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Aging assessment at cell level of battery packs with forced air cooling and immersed cooling system

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In the related work (P3-008) by Marongiu et al, two identical battery packs were cycled until End-of-Life condition using different thermal management strategies, namely air cooling and immersed cooling. While the mentioned work examines the degradation on pack level, this work focuses on analyzing single cell degradation state after packs’ disassembly. The goal is to evaluate how different pack cooling strategies and thus different temperature distributions of parallel cells in a pack impact degradation of single cells.

Before aging, single cell check ups were performed, consisting of a capacity test, a quasi OCV and 1C pulses for resistance calculation. Afterwards, 2 packs each with 25 cells connected in parallel were assembled – one air cooled and one immersed cooled. The packs performed cycles between 2.5V and 4.2V. After aging, packs were disassembled, and single cell check ups were run again.

In order to analyze the degradation of single cells, capacity loss and resistance increase were calculated and evaluated taking the position in the pack into account. Moreover, differential voltage analyses were conducted for all cells.

The cells from air cooled pack show higher and more inhomogeneous capacity loss and resistance increase than the cells from the immersed cooled pack (2.1% higher mean capacity loss, 15.4% higher mean inner resistance increase). The differential voltage analysis indicates that additional to loss of cyclable lithium loss of active mass of anode occurred in both packs. Compared to loss of cyclable lithium, loss of active mass of cathode is estimated relatively low in both packs. Therefore, the higher capacity loss of cells from air cooled pack is caused mainly by a higher loss of cyclable lithium. The cell outliers in the immersed cooled pack show remarkably higher loss of active mass of anode.

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