A battery management system for a battery pack comprising multiple battery modules is disclosed. Each of the battery modules includes multiple battery cells. The battery management system includes multiple first balancing units, multiple first controllers, a second balancing unit including multiple second balancing circuits, and a second controller coupled to the battery modules and the second balancing circuits. The first controllers are operable for controlling the first balancing units to adjust voltages of battery cells in the battery module if an unbalance occurs between the battery cells. The second controller is operable for controlling said second balancing circuits to adjust voltages of said battery modules if an unbalance occurs between battery modules.