A battery separator composed of a microporous sheet product having first and second major surfaces and a thickness of less than about 50 mils, formed from a uniform mixture of a polycarbonate polymer and a filler or a polycarbonate and a structure enhancing agent, said sheet product having a porous sheet embedded within said mixture and between the first and second major surfaces. The porosity of the sheet product is at least about 50 volume percent composed of pores such that the nominal pore diameter increases incrementally from each major surface towards the interior central portion of the sheet product's thickness.