Simulation of voltage imbalance in large lithium-ion battery packs influenced by cell-to-cell variations and balancing systems

Abstract

Abstract Due to manufacturing tolerances, lithium-ion cells usually suffer from varying capacities, impedances, self-discharge currents and intrinsic aging rates, which are often claimed to be the reason for the voltage imbalance and subsequently deteriorated utilization of the battery pack. However, the true influence of such cell-to-cell variations is still not completely understood. This work presents a lean battery pack modeling approach combined with a holistic Monte Carlo simulation. Using this method, the presented study statistically evaluates how experimentally determined parameters of commercial 18650 nickel-rich/SiC lithium-ion cells influence the voltage drift within a 168s20p battery pack throughout its lifetime. Major degradation mechanisms were represented through the manipulation of the half-cell potentials of the anode and the cathode. A low-DOD cycle profile was used for the aging. Additionally, cell impedance and reversible self-discharge were taken into account. The results obtained in this work reveal that the intrinsic variation of aging rates has the biggest influence on the pack utilization. Furthermore, initial variations of the capacity and impedance of state of the art lithium-ion cells play a rather minor role in the utilization of a battery pack, due to a decrease of the relative variance of cell blocks with cells connected in parallel. Although different self-discharge and aging rates evoked a voltage drift, the utilization of battery packs with and without dissipative balancing remained almost the same, assuming no cells with internal defects were present.