Study on battery pack consistency evolutions and equilibrium diagnosis for serial- connected lithium-ion batteries

The consistency among lithium-ion battery pack is an important factor affecting their performance. The paper analyzes the impact sensitivity of parameters consistency including capacity, internal resistance and state of charge (SOC) on energy utilization efficiency of the battery pack. It turns out that SOC variations is the most significant influence on battery consistency, and hence is employed as evaluation index to characterize battery consistency level. Then the SOC evolution is explored under four scenarios, and the result reflects that the columbic efficiency is associated with prominent accumulative effect on SOC divergence of the battery group in use. The OCV consistency model is established based on long-term battery data of two trolley buses. It is observed that the fitted results match with the operational data very well. Finally, the mapping relation between the OCV distribution and the SOC distribution of the battery pack using dichotomy method is proposed. The calculation error of battery pack energy utilization efficiency by using such method is within 1.5%, which can be used for battery equilibrium diagnosis and prognosis at a certain aging state.