A battery management system includes a sensing unit, a controller, an Open Circuit Voltage (OCV) calculator, and a State of Charge (SOC) determination unit. The sensing unit measures a total voltage of the secondary battery. The controller maintains the secondary battery in an open circuit state for a first period of time after a power supply switch is turned on, and couples the secondary battery to an external device when the first period of time ends. The OCV calculator receives the measured voltage from the sensing unit and calculates an OCV of the secondary battery during the first period of time. The SOC determination unit receives the calculated OCV from the OCV calculator and determines an SOC corresponding to the OCV.