Control Strategies for Battery/Supercapacitor Hybrid Energy Storage Systems

Batteries are one of most cost-effective energy storage technologies. However, the use of batteries as energy buffers is somehow problematic, since it is hard, if not impossible, to recover from rapid power fluctuations without dramatically reducing the batteries' lifetimes. In a supercapacitor, energy storage is by means of static charge rather than of an electrochemical process as in a battery; thus the supercapacitor has a higher power density than a battery. It is then advantageous to combine these two energy storage devices to accomplish better power and energy performances. This paper presents an active hybrid energy storage system that comprises a rechargeable battery, a supercapacitor bank and two corresponding DC/DC power converters. The battery and the super-capacitor may be charged or discharged simultaneously with the current or power appropriately split between them. The battery may be predominant in either the charging or discharging mode. Three different control strategies for power sharing between them are developed for the hybrid energy storage system. These control strategies are verified and compared against each other under some certain operating conditions. The effects of controller parameter variations on the system performance are also studied.