Internal Resistance:
-Caused by SEI buildup
The most important test is a lengthy capacity test, as this parameter varies much more than the internal resistance for various cells.
The article below discusses the effects of internal resistance mismatch of parallel connected lithium cells. Internal resistance mismatch between cells can lead to sudden capacity losses (Fig 8)
http://web.mit.edu/bazant/www/papers/pdf/Gogoana_2013_J_Power_Sources.pdf
We do not expect to be using multiple hundreds of cycles on our pack. The capacity loss shown in the figure happens after around 100 cycles, and is due to the mismatched cells being exposed to large charge/discharge currents as the ends of the charging and discharging cycles due to differing SoC-OCV curves cause by the mismatched resistance. The cells in these tests were LiFePO4 cells, and the internal resistance measured by a 15s 40A pulse, with a distribution shown above from 13.5 - 21.5 mOhm. While this is not a large spread, the currents that the pack were tested at are much larger than anything our cells will ever see. A high current test such as this decreases the time that the mismatch effects take to show up as the cells are performing at their peak characteristics.