Charging Electrochemistry, Electronics, and Fluid Requirements for Redox Flow Batteries

The Vanadium redox flow battery has been intensively examined since the 1970s. Here we address theoretically the charging requirements for redox flow batteries (RFBs), and in particular the all-vanadium type. Electrolyte concentrations, overpotential, current density, fluid flow, are all treated as controllable variables, capable of being modelled analytically. Electrolyte fluid contained in the battery stack cells, pumps, external pipes, and tanks are all considered. Using the Butler-Volmer equation, an analytical expression is developed which conveniently relates the overpotential to the total battery terminal current. This terminal current is directly connected to the chemical reaction current density through the battery cross-sectional area. Estimates of charging times to first take the battery from an initial unprepared mixed electrolyte, to a battery beginning charged state are determined. Motivation for this study comes from the need for tractable approaches to find battery terminal voltages, currents, and fluid flow for medium sized 10–50 kW RFBs. An experimental charging circuit used by us is presented and theoretically analyzed which may be useful for vanadium RFBs. Chemistry for initially preparing electrolytes is also provided.