In this article the energy and efficiency losses during cyclic operation of vanadium redox flow battery (VRFB) are studied. A theoretical model, based on the equation system of fluid dynamics, including convective and diffusive transfer of species in electrolytes, and electrochemical interaction of ions, is developed. The reaction rates are calculated on the basis of the Butler–Volmer equation. The electric field distributions in half-cells are found by solving of the equation system, which is obtained by Ohm’s law. A test battery was constructed. The parameters of the battery – voltage, current, power, energy and capacity during cyclic operation are measured and calculated. The numerical model was validated with the experimental data. The results of the calculation and of the measurement agree within ± 2 % , which corresponds to the measurement accuracy. The decreasing of the energy efficiency during cyclic operation of the battery is observed and simulated. It was found that the capacity decrease is caused mainly by the imbalance of the positive and the negative electrolytes. The increasing of the internal resistance depends on the passivation of the negative collector surface. An in situ regeneration of the energy efficiency, based on an inverse electric potential in the negative half-cells during the cyclic operation, was developed. The reduction of the internal resistance approximately of 2.4 times was measured after the reactivation of the negative collector surface.