Analysis of Cell Balancing to improve Performance and Life Expectancy for Lithium Ion Energy Storage Systems

摘要

Lithium-ion batteries are more and more gaining importance in automotive industry. Battery packs for hybrid electric vehicles (HEV) consist of more than 100 cells in series. Lithium-ion batteries in particular require cell voltages to be strictly kept in boundaries to ensure safe operation. Therefore power performance is always limited by the first cell which goes beyond the boundaries. Production deviations from cell to cell and inhomogeneous ageing and temperature within the pack result in imbalances. States of charge (SOC), capacities and internal resistances will deviate. On the other hand, main reasons for inhomogeneous temperature distribution are different internal resistances causing different cell losses and the position of the respective cell within the pack causing different heat dissipation. Even forced air or water cooling cannot avoid temperature spreads. Figure 1 schematically shows the distribution of the SOCs of a battery pack consisting of 5 cells for the ideal case (left) and after some time of operation (right) without balancing.