For over a hundred years from its conception, the lead–acid cell was normally operated with unrestricted access between the surface of its electrolyte and the external atmosphere so that, during periods of overcharge, hydrogen and oxygen were lost from the cell via electrolysis. As a result, periodic additions of distilled water were necessary. Since about 1970, an alternative to the traditional ‘flooded’ cell has been available —one that avoids the need for water maintenance. Moreover, acid is immobilized in the new design and this endows the cell with the additional advantages of being ‘spill-proof’ and able to operate in any orientation (upright, on its side, or even upside down). The change to the so-called ‘valve-regulated lead–acid’ (VRLA) technology has not, however, been accomplished without some difficulty. Experience has demonstrated forcibly the fundamental differences between the two systems, and the lead– acid battery manufacturing industry has faced major challenges in investing the VRLA version with a performance to match that of its flooded predecessor. Nevertheless, research into understanding the electrochemisty, producing improved cell components and optimizing charge strategies has resulted in VRLA batteries becoming well-established and reliable devices. Operators now take advantage of the particular properties of these batteries for the storage of electrical energy in a wide variety of stationary applications.