State-of-Charge Monitoring and Electrolyte Rebalancing Methods for the Vanadium Redox Flow Battery

Abstract

A major issue with all flow batteries is the control of the imbalance between the two half-cell electrolytes that arises as a result of the differential transfer of ions across the membrane and the inevitable gassing side reactions that can occur during charging. While a number of methods are available to rebalance electrolyte state of charge and restore capacity, reliable methods are needed to monitor the state-of-charge of each individual half-cell solution in order to determine the appropriate action to be taken by the battery control system. In this study different methods of state-of-charge monitoring have been considered for application in the All-Vanadium Redox Flow Battery (VRB). Half-cell potentials and electrolyte conductivities were calibrated as a function of state-of-charge and evaluated for state-of-charge monitoring of individual half-cell electrolytes for the purpose of capacity restoration and control. An empirical model based on experimental conductivity data has been shown to provide accurate predictions, with an average error of 0.77%, of the conductivity of the positive half-cell electrolyte as a potential state-of-charge detection tool. Separate monitoring of the two half-cell electrolyte potentials has also been used to determine the state-of-charge of each half-cell solution in order to detect system imbalance. This was used in small laboratory cell tests to determine necessary actions to restore capacity by either remixing the two solutions, or by using chemical rebalancing methods, depending on the cause of the solution imbalance.