Measuring Cylindrical Cell‘s expansion by Stereo Camera Based Methods

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The increasing use of Lithium-ion batteries in many aspects of human life is accompanied by a growing need for continuous battery supervision and recognition of safety-critical states of operation. Mechanical alterations inside the cell, e.g., caused by lithium plating, are possibly indicative of such states and are thus desirable to recognize. Furthermore, as most mechanical alterations are accompanied by a change of volume, measuring the strain of a round cell’s outer shell can be a proxy for the cell’s mechanical state.
In previous research, the availability of strain gauges to measure the volume expansion of cylindrical cells was shown by Willenberg et al. This work investigates the correlation between the cylindrical cell’s can strain and its state, as can be partially described by its state of charge and health. Therefore, a systematic analysis of different conditions during cycle and calendar aging is conducted.
The test matrix consists of two parts, focusing on cyclical and calendar aging, respectively. For the former part, 32 round cells of type LG INR18650-MJ1 with a silicon-graphite anode are cycled at different current rates at an ambient temperature of 25 °C. Multiple distinct states of charge ranges are covered based on differential voltage analysis of the cell. Check-up routines are employed regularly to obtain the cell’s quasi-open circuit voltage curve, capacity, and inner resistance. In the latter part, focused on calendar aging, 16 cells are stored at two different temperatures under constant-voltage conditions, which are ensured by custom-built hardware, allowing for continuous measurement of the cell’s floating current. Every three months, the cells are characterized utilizing the previously described check-up routine at an ambient temperature of 25 °C.
Systematic test matrices with continuous expansion measurement during cycling and aging are only possible with relatively low-cost measurement devices like strain gauges. However, these measurements are locally and directionally limited, giving limited insight into the cell’s three-dimensional expansion. Therefore, in-operando optical strain analysis and computer tomography measurements will be employed for selected cells to analyze the entire cell’s expansion further and investigate under which conditions the strain gauge’s measurements act as a reliable proxy.

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