NOVELTY - A monolayer microporous membrane comprises a polyolefin having weight average molecular weight (Mw) greater than 1x 106, the membrane having a normalized air permeability less than or equal to 4x 102 seconds/100cm3/20 mu m, and a heat shrinkage at 130 degrees C of less than or equal to 15% in at least one planar direction.    USE - As battery separator film; where battery used as power source for electric vehicle or hybrid electric vehicle (claimed).    ADVANTAGE - The membrane has a maximum shrinkage in the molten state in at least one planar direction of less than or equal to 10%. The membrane has thickness of 1-50 mu m, porosity of 25-80%, normalized pin puncture strength of greater than or equal to 3x 103 (preferably greater than or equal to 3.5x 103) mN/20 mu m, a tensile strength of greater than or equal to 4x 104 kPa, TD tensile elongation greater than or equal to 100%, meltdown temperature greater than or equal to 145 (preferably 170) degrees C, a shutdown temperature less than or equal to 140 degrees C, a thickness variation after heat compression less than or equal to 20%, air permeability after heat compression less than or equal to 7x 102 sec/100 cm3, or TD heat shrinkage at 105 degrees C in the range of 0.25-1.5 (1-2.3)%. The microporous membrane has good balance of important properties including resistance to heat shrinkage at elevated temperature, high porosity, with suitable mechanical strength, permeability, and compression resistance.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for following:    (1) manufacturing a microporous membrane involving stretching an extrudate in at least one of machine direction (MD) or transverse direction (TD), the extrudate comprising diluent and the polyolefin, and then removing at least portion of the diluent from stretched extrudate to form a membrane having a first length along MD and first width along TD; stretching the membrane in MD from the first length to second length larger than the first length by a first magnification factor of 1.1-1.5 and stretching the membrane in TD from the first width to a second width that is larger than the first width by a second magnification factor of 1.1-1.3; and then reducing the second width to a third width, the third width is in the range of from the first width to 1.1 times larger than the first width; and    (2) a battery comprising an anode, a cathode, an electrolyte, and the monolayer microporous membrane; where the microporous membrane separates at least the anode from the cathode.