IN PROGRESS
The goal of cell testing:
The more data and the more informed we are about the status of our pack, the better.
What parameters can be measured, and do they cause pack imbalance?
As-Received OCV
Cell Weight
Ir
Im
Self-Discharge (https://literature.cdn.keysight.com/litweb/pdf/5992-2517EN.pdf?id=2911018)
...
Effects of an Internal Resistance imbalance in a moduleThe article below discusses the parallel connected cells
A cell-cell variation of internal resistance is +/- 15% (https://www.sciencedirect.com/science/article/pii/S2352152X18308156, Table 2), and follows a roughly normal distribution. This matches with results in several other reports, while a +/- 5% variation in capacity is noted from the same table.
The effects of internal resistance mismatch of parallel connected lithium cells. Internal resistance mismatch between cells can lead to sudden capacity losses and a decrease in overall cycle life (Source 2). 
of around 40% (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 This is not a fairly large spread (50%), 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. .
Internal resistance mismatch causes current mismatch on charge and discharge. The current mismatch creates voltage drop difference and thus a difference in SoC between the cells. Thus there is an SoC mismatch between the cells when current is being drawn.
Thoughts on Temperature Variation:
As the temperature of a battery pack increases, the self-discharge rate also increases due to the increased rate of the chemical reactions inside the cell. An increase in 10 degrees Celsius will double the amount of self-discharge current.
Sources:
1 - https://iopscience.iop.org/article/10.1088/1742-6596/795/1/012036/pdf
2 - http://web.mit.edu/bazant/www/papers/pdf/Gogoana_2013_J_Power_Sources.pdf
Module Testing
Once we build our modules, the next step is to test every module to ensure an even capacity in series-connected strings.