 The vanadium crossover through the membrane can have a significant impact on the capacity of the vanadium redox flow battery over long-term charge–discharge cycling. The different vanadium ions move unsymmetrically through the membrane and this leads to a build-up of vanadium ions in one-half cell with a corresponding decrease in the other. In this paper, a dynamic model is developed based on different crossover mechanisms (diffusion, migration and convection) for each of the four vanadium ions, water and protons in the electrolytes. With a simple to use approach, basic mass transport theory is used to simulate the transfer of vanadium ions in the battery. The model is validated with own measurements and can therefore predict the battery capacity as a function of time. This is used to analyse the battery performance by applying a continuous overflow from one-half cell to the other. Different overflow rates were analysed with regard to an impact of the performance and electrolyte stability. It was observed that a continuous overflow increases the capacity significantly but that the electrolyte stability plays an essential role using a membrane with a big vanadium crossover. Even with a good performance, a complete remixing of the tanks is necessary to prevent electrolyte precipitations.