Monitoring the state of charge of all-vanadium redox flow batteries to identify crossover of electrolyte

During charging and discharging of an all-vanadium redox flow battery electrolyte components cross the membrane in the battery cell. This so called crossover leads to partial discharging and capacity loss. For the identification of electrolyte crossover and efficient operation of the battery the accurate and reliable determination of the state of charge is essential. In this study, state of charge estimation from open cell voltage measured currentless at a reference cell as well as from open circuit potentials measured at flow cells in the positive and negative electrolyte loop is discussed. Comparison of the state of charge obtained from the different potential measurements suggests, that monitoring the half-cell potentials of the battery allows timely detection of crossover processes and furthermore the determination of the direction of crossover between the half-cells.