The improvement and development of the separator discussed in this paper proceeded in accordance with the changes in the specifications for the batteries which were first made with wooden separators, then progressed through microporous rubber separators, cellulose separators and synthetic pulp separators with glass mats (hereafter, abbreviated as the SPG separator), and now polyethylene separators (hereafter termed the PE separator) have evolved. This sequential change in separator technology has provided continuous improvements in the charge and discharge efficiency of batteries and has given high vehicle-starting capability and reliability. Moreover, short-circuits (caused by particles of active material dislodged from the battery plates) are prevented due to the smaller pores and excellent electrochemical oxidation resistance of the PE separator. These features also contribute greatly to the improvement in battery life.