A system for estimating a power of a secondary battery, comprising: a secondary battery; and an apparatus for estimating a power of the secondary battery with: a discharging means configured to discharge the secondary battery under discharge conditions of a plurality of discharge rates different from each other; a sensor means configured to measure a plurality of discharge termination voltages corresponding to the discharge conditions of the plurality of discharge rates, each discharge termination voltage representing a voltage of the secondary battery measured immediately after discharge of the secondary battery; and a control means configured to approximate a correlation between the plurality of discharge rates and the plurality of discharge termination voltages by a two-dimensional linear equation, calculate a maximum discharge rate of the secondary battery corresponding to a minimum discharge voltage using the linear equation, and estimate a maximum power of the secondary battery from the calculated maximum discharge rate, characterized in that the secondary battery includes a blended cathode material comprising at least a first cathode material and a second cathode material and the discharge rates of the plurality of discharge rates are set larger than a transition discharge rate which is a discharge rate at which a cathode material, into which the working ions are mainly intercalated, changes from the first cathode material to the second cathode material during discharge of the secondary battery. a secondary battery; and an apparatus for estimating a power of the secondary battery with: a discharging means configured to discharge the secondary battery under discharge conditions of a plurality of discharge rates different from each other; a sensor means configured to measure a plurality of discharge termination voltages corresponding to the discharge conditions of the plurality of discharge rates, each discharge termination voltage representing a voltage of the secondary battery measured immediately after discharge of the secondary battery; and a control means configured to approximate a correlation between the plurality of discharge rates and the plurality of discharge termination voltages by a two-dimensional linear equation, calculate a maximum discharge rate of the secondary battery corresponding to a minimum discharge voltage using the linear equation, and estimate a maximum power of the secondary battery from the calculated maximum discharge rate, a discharging means configured to discharge the secondary battery under discharge conditions of a plurality of discharge rates different from each other; a sensor means configured to measure a plurality of discharge termination voltages corresponding to the discharge conditions of the plurality of discharge rates, each discharge termination voltage representing a voltage of the secondary battery measured immediately after discharge of the secondary battery; and a control means configured to approximate a correlation between the plurality of discharge rates and the plurality of discharge termination voltages by a two-dimensional linear equation, calculate a maximum discharge rate of the secondary battery corresponding to a minimum discharge voltage using the linear equation, and estimate a maximum power of the secondary battery from the calculated maximum discharge rate, characterized in that the secondary battery includes a blended cathode material comprising at least a first cathode material and a second cathode material and the discharge rates of the plurality of discharge rates are set larger than a transition discharge rate which is a discharge rate at which a cathode material, into which the working ions are mainly intercalated, changes from the first cathode material to the second cathode material during discharge of the secondary battery.