The types and properties of separators used for lead-acid batteries are reviewed. Attention is focused on the pocket-type polyethylene (PE) separator as this is widely used in present-day automotive batteries, i.e. in low-maintenance batteries with expanded lead-calcium grids. An improved PE separator has been developed by using a PE resin of high molecular weight. The resistance of the separator to attack by hot sulphuric acid is increased by a factor of 1.5. Batteries using the improved separator show a 40% increase in lifetime under the SAE 75 degreesC life-cycle test.