Mechanical pressure applied to the plate group is known to increase the cycle life of lead-acid batteries. However, the mechanism of the active material stabilisation process is not completely understood yet. The evolution of mechanical pressure in a lead-acid battery during a cycle and through cycle life is also a topic with still many open questions. Additionally, the separation systems available today are not able to exert high compression on the electrode plates: glass mats crush when compressed too much and the gel design does not allow any significant pressure generation at all. Daramic have developed the new acid jellying separator (AJS) allowing the application of mechanical pressure on the plate group. This paper reports on I. The capacity evolution during cycling for cells with different separation systems namely AGM, gel and AJS, under a variety of initial compression levels. 2. The evolution of mechanical pressure on the cell walls during a cycle and through cycle life for different separation systems, and initial compression levels. 3. The condition of the active materials at the end of Life for cells cycled under compression. 4. The effect of the addition of phosphoric acid to the electrolyte. 5. The effect of the application of mechanical pressure on the corrosion of pure lead based on cyclic voltammetry measurements under compression. The results of these studies are presented together with some conclusions about the mechanism and effect of compression on a VRLA battery. (C) 2001 Elsevier Science B.V. All rights reserved.