This paper proposes an improved state-of-charge (SOC)-based adaptive droop control strategy for photovoltaic DC microgrids with distributed energy storage units (DESUs). To overcome the slow SOC equalization and limited adaptability of conventional droop control methods, a nonlinear arctangent-based droop coefficient adjustment mechanism is introduced to enhance regulation sensitivity under small SOC deviations. In addition, a capacity compensation factor and an acceleration term are incorporated to improve proportional power sharing and SOC convergence speed among heterogeneous storage units. To further evaluate the engineering significance of SOC balancing performance, a time-integrated SOC deviation index is introduced to analyze the cumulative imbalance effect on battery degradation and lifecycle operation. By reducing the SOC imbalance duration, the proposed strategy contributes to mitigating uneven battery aging and may potentially reduce long-term battery replacement costs. Simulation results under discharging, charging, and irradiance-variation scenarios demonstrate that the proposed strategy significantly improves SOC-balancing performance compared with conventional SOC-based droop control. Across the three operating modes, the average SOC balancing time has been reduced by 52.6%. The proposed method provides an effective and economically sustainable control framework for distributed energy storage coordination in DC microgrids.