Disclosed is an apparatus for estimating a voltage of a secondary battery which includes a cathode comprising a first cathode material and a second cathode material with different operating voltage ranges, an anode comprising an anode material and a separator for separating the cathode from the anode. The apparatus comprises a control unit configured to estimate a voltage of a secondary battery based on a circuit model including a first cathode material circuit, a second cathode material circuit and an anode material circuit, each circuit modeled to change its voltage according to State Of Charge (SOC) of the electrode material corresponding the circuit and a current flowing through the circuit.