A power management system comprises a power conversion stage and a controller. The power conversion stage has two terminals. The first terminal is coupled to a first power source which provides a first voltage. The second terminal is selectively coupled to a second power source which provides a second voltage. The controller coupled to the power conversion stage can select a mode from at least a first mode and a second mode. In the first mode, the power conversion stage receives the first voltage at the first terminal and generates a step-up voltage at the second terminal. The generated step-up voltage is greater than the first voltage. In the second mode, the power conversion stage receives the second voltage at the second terminal and generates a step-down voltage at the first terminal for charging the first power source. The generated step-down voltage is less than the second voltage.