Disclosed is a battery system for a secondary battery including a blended cathode material, and an apparatus and method for managing a secondary battery having a blended cathode material. The blended cathode material includes at least a first cathode material and a second cathode material. The first and second cathode materials have different operating voltage ranges. When the secondary battery comes to an idle state or a no-load state, the battery system detects a voltage relaxation occurring by the transfer of operating ions between the first and second cathode materials.