Abstract: [Objectives]　Distributed energy storage systems exhibit slow state of charge (SOC) equalization and low current distribution accuracy due to inconsistency in the SOC and capacity among battery cells. To address these issues, a control strategy for energy storage systems that considers SOC for energy storage units with different capacities is proposed. [Methods]　The strategy improves droop control by integrating the SOC difference with the droop coefficient using an inverse tangent nested power
function. The variable acceleration factor in the droop coefficient is dynamically adjusted according to the SOC difference to achieve better equalization. Additionally, virtual voltage and DC bus voltage compensation are incorporated to achieve accurate output current distribution while ensuring bus voltage stability. [Results]　The variable acceleration factor derived from the proposed strategy reduces the SOC  equalization time by approximately 50% compared to a fixed acceleration factor. The output current remains consistent with the capacity ratio without compromising SOC equalization performance. [Conclusions]　This control strategy improves SOC equalization speed and output current distribution accuracy among energy storage units with different capacities.

Key words: distributed energy storage system, energy storage unit, state of charge, droop control, variable acceleration factor, equalization speed, voltage compensation, output current distribution