The present invention discloses a power bank and a control method for supplying power. The power bank includes a rechargeable battery, a charging switch for controlling a charging path to the rechargeable battery, a discharging switch for controlling a discharging path from the rechargeable battery, and a control circuit for controlling the charging switch and the discharging switch according to current information of the charging path, current information of the discharging path, and battery capacity information of the rechargeable battery. The power bank is adapted for being connected in series between a power supply apparatus and a load. The control method detects the supply current from the power supply apparatus and the current required by the load to determine the charging and discharging operations of the power bank.