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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AC Im (longer setup) Reject cells with elevated Im
The If balancing modules based on cells tests, one of the most important items to test is a lengthy capacity test, as this parameter varies much more than the internal resistance for various cells, and will cause larger pack imbalance. (show Taiping's linked page)the individual cell capacity. Building the modules based on a minimum capacity deviation will allow us to perform less testing on the modules as the capacity is already determined. This will require more test time at the cell level on a tradeoff for less test time at the module level. With access to the right equipment (a large cell cycling machine, around 200 slots), it will take less time to test the cells than to test the 48 modules on 1 or 2 high current cyclers. Building modules with a capacity metric in hand will allow us to create packs with equal useable energy.
Why single cell testing is ideal:
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Every cell inside a pack with matched IR will be able to deliver the full energy in the cell due to the change in SOC-OCV curve at higher internal resistance .(Source 1)(https://iopscience.iop.org/article/10.1088/1742-6596/795/1/012036/pdf).
3 - Once we put the cells inside the modules, they cannot be taken out. This is one of the reasons that we bought additional cells. Detecting a singular bad cell inside a module is possible in some cases, but we would never be able to identify the offending cell and replace it. If all the metrics (except capacity) of the individual cells are tested before being put into modules, we will know that there are no manufacturing defects in the cells.
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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 -
Module Testing
Once we build our modules, the next step is to test every module to ensure an even capacity in series-connected strings.