Strain-based Investigation of Current Imbalance and Lithium Intercalation Stages in Parallel-Connected Lithium-Ion Cells

Battery modeling approaches typically focus on single-cell behavior and then scale up to represent entire battery packs. However, in real-world applications, battery packs—rather than individual cells—are used in larger systems such as electric vehicles. When multiple cells are connected in parallel, uneven current distribution can occur, potentially affecting performance and reliability. A root cause of the dynamic current imbalances between cells may be the changing internal resistance of the cells, influenced by the de-intercalation of lithium ions out of the anode during discharge. Moreover, the de-intercalation of lithium ions out of the anode causes a shrinking of the cell, which can be transduced as a measurable strain in the hoop axis of a cylindrical battery cell. This research examines the relationship between current distribution and mechanical deformation of individual cells in a battery pack made up of three cells in parallel. The goal is to better understand the system’s dynamic discharge behavior while also integrating knowledge of the cell’s electro-mechanical coupling. By experimentally measuring the current flow through individual cells and correlating it with strain measurements, this study identifies measurable factors contributing to cell imbalances. Nickel-Manganese-Cobalt Oxide (NMC) cells (Samsung 30Q cell) are tested under C/10 load conditions. This work measured current values that showed consistent imbalance between cells, with one cell providing up to 15% more current than the pack average during discharge. Corresponding strain measurements showed much higher strain in the cell that provided 15% more current; compared to other cells. The most stable cell in regards to current ended with the largest difference in state of charge (SOC) of +2% from average, more than 3% higher than the other two cells which ended with ≤−1%. The difference in strain also varied widely between cells, two differed by as much as 415 µ�� which is 60% of the maximum strain observed. Understanding the coupled electro-chemical-mechanical behavior of batteries in parallel can improve model accuracy for predicting system reliability. The limitations of this study and potential implications for battery management are also discussed.