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I'm gonna build battery packs. I have quite a lot of experience with 18650 Li-ion cells, but there's one thing I still don't know.

I will buy CCA 100 new cells from seller (for example LGEBHE21865). There are multiple batches of cells (the number below battery model). Although the batteries are 2500 mAh nominal, actual capacity differs ±100 mAh.

My question is:

It is better to build a pack (5S2P for example) using only cells from the same batch or measure actual capacity of each cell and use cells with most similar capacity? Or is there another important aspect like internal resistance?

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  • \$\begingroup\$ How, exactly, do you measure capacity ?? \$\endgroup\$ – Kripacharya Aug 5 '19 at 18:25
  • \$\begingroup\$ You need to use a proper BMS whenever you connect cells like this. Internal resistance is important as well, but you must assume that the cells will not age in identically the same way so it doesn't matter if they are identical when you assemble the pack. Use a battery management system for lithium batteries or you will put your safety at risk. \$\endgroup\$ – Elliot Alderson Aug 5 '19 at 19:16
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    \$\begingroup\$ I'm almost certain I've answered a nearly identical question years ago. The answer actually is: if its 100 cells, you don't buy from some seller, but from an official supplier. Otherwise, the compromises you need to make to ensure safety of unknown cells simply eat up your savings in cell cost. \$\endgroup\$ – Marcus Müller Aug 6 '19 at 0:13
  • \$\begingroup\$ You should use a cell balancing circuit [look that up] for a series pack. Cell matching is not really a viable technique. \$\endgroup\$ – Nick Alexeev Aug 6 '19 at 0:17
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There are lots of videos on YouTube that answer the question. They cover all sizes from small to automotive. One particularly prolific and experienced builder is jehugarcia He has done at least one video where he measured a whole batch of cells and picked the ones that nearest matched capacity. Also look at Instructables there may be more information there.

The problem is that if the cells don't match in capacity then you run the risk of overcharging the weakest and similarly reverse charging the weakest when you run the battery near flat. A good BMS will alleviate most of these problems, but applying it to 100 cella is probably too complicated.

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