In 1967 work was begun on the research and development of small, cylindrical lead/acid cells containing spirally-wound electrodes. Four years later the resulting products were offered for sale: a cell equivalent in size to the conventional manganese dioxide D-cell, and another having twice the capacity. These cells were the first to use a separator material consisting of microfiber glass paper, now generally termed 'absorbent glass mat' (AGM). The sulfuric acid electrolyte incompletely saturates this separator, permitting oxygen gas transport directly through the separator to react with the sponge lead negative plate during overcharge of the cell. Thus, a recombination reaction is achieved which is analogous to that used in the sealed nickel-cadmium cell. A number of technical developments were incorporated, including substantial compression of the plate-separator assembly. This greatly lengthened the service life of these first 'valve-regulated' cells. In the following years, many sizes of rectangular batteries, using the principles described, have been manufactured throughout the world.