Abstract: With the emerging technologies in the field of Energy Storage System Development, the interest for the development of Electric Vehicle (EV’s) is growing for future road transportation.In this paper the Battery plus Ultracapacitor (UC) Hybrid Energy Storage System(HESS)with an energy management control strategy is proposed. The proposed control strategy optimizes the energy consumption, improves vehicle performance and prolongs the battery lifetime for different vehicle Drive cycles.The proposed HESS topology incorporates the bidirectional DC-DC converter and a complimentary switch pair for interconnection of energy sources and interconnection with vehicle traction drive. The proposed HESS interconnected with BLDC motor and the control strategy is implemented for the acceleration and braking conditions of the vehicle. The DC- DC converter is of small size and itis used for sharing the energy between Battery and Ultracapacitor. It also balances the energy levels of the two sources based on the vehicle drive cycles. And the complimentary switch pair swaps the vehicle load between the two sources based on the vehicle driving condition for optimum energy consumption. The Ultracapacitor contributes to the rapid energy recovery associated with regenerative braking and rapid energy consumption associated with vehicle acceleration. This power system allows the acceleration and deceleration of the vehicle with minimal loss of energy and minimizes the stress on the main batteries by reducing high power demands away from the batteries.The objective of the control strategy developed is to provide uninterrupted and adequate power from HESS to the vehicle motor drive unit for all drive cycles of vehicle. The control logic considers the Battery Voltage, Ultracapacitor Voltage and vehicle speed in order to provide a smooth control, reliable and efficient energy sharing between HESS and vehicle motor drive unit. The Battery SOC and open circuit voltage(OCV) are important parameters for ascertaining battery life because fully charged batteries do not accept any current and hence, under this condition, the Ultracapacitor should be available discharged (that means no more than 15-20 % of its full capacity) to store the energy generated due to regenerative braking at high speeds. By contrast, if the battery state of charge is poor, the ultracapacitor should be available fully charged (that means more than 90 %of its full capacity) to power the traction motor for sudden acceleration.To achieve this requirement both the sources are interconnected through a bidirectional DC-DC Converter which is controlled by the speed of the vehicle.And thus the UC voltage is maintained at required level based on the vehicle speed by sharing the energy between Battery and UC for different drive cycles of the vehicle.

Keywords: Bidirectional DC-DC Converter, BLDC Motor, Ultracapacitor, Battery and Hybrid Energy Storage System (HESS).