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 - Assuming the cells were manufactured at the same point on the voltage curve, the as-received OCV is an indicator of the amount of self-discharge current of the cell. See the self-discharge method.
Cell Weight - The weight of a cell is an indicator of how much material is inside the cell. According to this paper ( ), the weight of a cell correlates loosely with the capacity of the cell.
Ir
Im
Self-Discharge (https://literature.cdn.keysight.com/litweb/pdf/5992-2517EN.pdf?id=2911018). Keysight has developed a self-discharge measuring unit, which incorporates a super-high precision DC voltage source and a super high precision current measurement device (accurate to the tens of uA). The bucket method described in the document provides a quick test of the self-discharge current. A single cells having a significantly higher self-discharge current will cause the module to have a larger self-discharge current. Over long periods of time (such as sitting idle in the bay for a month), this self discharge current will cause an imbalance in SoC of the cells and lead to decreased pack capacity.
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AC Im (longer setup) Reject cells with elevated Im - as per this paper, can be an indicator of manufacturing defects.
Do we want to make a solar car, or do we want to make the best solar car? - 480mAh (20mAh difference per cell assumed) difference between modules = 0.48Ah x (36x3.7V) = 63Wh of energy. Assuming we travel at the efficiency of XXX, we will have XXX Km less range with a pack imbalanced by 0.48Ah
If balancing modules based on cells tests, one of the most important items to test is 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.
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