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. <EOS>