Polyethylene separators currently represent about 90% of the lead-acid separator market. They are composed of precipitated silica, ultra-high molecular weight polyethylene (UHMW-PE), process oil and minor ingredients. Obviously, the dispersion of precipitated silica will affect the separator electrical resistance. However, results show that the porosity is constituted of two types of pores in membranes: pores wettable by water and pores non-wettable by water. These two porosities can be distinguished using suitable measurement procedures. Only the porosity wettable by water is related to the electrical resistivity of the separator. Moreover, this porosity is strongly dependent on the quality of precipitated silica dispersion and distribution. Finally, it is possible to obtain a quantitative measurement of the effect of porosity and filler dispersion on the electrical resistivity by a simple separator porosity characterization.