The present invention relates to a battery management system and method. In some embodiments the battery management system includes a sensing unit and a micro controller unit (MCU). The sensing unit measures the current, the voltage and the temperature of a battery. The MCU receives the voltages, the currents, and the temperatures, calculates an estimated cell voltage, by applying a pack voltage sensing error and the cell voltage deviation to the cell voltages, and calculates an estimated cell resistance, by adding a pack assembly resistance to an internal resistance of the cell, and calculates an pack output of the battery by using the estimated cell voltage and the estimated cell resistance.