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I'm trying to test the capacity of 18650 cells. I don't know which numbers are important. I get different results on the same cells from discharge and the charge.

For example on same 18650 cell: (Panasonic NCR18650BD 3180mAh - 10A)

100% discharge register 3V - 3362mAh 06:26h 100% charge register 4,2V - 3538mAh 03:45h 62mΩ

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    \$\begingroup\$ Please be specific. What values are you getting? Capacity? And what else? Edit your question and clarify. Do so soon, before people flag your question as "lacks clarity" and close it. \$\endgroup\$
    – Davide Andrea
    Commented Aug 19, 2023 at 14:25
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    \$\begingroup\$ Are you referring to energy capacity, with units in watt-hours (or joules), or are you talking about charge, in amp-hours? \$\endgroup\$
    – Simon Fitch
    Commented Aug 19, 2023 at 15:54

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It is possible that a Li-ion cell takes in more charge during a full charge than it gives during a full discharge.

The ratio of the two values is called the "Coulombic Efficiency" (Or Faraday efficiency).

CE = (charge taken during a full charge) / (charge given in a full discharge)

Normally, a Li-ion cell has a CE of 99.9 % at room temperature. That means, for every 1000 electrons you put into it, you get back 999 electrons. (The missing electron is lost due to a Li+ ion being lodged in an electrode as Li metal. It's non-reversible.)

The CE gets worse at low temperatures, on the order of 95 % at 0 degrees C.

If your cells show a CE noticeably less than 99 % at room temperature, either the test is flawed, or the cells are really in bad shape.

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