Vanadium Redox Flow Batteries are one of the most interesting options for the large energy storage. In spite of the tremendous improvements that have taken place in recent years concerning materials and architectures, laboratory tests under controlled conditions have almost always been confined to small-scale single-cell devices. This paper reports the experimental results obtained on an industrial-size 9 kW/26 kW h 40-cell 600-cm(2) stack, during charge and discharge in two working conditions, i.e. fast mode and steady-state mode, representative of real grid operations, which occur e.g. in primary frequency regulation and peak shaving. Results show that current densities as high as 665 mA cm(-2) and power densities of 370 mW cm(-2) per cell at a specific flow rate of 20.5.10(-3) cm s(-1) can be achieved in the stack at high state of charge, thanks to proper control strategies. A procedure for qualifying such performance has been developed and is here presented.