NOVELTY - Producing in situ, from vanadium pentoxide, an acid vanadium sulfate solution, comprises, separate or unified steps of: mixing vanadium pentoxide powder with sulfuric acid and water of dilution to form a suspension of vanadium pentoxide powder in sulfuric acid solution; and mixing the vanadium pentoxide suspension in sulfuric acid with negatively charged electrolyte solution, having a non null state of charge, of the all-vanadium flow redox battery system, for dissolving and reducing the vanadium pentoxide, producing fresh acid vanadium sulfate solution from it.    USE - The method is useful for producing in situ, from vanadium pentoxide, an acid vanadium sulfate solution which is used as electrolyte solution in separated positively and negatively charged electrolyte solution circulation circuits of an all-vanadium flow redox battery system (claimed).    ADVANTAGE - The method: provides electrolyte solution in high yield which increases storage capacity of the flow redox battery system, in simple, fast and cost effective manner using small equipments.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a modified all-vanadium flow redox battery system comprising: a flow redox battery system composed of many flow redox cells having a positive electrode flow compartment and a negative electrode flow compartment, separated by an anionic membrane, separate circulation circuits, including at least a storage tank (V-Neg, V-Pos) and a pump (P1, P2), of an acid vanadium sulfate electrolyte solution in the two electrode compartments of the cells for positively charging the electrolyte solution of one circuit and negatively charging the electrolyte solution of the other circuit; a stirred vessel (R-1) for blending together sulfuric acid, water of dilution and vanadium pentoxide powder forming an acid suspension of it; a stirred reactor (R-2) having an inlet and an outlet coupled to the circulation circuit of the negatively charged electrolyte solution of the battery system; a sulfur dioxide fed absorber (TW1) coupled to the circulation circuit of positively charged electrolyte solution, through which part of the circulating electrolyte is diverted for absorbing sulfur dioxide and chemically reducing vanadium+5 (VO2+) contained in the positively charged electrolyte solution to vanadium+4 (VOSO4); an air fed packed column stripper (TW2) of un-reacted residue of sulfur dioxide, connected in cascade of a liquid outlet of the absorber and having a liquid outlet connected to the respective circulation circuit for returning the diverted stream of positively charged electrolyte solution; and means adapted to dose the vanadium pentoxide suspended in sulfuric acid solution into the negatively charged electrolyte solution flowing through the stirred reactor for dissolving and reducing the suspended vanadium pentoxide to soluble compounds of vanadium+3 (V+3) and vanadium+4 (V+4) at a rate adapted to maintain a balanced state of charge between the two electrolyte solutions circulating in the respective circuits of the flow redox battery system.    DESCRIPTION OF DRAWING(S) - The figure shows a basic scheme of an all-vanadium energy storage flow redox battery system with the ancillary equipment adapted for producing in situ an acid vanadium sulfate solution from commercial vanadium pentoxide powder.    Pump (P1, P2)    Stirred vessel (R-1)    Stirred reactor (R-2)    Sulfur dioxide fed absorber (TW1)    Air fed packed column stripper (TW2)