Modular battery energy storage systems (MBESS) allow individual battery power control, with reduced voltage ratings of the converters, and enhanced system reliability. The power is shared amongst different battery modules based on the state of charge (SoC) of each module. In this paper, an adaptive power sharing control method is proposed for an MBESS that is connected to the DC bus voltage of a distributed generation system. The adaptation process considers the efficiency and balancing speed of the system at different battery loads. Simulation results (MATLAB/Simulink) are presented to compare the proposed systems performance in terms of balancing speed and efficiency, with conventional and adaptive power sharing methods.