This project involves the comprehensive design of an energy storage system (ESS) for a premium plug-in electric vehicle (PHEV), addressing specific requirements such as test cycle energy, range, and power demands. The design process includes a thorough evaluation of various energy storage technologies, leading to the selection of lithium-ion batteries due to their high energy density and power capabilities. The project outlines the design of the battery pack, including cell selection, pack arrangement, and high-voltage architecture, while also considering safety systems and charging mechanisms. Furthermore, it addresses vehicle integration aspects like mass, volume, and thermal management. The solution demonstrates an understanding of energy storage technologies, architecture selection, and practical vehicle applications. The project includes calculations to meet the specified requirements, design considerations for the DC-DC converter, inverter, AC motor, and transmission. The project also considers the use of solar panels as an energy backup system. The project aims to provide a complete design with a focus on minimizing cost and weight, maximizing autonomy, and achieving optimal performance.