A method and an apparatus for regulating state of charge in a battery assembly in which respective voltages across unit cells can be equalized in a short time. Among a plurality of unit cells B1 to B4 . . . Bn, the unit cell having the largest voltage across the cell is referred to as the largest unit cell, and the unit cell having the smallest voltage across the cell is referred to as the smallest unit cell. Electric charge is transferred from the largest unit cell to a capacitor CB so that the voltage across the capacitor CB may be made higher than the voltage across the largest unit cell. Thereafter, the electric charge is transferred from the capacitor CB to the smallest unit cell thereby to equalize the voltages across the unit cells.