An apparatus for estimating a battery (B) free capacity, comprising: a sensing unit configured to measure a voltage of a battery; and a processor operably coupled to the sensing unit, wherein the processor is configured to detect first inflection points of first SOC-voltage data based on the first SOC-voltage data of the battery in which a first SOC of the battery and the voltage of the battery are mapped, characterised in that the processor is configured to determine a first SOC region based on the first SOCs corresponding to the first inflection points, and in that the processor is further configured to select second inflection points satisfying an attribution electrode condition among second inflection points detected from second SOC-voltage data of a half cell for each of a positive electrode and a negative electrode of the battery, wherein the attribution electrode condition is satisfied when the second SOC corresponding to the detected second inflection point is included in the first SOC region, and in that the processor is configured to estimate a free capacity of the half cell corresponding to the selected second inflection points by using a SOC ratio of a first SOC difference value between first SOCs corresponding to the first inflection points and a second SOC difference value between second SOCs of the half cell corresponding to the selected second inflection points, and in that the processor is further configured to increase or decrease the second SOCs corresponding to the selected second inflection points by using the SOC ratio so that the second SOC of the battery is revised to a first revised SOC of the battery. a sensing unit configured to measure a voltage of a battery; and a processor operably coupled to the sensing unit, wherein the processor is configured to detect first inflection points of first SOC-voltage data based on the first SOC-voltage data of the battery in which a first SOC of the battery and the voltage of the battery are mapped, characterised in that the processor is configured to determine a first SOC region based on the first SOCs corresponding to the first inflection points, and in that the processor is further configured to select second inflection points satisfying an attribution electrode condition among second inflection points detected from second SOC-voltage data of a half cell for each of a positive electrode and a negative electrode of the battery, wherein the attribution electrode condition is satisfied when the second SOC corresponding to the detected second inflection point is included in the first SOC region, and in that the processor is configured to estimate a free capacity of the half cell corresponding to the selected second inflection points by using a SOC ratio of a first SOC difference value between first SOCs corresponding to the first inflection points and a second SOC difference value between second SOCs of the half cell corresponding to the selected second inflection points, and in that the processor is further configured to increase or decrease the second SOCs corresponding to the selected second inflection points by using the SOC ratio so that the second SOC of the battery is revised to a first revised SOC of the battery. <EOS>