The present invention relates to a high performance lead-acid storage battery having a special microporous separator between a positive and a negative plate in a cell of the battery. The separator is comprised of a microporous embossed thin sheet having many small projecting embossed parts which provides unique characteristics in a cell including low electric internal resistance in the cell, sufficient electrolyte between the plates in the cell and sufficient elasticity of the separator to prevent active material shedding, thereby giving a high performance at a high rate of discharge and long life.