In a rechargeable battery set, an average voltage is computed from average voltage drop produced by the sum of internal resistances of each battery cell. A calculated internal resistance of each battery cell is then computed from a measured voltage of each battery cell and the average voltage. Finally, the measured voltage of each battery cell is calibrated according to the calculated internal resistance and a calibrated voltage of each battery cell may be computed. In such way, the calibrated voltage of each battery cell is much closer to a real voltage of the battery cell, which provides a battery control unit more precise information when charging/discharging the battery cells.