In a battery management system and a driving method thereof, the system includes a sensor and a micro control unit (MCU). The sensor senses a voltage and a current of a battery, and generates an estimation current of the battery using a result of cumulatively calculating the battery current by a unit of a predetermined period. The MCU receives the battery voltage and the estimation current, sets a voltage of the battery in a key-on state as a first voltage, sets a voltage of the battery after a first period as a second voltage, and calculates an internal resistance of the battery using a difference between the first and second voltages and an average value of the estimation current.