A quantitative comparative study of efficiency for battery-ultracapacitor hybrid systems

This paper provides a quantitative and comparative study on efficiencies of a battery semi-active hybrid energy storage system (HESS) and a battery-only system. The discussion is based on the equivalent series resistance (ESR) circuit models and a pulsed current load. It is theoretically proved that the efficiency difference between the two systems depends on the variance of the load current, the average load current, and the internal resistance of the battery pack. A threshold variance is then accurately derived, above which the battery semi-active HESS becomes more energy efficient. This threshold decreases with an increasing internal resistance of the battery pack and/or a decreasing average load current. Therefore, the best usage of ultracapacitors is to combine with batteries designed for low cost (i.e. a large internal resistance), and supply the dynamic part of a current load with a high peak-to-average ratio. Finally, JC08 driving cycle-based simulation validates the theoretical analysis.