The vanadium redox flow battery pioneered by Skyllas-Kazacos et al. at the University of New South Wales (UNSW) is currently considered as one of the few electrochemical energy storage systems suitable for use in the large-scale utility applications that are emerging in response to the increasing global implementation of renewable energy technologies for the mitigation of greenhouse gas emissions. While the original all-vanadium redox flow battery (G1 VRF) has already been successfully implemented in a wide range of stationary field trials in Japan, U.S.A., Austria, Italy and Australia, further cost reduction has been needed for its widespread market up-take. In this paper, up to 80% efficiency is reported for the latest 5–10 kW G1 VRF battery stack employing low-cost stack materials that are expected to achieve the necessary cost structure for the majority of stationary energy storage applications. While the G1 VRF battery has shown high efficiencies and excellent cycle life, however, its low energy density has restricted its use in mobile applications. The new Generation 2 vanadium bromide redox battery (G2 V/Br) patented by UNSW in 2001 has been shown to potentially double the energy density of the G1 VRF battery, allowing mobile applications to be considered. In this paper, the performance of the G2 V/Br is presented at a range of temperatures and the use of complexing agents is shown to successfully bind any bromine produced during charging to prevent the formation of bromine vapours. Copyright © 2009 John Wiley & Sons, Ltd.