There is described a battery separator comprising: a porous membrane (preferably of a cellulose film) having a mean thickness less than about 25 microns (preferably 15 microns) and optionally with one or more of the following properties: a) a tensile strength when dry (measured in the transverse direction of the membrane web) of greater than about 120 Nmm<-2>; b) a wet ionic resistance of less than about 20 mOmega.cm<2>; c) a membrane degree of polymerisation (D.P.) of greater than 350 chain units after being immersed for four hours in a bath of sodium hydroxide and hydrogen peroxide; and/or a) a tensile strength when dry (measured in the machine direction of the membrane web) of greater than about 200 Nmm<-2>. Preferred membranes comprise biopolymeric films, such as films derived from cellulose and/or polylactic acid, more preferably a regenerated cellulosic film made using NMMO as regeneration liquid.
