Abstract: The development of electric vehicles (EVs) put pressure on the power grid, such as overloading, reducing power quality and reliability. Equipping battery energy storage systems at charging stations (EVCSs) is an effective measure to relieve the pressure. This paper presents a planning method for determining the optimal capacity of battery energy storage systems in EVCSs. The proposed method minimizes the cost of the energy storage system, while enhancing the resiliency of the electric vehicle at the same time. This problem is formulated as a multi-objective optimization which is solved by particle swarm algorithm. Eastern Massachusetts highway network is utilized to verify the validity of this method. The capacities of four charging stations in the road network are optimally determined. Power failure of the charging station is simulated, we compare the resilience of electric vehicles with and without considering the optimal capacity planning. The simulation results show that the optimal planning of battery energy storage system at EVCSs could be a feasible solution to improve the resilience of electric vehicles.