A system configured to estimate a state of charge, SOC, of a battery, comprising: a secondary battery including (i) a cathode comprising a blended cathode material having a first cathode material and a second cathode material, wherein the first and second cathode materials have different operating voltage ranges; (ii) an anode comprising an anode material; and (iii) a separator for separating the cathode from the anode; and an apparatus comprising: a sensor configured to measure a dynamic voltage of the secondary battery during charging of the secondary battery; and the apparatus is characterised by a control unit configured to calculate an inflection point identifier from the measured dynamic voltage, V, determine a parameter corresponding to a transition region voltage pattern generated in a SOC region where the kind of a cathode material mainly reacting with operating ions changes when the inflection point identifier satisfies a condition under which an inflection point occurs, and estimate a SOC of the secondary battery from the calculated parameter by using a predetermined relationship between the parameter and the SOC of the secondary battery, wherein the inflection point identifier is a first-order differential value of the dynamic voltage with respect to time or the SOC of the secondary battery, and wherein the condition under which the inflection point occurs is when the first-order differential value becomes a maximum value. a secondary battery including (i) a cathode comprising a blended cathode material having a first cathode material and a second cathode material, wherein the first and second cathode materials have different operating voltage ranges; (ii) an anode comprising an anode material; and (iii) a separator for separating the cathode from the anode; and an apparatus comprising: a sensor configured to measure a dynamic voltage of the secondary battery during charging of the secondary battery; and the apparatus is characterised by a control unit configured to calculate an inflection point identifier from the measured dynamic voltage, V, determine a parameter corresponding to a transition region voltage pattern generated in a SOC region where the kind of a cathode material mainly reacting with operating ions changes when the inflection point identifier satisfies a condition under which an inflection point occurs, and estimate a SOC of the secondary battery from the calculated parameter by using a predetermined relationship between the parameter and the SOC of the secondary battery, wherein the inflection point identifier is a first-order differential value of the dynamic voltage with respect to time or the SOC of the secondary battery, and wherein the condition under which the inflection point occurs is when the first-order differential value becomes a maximum value. a sensor configured to measure a dynamic voltage of the secondary battery during charging of the secondary battery; and the apparatus is characterised by a control unit configured to calculate an inflection point identifier from the measured dynamic voltage, V, determine a parameter corresponding to a transition region voltage pattern generated in a SOC region where the kind of a cathode material mainly reacting with operating ions changes when the inflection point identifier satisfies a condition under which an inflection point occurs, and estimate a SOC of the secondary battery from the calculated parameter by using a predetermined relationship between the parameter and the SOC of the secondary battery, wherein the inflection point identifier is a first-order differential value of the dynamic voltage with respect to time or the SOC of the secondary battery, and wherein the condition under which the inflection point occurs is when the first-order differential value becomes a maximum value.