An apparatus for diagnosing a battery comprising: a sensing unit configured to measure a voltage of a battery; and a processor configured to: estimate a SOC of the battery, map the voltage value received from the sensing unit and the estimated SOC with each other to obtain mapped SOC-voltage data, detect a plurality of inflection points from the mapped SOC-voltage data to obtain a plurality of inflection data, calculate from the inflection data a differential coefficient as a value of a first-order derivative expressed as dV/dSOC at the detected inflection points; calculate a differential coefficient increase or decrease rate at each of the plurality of the inflection points from the detected inflection data as a difference between the calculated differential coefficient and a preset reference differential coefficient calculated from the inflection data detected from the SOC-voltage data of the battery in its beginning of life state without degradation, characterized in that said processor is further configured to diagnose that an electrode reaction resistance of the battery increased when at least one of the plurality of calculated differential coefficient increase or decrease rates is outside of a preset reference increase or decrease rate range, wherein the electrode reaction resistance is a parameter corresponding a reaction rate of an electrode reaction occurring at an interface between an electrode and a solution, and reduce the magnitude of a charge or discharge current of the battery based on a ratio corresponding to any one of the plurality of calculated differential coefficient increase or decrease rates, only when it is diagnosed that the electrode reaction resistance is increased. a sensing unit configured to measure a voltage of a battery; and a processor configured to: estimate a SOC of the battery, map the voltage value received from the sensing unit and the estimated SOC with each other to obtain mapped SOC-voltage data, detect a plurality of inflection points from the mapped SOC-voltage data to obtain a plurality of inflection data, calculate from the inflection data a differential coefficient as a value of a first-order derivative expressed as dV/dSOC at the detected inflection points; calculate a differential coefficient increase or decrease rate at each of the plurality of the inflection points from the detected inflection data as a difference between the calculated differential coefficient and a preset reference differential coefficient calculated from the inflection data detected from the SOC-voltage data of the battery in its beginning of life state without degradation, characterized in that said processor is further configured to diagnose that an electrode reaction resistance of the battery increased when at least one of the plurality of calculated differential coefficient increase or decrease rates is outside of a preset reference increase or decrease rate range, wherein the electrode reaction resistance is a parameter corresponding a reaction rate of an electrode reaction occurring at an interface between an electrode and a solution, and reduce the magnitude of a charge or discharge current of the battery based on a ratio corresponding to any one of the plurality of calculated differential coefficient increase or decrease rates, only when it is diagnosed that the electrode reaction resistance is increased. estimate a SOC of the battery, map the voltage value received from the sensing unit and the estimated SOC with each other to obtain mapped SOC-voltage data, detect a plurality of inflection points from the mapped SOC-voltage data to obtain a plurality of inflection data, calculate from the inflection data a differential coefficient as a value of a first-order derivative expressed as dV/dSOC at the detected inflection points; calculate a differential coefficient increase or decrease rate at each of the plurality of the inflection points from the detected inflection data as a difference between the calculated differential coefficient and a preset reference differential coefficient calculated from the inflection data detected from the SOC-voltage data of the battery in its beginning of life state without degradation, characterized in that said processor is further configured to diagnose that an electrode reaction resistance of the battery increased when at least one of the plurality of calculated differential coefficient increase or decrease rates is outside of a preset reference increase or decrease rate range, wherein the electrode reaction resistance is a parameter corresponding a reaction rate of an electrode reaction occurring at an interface between an electrode and a solution, and reduce the magnitude of a charge or discharge current of the battery based on a ratio corresponding to any one of the plurality of calculated differential coefficient increase or decrease rates, only when it is diagnosed that the electrode reaction resistance is increased.