Limitations of cell imbalances on the operation of a large-scale battery storage system: example LTO batteries

Large-scale battery energy storage systems (BESS) are increasingly being used for a variety of applications, including system services and energy trading. The performance and lifetime of BESS depend on the health and homogeneity of individual battery cells. This study presents a comprehensive evaluation of cell variations within the large-scale BESS M5BAT to determine the state of balancing (SOB) and to optimize performance. M5BAT features different battery technologies such as lithium-titanate-oxide (LTO). Quasi-open circuit voltage (qOCV) measurements and differential voltage analysis (DVA) are used to identify shifts in cell state of charge (SOC) and differences in cell state of health (SOH). The SOH was identified to be within the range of 90%, while the cell imbalances were found to be at around 40% for the M5BAT LTO battery unit. Using the M5BAT LTO battery unit as an example, the imbalance could be reduced to about 18% through a manual balancing process, thus increasing the usable capacity. Furthermore, different SOC states can result in aging differences and thus reduce the service life. Therefore, effective cell balancing algorithms are particularly important in a large-scale BESS. This helps to improve the development of algorithms to address the issue of cell imbalance during operation and optimize the operation of large-scale BESS.