NOVELTY - A membrane (A1) comprises (wt.%): polyethylene ( greater than or equal to 1) and polypropylene (4-20) having a molecular weight of greater than or equal to 5.0x 105 and a heat of fusion of greater than or equal to 80.0 J/g, where the weight percentage is based on the weight of polymer in the membrane, and the membrane is microporous and has a thickness of less than or equal to 12.0 mu m.    USE - As membrane used as battery separator film for making membrane product; in battery e.g. lithium ion polymer battery, which is used for a motor system of electric vehicle or hybrid electrical vehicle (claimed); in nickel-hydrogen battery, nickel-cadmium battery, nickel-zinc battery, silver-zinc battery or lithium-ion battery.    ADVANTAGE - The membrane is microporous and has improved meltdown temperature and sufficient electrolyte affinity. The thin polymeric membrane has affinity for polymer used as electrolyte medium in polymer batteries and that dimensionally stable at elevated temperature. The membrane shows a thickness of less than or equal to 12.0 (preferably less than or equal to 10) mu m; a meltdown temperature of greater than or equal to 145 degrees C; a normalized electrolyte affinity of less than or equal to 2.5 seconds/ mu m; porosity of 20-80%; a normalized air permeability of less than or equal to 50.0 seconds/100 cm3/ mu m; a normalized pin puncture strength of greater than or equal to 2.5x 102 mN/ mu m; and a machine direction (MD) and a transverse direction (TD) tensile strength of greater than or equal to 1.4x 105 kPa.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) producing a microporous membrane involving: a) extruding a mixture of diluent and polymer, where the polymer comprises polyethylene ( greater than or equal to 1 wt.%), and polypropylene (4-20 wt.%), where the weight percentages are based on the weight of polymer in the polymer-diluent mixture, b) stretching the extrudate in at least one planar direction, and c) removing at least a portion of the diluent from the stretched extrudate; and    (2) a battery comprising an anode, a cathode, electrolyte, and a battery separator comprising the membrane (A1) located between the anode and the cathode.