A method of balancing rack voltages of a battery pack configured to have a structure in which a plurality of racks is connected in series to each other, and a plurality of unit modules is connected in series to each other in each of the racks, the rack voltage balancing method comprising: (a) a voltage measurement step of measuring voltages of the racks; (b) a sorting step of sorting the voltage values of the racks in an ascending order; (c) a comparison step of comparing a difference between a maximum voltage value and a minimum voltage value with a set voltage; wherein if the difference between the maximum voltage value and the minimum voltage value is lower than the set voltage, the racks are charged with the maximum voltage value; the method being characterised by comprising the following steps that are carried out only when the difference between the maximum voltage value and the minimum voltage value is higher than the set voltage: (d) a charging/discharging count step of increasing either a charging count or a discharging count based on the result of a comparison between the voltage values of the racks and a reference voltage, the discharging count being increased each time the voltage value of a rack is found lower than the reference voltage and the charging count being increased each time the voltage value of a rack is found higher than the reference voltage; and (e) a charging/discharging step of charging or discharging the racks according to the charging or discharging count, wherein step (e) comprises: - sequentially discharging the racks from a rack having the maximum voltage value to a rack having the minimum voltage value, when the discharging count is higher than the charging count and the minimum voltage value is higher than the reference voltage; or - sequentially charging the racks from a rack having the minimum voltage value to a rack having the maximum voltage value, when the discharging count is higher than the charging count and the minimum voltage value is lower than the reference voltage; or - sequentially discharging the racks from a rack having the maximum voltage value to a rack having the minimum voltage value, when the charging count equals the discharging count; or - sequentially charging the racks from a rack having the minimum voltage value to a rack having the maximum voltage value, when the charging count is higher than the discharging count. (a) a voltage measurement step of measuring voltages of the racks; (b) a sorting step of sorting the voltage values of the racks in an ascending order; (c) a comparison step of comparing a difference between a maximum voltage value and a minimum voltage value with a set voltage; wherein if the difference between the maximum voltage value and the minimum voltage value is lower than the set voltage, the racks are charged with the maximum voltage value; the method being characterised by comprising the following steps that are carried out only when the difference between the maximum voltage value and the minimum voltage value is higher than the set voltage: (d) a charging/discharging count step of increasing either a charging count or a discharging count based on the result of a comparison between the voltage values of the racks and a reference voltage, the discharging count being increased each time the voltage value of a rack is found lower than the reference voltage and the charging count being increased each time the voltage value of a rack is found higher than the reference voltage; and (e) a charging/discharging step of charging or discharging the racks according to the charging or discharging count, wherein step (e) comprises: - sequentially discharging the racks from a rack having the maximum voltage value to a rack having the minimum voltage value, when the discharging count is higher than the charging count and the minimum voltage value is higher than the reference voltage; or - sequentially charging the racks from a rack having the minimum voltage value to a rack having the maximum voltage value, when the discharging count is higher than the charging count and the minimum voltage value is lower than the reference voltage; or - sequentially discharging the racks from a rack having the maximum voltage value to a rack having the minimum voltage value, when the charging count equals the discharging count; or - sequentially charging the racks from a rack having the minimum voltage value to a rack having the maximum voltage value, when the charging count is higher than the discharging count. - sequentially discharging the racks from a rack having the maximum voltage value to a rack having the minimum voltage value, when the discharging count is higher than the charging count and the minimum voltage value is higher than the reference voltage; or - sequentially charging the racks from a rack having the minimum voltage value to a rack having the maximum voltage value, when the discharging count is higher than the charging count and the minimum voltage value is lower than the reference voltage; or - sequentially discharging the racks from a rack having the maximum voltage value to a rack having the minimum voltage value, when the charging count equals the discharging count; or - sequentially charging the racks from a rack having the minimum voltage value to a rack having the maximum voltage value, when the charging count is higher than the discharging count.