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Current Distribution Measurements in Parallel-Connected Lithium-Ion Cylindrical Cells under Non-Uniform Temperature Conditions

UCD-ITS-RP-17-14

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Suggested Citation:
Klein III, Matthew Paul and Jae Wan Park (2017) Current Distribution Measurements in Parallel-Connected Lithium-Ion Cylindrical Cells under Non-Uniform Temperature Conditions. Journal of the Electrochemical Society 164 (9), A1893 - A1906

Understanding internal state non-uniformity that occurs across the electrodes in large-format Lithium-ion batteries, and among parallel-connected cells, is a critical part of the cell and battery module design process. Two separate groups of parallel-connected 18650 cells were tested using LiFePO4/C6 (LFP), and LiNiMnCoO2/C6 (NMC) chemistries. Pulse and full-capacity discharges were performed at various States of Charge (SOC), C-rates, average temperatures, and levels of temperature non-uniformity. Current non-uniformity for the pulse testing was always lower for the LFP group compared to the NMC group. The hottest cell in the LFP group produced up to 40% more current than average, while this was up to 80% for NMC. Conversely, under charge depleting conditions the NMC group experienced less current non-uniformity, and in certain cases provided a nearly uniform current distribution in the presence of non-uniform temperature. The results indicate that higher temperature sensitivity in the impedance of a cell will cause larger current non-uniformity under pulse conditions. However, due to the presence of non-uniform SOC for charge depleting, the Open Circuit Voltage (OCV) versus SOC gradient plays a significant role in dictating the current distribution behavior, where steeper OCVs provide a corrective action that minimizes the effect of the non-uniform impedance.