Control of Ultracapacitor-Battery Hybrid Power Source for Vehicular Applications

The energy storage system in electric vehicles (EV) must supply variable power levels and take regenerative power from braking. Ultracapacitors (UC) are more efficient than batteries for variable loads and recharging, but have a much lower energy density; the combination of these into a hybrid source can deliver better performance in an EV. We present several control systems, compare three active control schemes in-depth, and suggest a design. We describe a superior UC voltage control algorithm and a method of choosing optimal system parameters. Simulation validates the control approach of the complete system, and shows performance improvement of 48% by one metric. The test system includes a DC-to-DC converter with 97-98% typical efficiency.