NOVELTY - The system has a memory unit (202) that memorizes correlation with physical-property value having charge condition of calculated electrolyte solution to be examined. A charge condition calculating unit (203) calculates charge condition of electrolyte solution to be examined using the measured correlation with physical-property value having charge condition of electrolyte solution to be examined, and the memorized correlation. A charge-and-discharge control unit (204) controls charge or discharge of main portion based on the result depended on charge condition calculating unit.    USE - Redox flow cell system for use as secondary battery system in high capacity electric power generation system (all claimed) such as solar power generation and wind power generation systems, and for providing momentary low and countermeasure against power failure and load leveling.    ADVANTAGE - The redox flow cell system is provided to grasp the exact charge condition of redox flow cell correctly and in efficient manner. The redox flow cell system is charged and/or discharged by making the battery cell circulate-supply positive electrode electrolyte solution and negative electrode electrolyte solution. The overcharge and overdischarge of redox flow cell system is prevented reliably. The electric power generation system is provided to perform output stabilization and supply-and-demand smoothing appropriately. The operation of the electric power generation system is controlled optimally.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) an electric power generation system;    (2) a method for controlling redox flow cell system; and    (3) a method for controlling electric power generation system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the redox flow cell system. (Drawing includes non-English language text)    Cell (100)    Measurement unit (201)    Memory unit (202)    Charge condition calculating unit (203)    Charge-and-discharge control unit (204)