 A 1 kW–4 kWh zinc-air flow battery has been built at Técnicas Reunidas facilities. The battery is divided in three different stacks connected in parallel, each of them comprising 20 cells connected in series and 0.25 m3 of electrolyte. The main challenges found on scaling up include the necessity of using three electrodes per cell, electrolyte leakage, ohmic resistance and gas diffusion electrode flooding. The three electrode solution is not optimal because it adds mechanical complexity and cost but it is a robust solution to evaluate the performance of the system as a baseline. During pilot plant operation, a phenomenon called shunt currents has revealed to reduce coulombic efficiency by 18%. In order to better understand this phenomenon, a mathematical model has been developed and it has been validated using real measurements taken on one of the 20-cell stacks. The major part of the challenges were solved and technical viability has been evaluated. However, efficiency, durability and the bifunctional air electrode are the challenges to overcome before this technology becomes ready for commercialization.