Disclosed is an apparatus for estimating a state of charge (SOC) of a secondary battery which includes (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. The apparatus includes a sensor configured to measure a dynamic voltage of the secondary battery during charging of the secondary battery, and a control unit configured to identify a dynamic voltage profile of the secondary battery as a transition region voltage pattern, calculate a parameter of the transition region voltage pattern, and estimate a SOC of the secondary battery from the calculated parameter by using a predetermined relationship between the parameter and the SOC.
