Modern batteries provide high discharging efficiency, but the rate capacity effect in these batteries drastically decreases the discharging efficiency as the load current increases. Electric double layer capacitors, or simply supercapacitors, have extremely low internal resistance, and a battery-supercapacitor hybrid may mitigate the rate capacity effect for high pulsed discharging current. However, a hybrid architecture comprising a simple parallel connection does not perform well when the supercapacitor capacity is small, which is a typical situation because of the low energy density and high cost of supercapacitors. This paper presents a new battery-supercapacitor hybrid system that employs a constant-current regulator isolating the battery from supercapacitor to improve the end-to-end efficiency from the battery to the load while accounting for the rate capacity effect for the Li-ion batteries and the conversion efficiency data for the regulator. We optimize the system in terms of a delivered energy density which is an end-to-end energy delivery per unit volume of the energy storage elements. We evaluate the delivered energy density with the aid of detailed simulations and develop a design space exploration algorithm based on the characteristics of the proposed architecture. We achieve 7.7% improvement in deliverable energy density over conventional parallel connection of battery and supercapacitor.