An exemplary power source system includes a power source, controller, and a voltage fine adjusting unit. The power source includes a number of cells and a number of switches configured for connecting the cells in series or in parallel. The controller is configured for coarsely controlling an output of the power source by selectively turning on and off the switches. The voltage fine adjusting unit is configured for further and finely adjusting the output of the power source on condition that the coarsely controlled output of the power source is outside a predetermined acceptable range of output.