There are four major requirements for a separator in a flooded lead-acid battery:
- prevent shorting between electrodes
- stable in sulfuric acid
- allow ionic conduction without dendrite growth
- provide mechanical spacing between electrodes
Historically, lead acid battery separators have included cellulose, polyvinyl chloride, organic rubber, and polyolefins. Today, most flooded lead acid batteries utilize “polyethylene separators” — a misnomer because these microporous separators require large amounts of precipitated silica to be acid-wettable. Silica is responsible for the separator’s electrical properties; polyethylene is responsible for the separator’s mechanical properties. The porosity range for polyethylene separators is 50-65%.