A six-parameter model of the open-circuit voltage (OCV) for all-vanadium redox flow battery (VRB) was proposed.This model indicated that OCV was associated with the electromotive force of the cell

VO2+/VO2+ electrode potential

and the transport behavior of vanadium ions in electrolyte across the membrane.Two plateaus and a turning point were obtained in OCV curve by this model

indicating that the calculated results were in good agreement with the experimental data.The dependence of concentrations of four kinds of vanadium ions on time was also investigated.The result showed that the electrolyte imbalance was caused by the differences of Donnan balance and diffusion coefficients of different vanadium ions across the membrane.This model can provide theoretical fundamentals for operation process and engineering guidance for electrolyte management of VRB.