Experimental Study and Numerical Simulation of Parallel-Connected Li-ion Battery Array

Lithium-ion (Li-ion) batteries are the most promising energy storage for electric vehicle battery packs due to their high-power density and long cycle life. Generally, a battery pack installed in electric vehicles consists of several identical Li-ion batteries connected in parallel to increase the capacity. This study investigates the impact of uneven current distribution on the thermal behavior of a parallel-connected 18650 cylindrical Li-ion battery array, utilizing both experimental methods and numerical simulations. The battery array was cycled at various charge-discharge rates during the experiment to observe voltage and temperature changes, which were used as inputs for the simulation and to validate the model. Results indicate a significant current imbalance among the cells, resulting in an uneven temperature distribution. During discharge, the hottest cell tends to draw more current as discharge continues, causing current mismatch and increasing the temperature gradient. Rising cell temperatures create resistance differences that speed up discharge. This results in higher polarization resistance, temporarily balancing the current and then reintroducing severe current divergence at low SOC, especially under higher current rates. During charging, the current difference among the batteries gradually decreases as SOC increases and fluctuates near the end of the cycle. The highest temperature consistently occurs at the end of each cycle, with both maximum temperature and temperature non-uniformity increasing as the current rate increases. The validated simulation model accurately reproduces the thermal behavior of the battery array, showing localized heating in the central cell. These findings underscore the need to address the current imbalance in battery management system (BMS) design and thermal management strategies, particularly for module-level applications in electric vehicles operating at high current rates.