Ribs are formed on a nonwoven mat to produce a battery separator. The ribs are formed by extruding a foamed polymer onto the nonwoven mat by extruding the foamed polymer through a multihole die as the nonwoven mat follows an arc passing over a mat backup plate. After the ribs have been extruded and the foamed polymer is still soft, the nonwoven mat with the extruded ribs may be passed through a fixed gap such as a pair of calendar rolls to produce a battery separator of uniform thickness.