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 (30; 130; 230) 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, characterized in that 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, wherein the characteristic point indicates a maximum point, a minimum point or an inflection point, wherein the secondary battery system is configured to determine, based on the dV/dQ value, at least one of whether or not the secondary battery has reached a state corresponding to the characteristic point in the Q-dV/dQ curve and whether or not the secondary battery has reached a state corresponding to the characteristic point in the V-dV/dQ curve, and the secondary battery system is further configured to detect the state of the secondary battery system based on the storage amount value Q at the characteristic point in the Q-dV/dQ curve, a difference value of the storage amount Q between two specific characteristic points selected from a plurality of characteristic points appearing in the Q-dV/dQ curve, or a difference value of the battery voltage V between two specific characteristic points selected from a plurality of characteristic points appearing in the V-dV/dQ curve. 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 (30; 130; 230) 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, 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, wherein the characteristic point indicates a maximum point, a minimum point or an inflection point, wherein the secondary battery system is configured to determine, based on the dV/dQ value, at least one of whether or not the secondary battery has reached a state corresponding to the characteristic point in the Q-dV/dQ curve and whether or not the secondary battery has reached a state corresponding to the characteristic point in the V-dV/dQ curve, and the secondary battery system is further configured to detect the state of the secondary battery system based on the storage amount value Q at the characteristic point in the Q-dV/dQ curve, a difference value of the storage amount Q between two specific characteristic points selected from a plurality of characteristic points appearing in the Q-dV/dQ curve, or a difference value of the battery voltage V between two specific characteristic points selected from a plurality of characteristic points appearing in the V-dV/dQ curve.