To address the circulating current issue among parallel battery clusters in energy storage systems， a series-connected step-up partial-power equalizer is proposed， which optimizes transmission efficiency and power density of the equalizer by introducing partial power conversion technology. Aiming at the control problem of two different power flows in the partial-power equalizer， a duty-cycle-based power flow control method is proposed， and a detailed analysis of the equalizer’s working states under the proposed control method is conducted， which realizes effective control and bi-directional flow capability for both the base power flow and the adjustment power flow. For the degradation of system performance caused by the inconsistency of battery clusters， a parallel-battery-clusters power equalization control method is proposed， which suppresses the power circulation issue among parallel battery clusters. Finally， experimental results verify the effectiveness of the proposed partial-power equalizer and its control method.