This article presents results of studies, carried out on chemical and electrochemical processes, resulting in energy and capacity losses in vanadium redox flow battery (VRFB). Two mechanisms, concerning losses, were evaluated: hydrogen evolution and imbalance of vanadium ions in electrolytes. Capacity losses of the battery were measured as the reduction of total electric charge, obtained from battery in discharging process, during charging/discharging cycle operation. Charge losses were verified using chronoamperometry measurements, where the electric current was measured and used to quantify the losses while keeping the battery at constant voltage. Methods for compensation of capacity losses and rebalancing of the battery were investigated and discussed. An electrolysis cell was developed to compensate the losses of charge in one half-cell only and, therefore, to rebalance the electrolytes. (C) 2013 Elsevier B.V. All rights reserved.