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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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 module level.
Self-Discharge is difficult to measure without extremely accurate and precise equipment (which may be available from Keysight). If one cell has an elevated self-discharge current of 20uA while all other cells have a self-discharge current of 10uA, a 24P module made up of 10uA self-discharge cells will have a self-discharge current of 240uA. If one cell in the module has an elevated self-discharge, then the total self-discharge will be 250uA.
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 due to the change in SOC-OCV curve at higher internal resistance. (voltage drop over IR)(Source 1)
(Source 1)
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.
Effects of an Internal Resistance imbalanced imbalance in a 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 . (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.
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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.