A power conversion system comprises a plurality of power converter modules, each including a bi-directional DC to DC converter and a current controller, wherein the bi-directional DC to DC converter is connected to the current controller, for charging or discharging a DC power source according to a distribution command received from the current controller, and a voltage controller, connecting to the plurality of power converter modules, for generating a current command to the current controller, wherein the voltage controller generates a current command to the current controller of the power converter module according to the detected capacity and voltage of the DC power source, whereby the current controller generates the distribution command to the bi-directional DC to DC converter with the received current command.