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The battery/supercapacitor hybrid energy storage system actively combines two energy storage devices to achieve better power and energy performances. This paper presents a detailed small-signal mathematical model that can represent the dynamics of the converter-interfaced energy storage system around the steady-state operating point. This model takes into account the effects on the currents of a variety of factors such as the voltage-current characteristics of individual energy storage devices, power converter and filter parameters, and controller parameters. The proposed model considers the variations in the battery current, supercapacitor current and DC bus voltage as state variables, the variations in the power converter duty cycles as the control input, and the variations in the battery voltage, supercapacitor voltage and load current as external disturbances. Frequency-domain model and control strategies for the power sharing between the battery and supercapacitor are developed based on the small-signal model of the hybrid energy system. Frequency-domain characteristics of the open-loop and closed-loop systems are analyzed. Time-domain simulation is used to verify the system operation. The effects of system and controller parameters on the system performance are also studied.