Choosing cells for modules/packs (grouping cells)

italianuser

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Feb 25, 2020
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130
I read a lot about choosing and grouping cells and found many different approaches. I would like some parameters from your to make my modules while I'm waiting for the first 400 cells to arrive from Ebay seller (I'm bit worried about his reviews, many good ones but many bad ones AHHH :censored:, fingers crossed) . Meanwhile I'm harvesting, desk is filling up with cells. My final goal is to have 4 modules 14S20P (@51.8V) but I did some calculations and could increment the P number to obtain 2.4KWh modules and a big total of 9.6KWh.

My choice is to stay around 2200mAh cells, but what is a good range to stay in? Plus/minus 200mAh? I see some people have cells with big differences in mAh in the same module but it seems reasonable to keep this difference as low as possible. Using second hands cells this could be difficult that's why your experience is so important.

Same question for IR. @daromer has an 80mOhm limit which seems reasonable, but being my first pack I prefer asking. Does it make sense to group all 20-40mOhm cells in a module, and then group 41-60mOhm in another module and so on?

TIA:giggle:
 

Korishan

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You want each pack to be similar Ohms as a whole. You don't want high current flowing easily through 1 pack, and then being held up at another one. This will cause excessive heating of that pack with the higher overall IR.
With that said, you also don't want too much difference within the pack, either, for the same reason. You don't want high current flowing through some cells, and higher IR cells slowing it down. In this case, the heating will be the other way around. More current than expected will flow through the lower IR cells, causing them to heat up, and ultimately shortening their life.

So overall, you want a fairly balanced IR across the cells in a pack, and across packs. +/-10-Ohms, probably won't make a difference. 50-Ohm difference will.
If you find that you have a majority of 100-ish Ohm cells, then build your packs around that number. If they are around 50 or so, then go that route.
The key is to measure IR and capacity before building any packs. This is the only way to get a very uniform battery build.

As far as measuring IR, it would best to measure it first, as it can weed out potential heaters or otherwise problem cells. This will save you time, energy, and power not having capacity test those. You can look at @Wolf 's threads on this topic for more information.
 

italianuser

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Feb 25, 2020
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130
Thx @Korishan I guess I'll have IR all over the place from 30 to 120mOhms. I'll start measuring IR next week, just bought 10 minutes ago a YR1030 from Germany, looking forward to that.
 

floydR

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Aug 23, 2017
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cool I think you will be happy with it. Much easier than the opus and much more accurate Than testing with a 4 slot tester. Make a slide cell holder if the YR1030 didn't come with one.
later floyd
 

Wolf

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Sep 25, 2018
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I'll start measuring IR next week, just bought 10 minutes ago a YR1030 from Germany, looking forward to that
Good choice! The YR1030 was my first IR meter and it measured 6000+ cells and was instrumental in choosing the cells for my frankenstein battery.
See my choice in mAh and IR criteria in my build thread. https://secondlifestorage.com/index.php?threads/wolfs-powirwall.7804/post-60439
That will give you an idea of the what I believe you should use for a spread in mAh and IR numbers. With a 14s20p pack though you want to try to maintain as close a tolerance as possible. The secret to well balanced packs is to have the same Ah and close IR results in all 14 packs.
Wolf
 
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