A battery management system and a driving method thereof. The battery management system manages a plurality of battery cells, and a plurality of cell relays respectively coupled to the plurality of cells. The battery management system includes a voltage detecting unit and an MCU. The voltage detecting unit receives a first voltage corresponding to an input voltage transmitted through a 3-contact relay coupled to a first cell corresponding to the turn-on first cell relay when the first cell relay is turned on, and generating a second voltage corresponding to the first voltage. The MCU calculates an effective gain in correspondence with a ratio of the second voltage to the input voltage and controls a connection of the 3-contact relay.