IN PROGRESS
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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)
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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, there are many other factors that could influence the resistance measured in a module such as contact resistance, spot weld connections, etcthe 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.
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