NOVELTY - The reduction-oxidation flow battery system comprises a first electrolyte storage tank, a first movable tank separator, a reduction-oxidation flow battery stack assembly (10), a second electrolyte storage tank, a second tank separator, a pump (32), a control system, and a second flexible bladder. The movable tank separator is configured to divide a volume of the first electrolyte storage tank into a first volume portion and a second volume portion and to float at an interface between a more-dense electrolyte and a less-dense electrolyte in the first and second volume portions respectively.    USE - The reduction-oxidation flow battery system is useful: in loads including a distributed electrical grid, a data center, an irrigation pump, a cellular telephone station, an electric vehicle, a vehicle charging system and a building; and in a solar collector or solar energy conversion system such as a concentrating photovoltaic array, solar thermal energy power plant or a solar hot water system.    ADVANTAGE - The cost-effective, renewable and simple reduction-oxidation flow battery system exhibits high storage capacity and power generation efficiency, long lifespan, excellent performance, increased output and less electrolyte breakdown and hydrogen generation.    DETAILED DESCRIPTION - The reduction-oxidation flow battery system comprises a first electrolyte storage tank, a first movable tank separator, a reduction-oxidation flow battery stack assembly (10), a second electrolyte storage tank, a second tank separator, a pump (32), a control system, and a second flexible bladder. The movable tank separator: is configured to divide a volume of the first electrolyte storage tank into a first volume portion and a second volume portion, configured to float at an interface between a more-dense electrolyte in the first volume portion and a less-dense electrolyte in the second volume portion and configured to have a density between a density of the more-dense electrolyte and a density of the less-dense electrolyte; and comprises blocks, a fluid layer, an inflatable balloon, a seal, a circumferential ring of greater thickness than a central region of the movable tank separator, a first flexible bladder and mechanically operable louvers. The reduction-oxidation flow battery stack assembly is joined in fluid communication with the first and second electrolyte storage tanks. The second tank separator is configured to divide a volume of the second electrolyte storage tank into two volume portions. The fluid layer is immiscible in the more-dense electrolyte and the less-dense electrolyte. The control system is configured to control a density of the inflatable balloon by controlling the pump to fill or empty the inflatable balloon. The seal is configured to movably engage a side wall of the first electrolyte storage tank. A ratio between a diameter of the movable tank separator and a thickness of a separator region closest to a side wall of the first electrolyte storage tank is 5:1. The second flexible bladder is placed within the first electrolyte storage tank, and defines a third volume portion. The first volume portion contains a charged electrolyte. The third volume portion contains a discharged electrolyte. The second volume portion contains a ballast fluid. The first flexible bladder defines the first volume portion within the flexible bladder and the second volume portion between the first flexible bladder and the first electrolyte storage tank. The first tank separator comprises a porous matrix configured to prevent convective mixing or agitation between the first volume comprising a top tank segment and the second volume comprising a bottom tank segment. An INDEPENDENT CLAIM is included for a method of reducing mixing of two electrolytes stored within one tank.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of a reduction-oxidation flow battery system.    Redox flow battery stack assembly (10)    Conductive surface (22)    Pump (32)    Electrical switch (44)    Tank separator. (502)