is the cheaper method of manufacturing sulphuric acid and probably used for battery grade. This link indicates that lead contaminants are generally in the 1-5 ppm range, though none are specified for battery grade. Indicates that contaminants come from the ore feedstock rather than lead chamber.
http://members.rogers.com/acidmanual/properties_acid_quality.htm (http://members.rogers.com/acidmanual/properties_acid_quality.htm)
Nice to see Yanks spelling 'sulphuric' properly.