State Determination of Cylindrical Lithium-Ion Batteries using Strain Gages

The precise determination of the state of charge (SOC) of lithium-ion batteries (LIBs) is essential for the safe and efficient operation of energy storage systems. This thesis explores the use of strain gages as a non-invasive method for SOC determination in cylindrical lithium-ion cells, focusing on the mechanical defor-mation that occurs during charge and discharge. Strain gages were placed direct-ly on the cell surface to measure deformation resulting from structural changes and volume expansion caused by lithium-ion intercalation and deintercalation in the active materials. Experimental investigations were conducted on cells in the 18650 and 4695 formats to assess how strain behavior correlates with SOC un-der different cell designs and chemistries.

The study showed that cell design and electrode composition significantly affect the reproducibility of strain measurements. An 18650-format cell with a graphite anode exhibited overall low volume expansion. Combined with uneven jelly roll packing and the influence of tab design, this led to inhomogeneous strain distribu-tion and even inverse strain behavior at certain positions. In contrast, another 18650-format cell with a silicon-graphite anode showed greater expansion and more consistent strain behavior that positively correlated with SOC. This higher expansion helped mitigate the effects of irregular geometry and tab placement. The 4695 cell, with its larger format and tabless design, exhibited the highest strain amplitudes and the most homogeneous response due to its uniform internal structure and improved mechanical symmetry.

To investigate whether SOC can be determined from strain measurements, an SOC determination method was developed using a Long Short-Term Memory (LSTM) neural network. The best predictive performance was achieved at the highest charging rate, yielding a mean absolute error of 4.68 %. Additionally, abu-sive overcharge tests demonstrated that strain measurements can detect safety-critical conditions at an early stage.

Overall, the results confirm the feasibility of strain-based SOC determination and highlight its potential for further development, particularly in the context of larger-format cylindrical cells and integrated safety monitoring systems.