State‐of‐Charge Monitoring for Vanadium Redox Flow Batteries

Abstract

The state of charge (SOC) is one of the most important parameters to monitor during battery operation. In the vanadium redox flow battery (VRFB) system, a common approach is to correlate the overall system SOC with the potential measured from an open-circuit cell. This approach provides accurate estimations when the system is balanced. However, the battery system can go out of balance during operation due to vanadium ions crossover and side reactions. It is important to monitor the SOCs of each half-cell separately to detect any electrolyte imbalances. One approach is to measure both half-cell potentials vs. a reference potential and thereby determining the half-cell SOCs. Several other approaches have been developed to determine half-cell SOCs based on the electrolyte physical properties, such as conductivity, density, and viscosity. Model-based estimation approaches, such as Coulomb counting and state observer, have also been developed to estimate the system SOC.