Ultracapacitor/battery hybrid energy storage system with real-time power-mix control validated experimentally in a custom electric vehicle

This paper experimentally demonstrates the benefits of integrating ultracapacitors (u-cap) into a custom electric vehicle. The new hybrid energy storage system (HESS) includes a 25 kW four-phase dc-dc converter connected in between the 33.8 kWh lithium phosphate battery pack and the u-caps. By combining the high energy-density of the battery and the high power-density and cycle-life of u-caps, the HESS delivers high power while minimizing peak battery current during acceleration and regenerative braking. Real-time power-mix control within the HESS using the dc-dc converter is the main challenge addressed in this project. In one experimental test-drive, the HESS not only improves the maximum power of the EV from 38 kW (50 hp) to 52 kW (70 hp), but also captures the majority of the regenerative braking energy, greatly reducing the stress on the battery pack.