Performance of inconsistency in lithium-ion battery packs for battery energy storage systems

Inconsistency is common in lithium-ion battery packs and it results in voltage differences. Data from a battery pack with 200 cells connected in serial in a battery energy storage system (BESS) are applied for study. According to the causes of the voltage difference, three cell inconsistencies can be categorized as state-of-charge (SOC), internal resistance and capacity. The divergences among the cell voltage curves present different trends due to different causes. Based on an equivalent circuit model and the characteristics of lithium-ion batteries, a simulation has been performed for a further study of the cell inconsistencies. The simulation result is similar to that of the 200-cell battery pack. Experiment results on an 8-cell battery pack show that internal resistance difference is the main cause for voltage difference and a low charge current helps to make full use of the pack capacity. Meanwhile, SOC difference can be easily detected at low SOC stage.