To enhance the utilization of ultracapacitor (UC) energy in a battery-UC based hybrid energy storage system (HESS) for electrified vehicle (EV) powertrains, the UC needs to be discharged down to its minimum operating terminal voltage. However, this leads to higher duty cycle operation of the power converter and excessive current at source side. Such conditions hamper the steady operation of the converter and necessitates high current rated magnetic and switching components. Addressing to this challenge, an improved flexible bidirectional multiport converter is proposed, which enables deep discharging of UC without increasing the operating duty cycle or the component ratings of converter, while ensuring flexible power flow among multiple ports. The proposed converter offers the unique feature of energizing the inductor co-operatively through both UC and battery in the same switching cycle, during certain transient events of peak power delivery or regenerative braking energy recuperation. This eventually facilitates in extending the range of UC operation, thereby augmenting the dynamic performance of EV. The flexible operation of the proposed converter along with extended utilization of UC is validated at standard EV load profiles through simulation and further verified by experimentation performed on a laboratory scale hardware prototype.