A power management unit for a battery-operated electronic device having a bus interface for the interconnection with another electronic device, the power management unit including an electric energy storage element coupled between a battery of the battery-operated electronic device and a voltage supply line of the bus interface, the electric energy storage element being operable to charge/discharge electric energy; a drive circuitry arranged to control a charge/discharge of the electric energy storage element. The drive circuitry is operable to cause an electric power supplied by the other electronic device through the voltage supply line to re-charge the battery; or, in case the other electronic device does not supply electric power, cause the battery supply electric power to the other electronic device through the voltage supply line. The power management unit is particularly adapted for battery-operated, mobile USB OTG devices.