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)
...
Why single cell testing is ideal:
1 - A manufacturing error will be impossible to detect once we put the cells in modules
Once we put the cells in modules, the IR will be extremely low. Finding differences in the IR of the modules due to 1 bad cell will be next to impossible due to small contribution each cell has to the overall Resistance. If we take the internal resistance of each cell to be 50mOhm, then the IR of a 24P module will be 2.08mOhm. If one cell had an IR of 70mOhm (which would cause increased currents during the ends of charge/discharge cycles due to mismatched SOC-OCV curves), the total resistance would be 2.11mOhm. A 100A load connected to the module would produce a voltage drop of 0.208V if all cells were 50mOhm, and 0.211V if one cells was at 60mOhm. While this level of precision if achievable with our multimeters, the same voltage drop (0.211V would occur if all cells had an Ir of 51mOhm).Thus, to determine any imbalances in Ir, individual cell testing must be conducted.
While the capacity of the modules might be able to tell of a faulty cell, if other cells were picked such that the capacity was increased more than the average, the faulty cell could not be detected on a pack level.
2 - The cell life can be prolonged by avoiding exposure of the cells to high charge and discharge currents.
Every cell inside a pack with matched IR will be able to deliver the full energy in the cell. (voltage drop over IR)
(Source 1)
Effects of an Internal Resistance imbalanced module
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 and a decrease in overall cycle life.
http://web.mit.edu/bazant/www/papers/pdf/Gogoana_2013_J_Power_Sources.pdf
(Source 2)
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.
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.