For decades, valve regulated lead acid batteries with gel electrolyte have proved their excellent performance in deep cycling applications. However, their higher cost, when compared with flooded batteries, has limited their use in cost sensitive applications, such as automotive or PV installations. The use of flooded batteries in deep or partial state of charge working conditions leads to limited life due to premature capacity loss provoked by electrolyte stratification. Different electrolyte formulations have been tested, in order to achieve the best compromise between cost and life performance. Work carried out included electrochemical studies in order to determine the electrolyte stability and diffusional properties, and kinetic studies to check the processability of the electrolyte formulation. Finally, several 12 V batteries have been assembled and tested according to different ageing profiles. (c) 2005 Elsevier B.V. All rights reserved.