Patent ID: 8742725

Claim:
A secondary battery system including a secondary battery, wherein the secondary battery includes an electrode body having a first electrode plate, a second electrode plate, and a separator, the first electrode plate including a first active material that makes a phase change by charge and discharge, and the second electrode plate including a second active material that performs two-phase coexistence type charge and discharge, the secondary battery system comprising: dV/dQ calculation means for calculating a dV/dQ value that is a ratio of a change amount dV of a battery voltage V of the secondary battery with respect to a change amount dQ of a storage amount Q of the secondary battery when the storage amount Q is changed during charge and discharge of the secondary battery, wherein a state of the secondary battery system is detected by use of a characteristic point appearing in a Q-dV/dQ curve representing a relationship between the storage amount value Q and the dV/dQ value or a characteristic point appearing in a V-dV/dQ curve representing a relationship between the battery voltage value V and the dV/dQ value, and the characteristic point is a maximum point, a minimum point, or an inflection point.