A hybrid energy storage system (HESS) consisting of high-energy-density batteries and high-power-density Ultra-capacitors (UCs) can perform better than a battery alone powering an Electric Vehicle. Hybrid energy storage can harness the benefits of the battery and UCs together and meet the energy storage as well as a vehicle’s power flow requirements. Recent research has focused on a variety of HESS circuit design topologies - and power management and control -. The passive, semi-active, and active HESS configurations have gained significant attention. The semi-active topology is the preferred choice due to its reduced circuit complexity and price compared to the active hybrid configuration, and for its better performance when compared with the passive hybrid configuration. In the semi-active topology, UCs power the traction motor directly, and batteries with mass amount of energy storage are connected in parallel with UCs through a bi-directional DC-DC converter. The UCs supply the dynamic component of the load current which decreases the battery size, limits the battery peak current, and extends its life. The UCs can be charged by the batteries when necessary, and vice versa. Most of the recent work on the subject of HESS -, - either presents the simulation and analysis of various HESS topologies or validates their feasibility on a test bench. However, this article describes the actual implementation of a semi-active HESS and demonstrates its performance in the Ford European Mondeo for the Integrated Starter Alternator (ISA) application. The Mondeo HESS is designed with a 48 V battery connected through a bi-directional DC-DC converter with 300 V UC bank which powers the ISA motor. The semi-active HESS topology also enables the flexibility of powering the motor at a higher voltage while operating the battery bank at lower voltage. Thus, the work presented in this article also presents a design for the future potential 48 V automotive power net required for meeting the ever increasing energy demands of the vehicle.