Despite the Covid-19 pandemic and supply chain challenges, including shortages of semiconductor chips, the electric vehicle (EV) market is expanding rapidly. Automobile manufacturers have been progressively developing business plans, considering electrification as an opportunity to acquire market share and preserve competitive advantages. The main barriers to EV adoption are high vehicle costs, range issues, and charging infrastructure. Installing a comprehensive EV fast charging network will help alleviate range and charging problems on long intercity drives. However, fast and ultra-fast charging directly from the grid will place a huge and unpredictable load on the power system. Therefore, energy storage systems (ESS) appear as a promising solution to prevent grid overload during charging and help reduce infrastructure costs. In this dissertation, the integration of the battery energy storage system (BESS) in a 450 kW EV charger is designed and validated via modeling, simulation and experiment. For the front end of the DC fast charger, the 3-phase 3-level T-type converter is selected as the PFC converter linked to the grid for its superior performance, especially for applications with low voltage and medium switching frequencies. An appropriate modulation method based on space vector modulation (SVPWM) and closed loop control will be illuminated and simulated via PLECS. Based on the comparison of several types of state-of-the-art cylindrical 18650 batteries, the battery energy storage system is designed from thermal issues and integration impact on the DC bus. In order to explore the heat dissipation and the temperature distribution across the pack, the thermal model based on the sub-model technique is developed via COMSOL, and a preliminary layout and cooling strategy are determined. Finally, the proposed structure of the EV charger integrated with the battery energy storage system is validated based on a 6 kW rated lab-scaled hardware test bench. In conjunction with the actual scenario, both the prospective steady states and transitions have been implemented. Hence, the feasibility of the concept of directly connecting the BESS to the DC bus is also verified by observing the response of the front-end AC/DC converter with the modulation and control strategies utilized.