The Energy Loss Due to Interconnections in Paralleled Cell Configurations of Lithium-Ion Batteries in Electric Vehicles

In Electric vehicles, the Li-ion battery reaches its end of life when the capacity is decreased to 80% of the initial rated capacity. However, a battery with only 20% used capacity does not mean the battery cannot be used in a secondary application with less current demand, in a controlled and secure environment. This necessitates a comprehensive understanding of the configuration of the end of life (EoL) batteries in module and pack level, in terms of inconsistencies in capacity and impedance of the cells forming the module and hence the battery pack. Accordingly, a safer and longer second life use can be granted. This paper investigates the impact of parallel connection on the impedance and capacity of four, pouch lithium-ion cells forming a battery module in 2P 2S configuration. The energy storage capacity and the AC impedance of each parallel pair and individual cells are recorded and compared. The results highlight that the capacity loss due to the parallel connection is 6% less than the sum of the individual capacity for each cell. With the help of a developed equivalent circuit model, the ohmic resistance of the pair and cell is estimated to demonstrate the contribution of interconnections in increasing the total impedance and hence the perceived loss of capacity in the parallel connection.