IN PROGRESS
Quick Overview and Recommendations:
The goal of cell testing:
The more data and the more informed we are about the status of our pack, the better.
Any imbalances present in the pack at the start will only get worse as the pack is cycled. Within series modules, this variance can be overcome with cell balancing, but within parallel cells there is no way to overcome this. (link study) Due to the nature of li-ion cells being a manufactured component, there is always some degree of tolerance from the manufacturer. This page will discuss the parameters that can vary and how they affect pack imbalance.
Also, make a battery basics page - links to MDPI pages
To get a better understanding of the equivalent circuit model of a cell, see this page (link).
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 - The internal impedance of a cell is a similar measure to the DC internal resistance
EIS Measurements
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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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.
Why single cell testing is
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required (instead of module/pack testing):
1 - A manufacturing error will be impossible to detect once we put the cells in modules
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Internal resistance mismatch causes current mismatch on charge and discharge. The current mismatch creates voltage drop difference and thus a difference in SoC between the cells. Thus there is an SoC mismatch between the cells when current is being drawn.
Thoughts on Temperature Variation:
Pack-Variation: 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. (Keysight link) The self-discharge will still be in the high-uA range (maybe low mA in the worst case), and thus not be an issue in module or pack balancing as the passive balancing circuit will take care of the pack-level variation in self-discharge. Assuming the temperature within the pack is only hotter when we are drawing current, the change in DC Internal Resistance will more than offset the change in self-discharge current.
Module-Variation: Temperature variation within a module, under extreme circumstances, can result in a positive feedback situation (until a certain delta OCV is reached). A decrease in internal resistance of the cell due to an increase in temperature will cause more current to be drawn from that cell, which will lower its OCV. Due to the close proximity of cells within a module, the temperature variation (unless caused by a cell with initially high IR) will be very small ans thus its effects negligible.
With temperature variation, the useable capacity of the cell will increase as the internal resistance decreases and thus we have less power loss in the cells due to IR. (Show IR-temp graph and Capacity-Temp Graph)
Module Testing
Once we build our modules, the next step is to test every module to ensure an even capacity in series-connected strings, and test for any possible manufacturing errors.