A battery separator for a lead acid (storage) battery is made from a thermoplastic sheet material. The sheet material has a central region flanked by peripheral regions. The central region includes a plurality of longitudinally extending ribs that are integrally formed from the sheet material. The peripheral regions are free of ribs and may include a densified structure. Also disclosed are a method of producing the foregoing separator, an envelope separator made from the sheet material, and a method of making the envelope separator.