TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The steps are performed in two separate steps and the mixing of the vanadium pentoxide powder sulfuric acid suspension with the negatively charged electrolyte solution is carried out with part of the negatively charged electrolyte solution purposely diverted from the negatively charged electrolyte circuit of the flow redox battery system and then returned to it. The steps are conducted in continuous mode using a stream of negatively charged electrolyte solution diverted from the respective circuit of the flow redox battery system, for passing through at least a stirred reactor, into which the vanadium pentoxide suspension is continuously dosed, before returning a volume of the negative electrolyte solution and fresh acid vanadium sulfate solution produced equivalent to the diverted volume to the negative electrolyte circuit of the working battery and collecting the freshly produced volume of acid vanadium sulfate solution. The method further includes: diverting a stream of positively charged electrolyte solution from the respective circulation circuit of the battery for passing it in an absorber fed with gaseous sulfur dioxide for reducing vanadium+5 (VO2+) contained in the positively charged electrolyte solution at a rate adapted to maintain a balanced state of charge of the two electrolyte solutions of the battery system during the process of producing fresh electrolyte solution from vanadium pentoxide; and percolating the positively charged electrolyte solution leaving the absorber through an air fed packed column stripper of any residue of un-reacted sulfur dioxide before returning the diverted stream of positively charged electrolyte solution to the respective circulation circuit. The collected volume of freshly produced acid vanadium sulfate solution is: distributed to the two separated electrolyte solution circuits of the flow redox battery system; and accumulated and successively distributed into the two separated vanadium electrolyte solution circuits of the flow redox battery system. Preferred Components: The absorber is gravity operated packed columns, tubular absorber with gas ejector and inner static mixing means, stirred mixer with adjustable overflow liquid outlet, turbulent flow tubular absorber or gas bubbler absorber. EXAMPLE - Two electrolyte circuits of 5 kW energy storage systems were filled with 40 l of standard electrolyte solution per tank sufficient to fill the battery circuit, namely cell compartments, pumps and piping (12 l) and to have sufficient liquid (28 l) in the tank for safely operating the recirculation pumps. (Standard electrolyte solution containing 1.6 mole/l of vanadium of which 0.8 mole/l of vanadium (V=3) and 0.8 mole/l of vanadium (V+4) or vanadium oxide (VO+2). Keeping the two electrolyte pumps in continuous operation (each pump operated with a flow of 3000 l/hour) the two electrolyte solutions (40 l/circuit) were charged up to a state of charge of 50% (open circuit battery voltage of 1.35 Volt). Once the 50% state of charge was reached, 88 l/hour of negatively charged electrolyte solution (out of a total of 3000 l/hour being pumped in the circuit) were diverted from the delivery pipe of the negative electrolyte pump to flow through a cylindrical 30 liter stirred plastic reactor, into which 8.55 kg/hour of vanadium pentoxide were added after premixing it with sulfuric acid 6.26 l/hour and water 51.6 l/hour inside a smaller 10 liter plastic vessel also provided with a stirrer. Both the 30 liter reaction vessel and the 10 liter mixing vessel were provided with a stainless steel cooling pipe coil. The overflow stream of solution (88.15 liter/hour) from the reactor was partially returned to the negative electrolyte tank to keep a constant level of electrolyte in it and a balance stream of 58.75 l/hour, collected and stored in a separate tank represented the fresh vanadium electrolyte solution produced.