cell1 4.176V 52.74mΩ 2089mAh
cell2 4.178V 51.20mΩ 2141mAh
cell3 4.178V 53.73mΩ 2072mAh
cell4 4.180V 53.30mΩ 2043mAh

As predicted the discharge of all cells was relatively even as the IR and mAh were closely matched so no cell was stressed more than the other. Temperature did rise about 4°C toward the end. Temperature on cell 1 and cell 4 was less as they did not have a cell on both sides such as cell 2 and cell 3.
 
Since these cells have a spec sheet of max 4.30V full charge and cutoff at 2.75V. I will run this test again with those parameters and see what happens under those circumstances.
Pictures of my testing lab.
  

Wolf
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#2
Hey Wolf

I´m currently in the process of assembling my 80-100p packs to get an even overall capacity.
something that came into my mind was the following:

I try to bring my cells together by internal resistance and not by there capacity to minimise the loadsharing and balancing curent between the 80 cells in one pack.
But the thing is the following:
If we have 2 cells with the same internal resistance but different capacity how does the loadsharing look like?
Cell 1 has 1500mAh
Cell 2 has 3000mAh

Do you have a nice graph to show that? But maybee I just need another coffee to find the solution Wink
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#3
PAF,

Good question.

Let me see if I can locate 4 cells with that criteria and run a test.

Wolf
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#4
the 3000 will in average have 2x the current. BUT this depends on the internal of the cells and will if different vary over time.
Ie in beginning one have 1.5x more than the other but in the end perhaps 2.5x

Several tech sheets out there talk about this and its also pure electrical theory.
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#5
A Comprehensive Approach for the Clustering of Similar-Performance Cells for the Design of a Lithium-Ion Battery Module for Electric Vehicles

https://www.sciencedirect.com/science/ar...9919307829

The SVC algorithmIn general, a support vector machine (SVM) is adopted for clas-sification (supervised learning). SVC is a slightly different algo-rithm from an SVM. In fact, SVC is an unsupervised learningclustering algorithm.The main idea of SVC is to map data space to a high-dimensionalfeature space using a Gaussian kernel function. Next, a sphere witha minimum radius is obtained and the sphere contains most of themapped data[41,42].

and

Analysis of the Effect of the Variable Charging Current Control Method on Cycle Life of Li-ion Batteries

https://res.mdpi.com/d_attachment/energi...-03023.pdf

Both of these are worth a read ..
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#6
PAF,

I have managed to find 4 cells of similar IR but widely differnt mAh.


Here they are:

They both have a maxV of 4.2 and a 2.5V cutoff so we will run them trough the gamut.
Let see what the Discharge tester will show.

Wolf
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#7
Those are made to be charged to 4.35V, so you can actually get a little more mAh out of them.
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#8
Hm maybe I got the wrong datasheets.
This is what I came up with.
Maybe you have some different ones that you could share with us.

UR18650H



LGABC41865 (INR18650C4)



Let me know if I got it wrong.

Thanks

Wolf
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#9
I was referring to ICR18650-28As.

Specs of the 30A here:
http://gamma.spb.ru/media/pdf/liion-lipo...50-30A.pdf
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#10
(06-09-2020, 11:36 AM)Overmind Wrote: I was referring to ICR18650-28As.

Specs of the 30A here:
http://gamma.spb.ru/media/pdf/liion-lipo...50-30A.pdf
Aha OK that makes sense I did a test on those in the begining.

Wolf
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