NOVELTY - The microporous membrane comprises an ethylene-based polyolefin (5-15 wt.%) produced with single site catalyst, a second polyethylene having a low melting point (Tm) of Tm greater than or equal to 131 degrees C and a weight average molecular weight (Mw) of 1x 105 to 9x 105, and a third polyethylene having a Mw greater than or equal to 1.0x 106. The membrane has a normalized air permeability of less than or equal to 5x 102 seconds/100 cm3/20 mu m and a shutdown temperature of less than or equal to 130.5 degrees C. The ethylene-based polyolefin has a Tm of 120-128 degrees C, and a Mw of 1x 104 to 1x 105. The ethylene-based polyolefin is a copolymer of ethylene and a second alpha-olefin.    USE - The microporous membrane is useful as a battery separator film in battery such as a lithium ion secondary battery, a lithium-polymer secondary battery, a nickel-hydrogen secondary battery, a nickel-cadmium secondary battery, a nickel-zinc secondary battery or a silver-zinc secondary battery (all claimed), which is useful as a power source in an electric vehicle, hybrid electric vehicle, power tool, computer, mobile phone and consumer electronics.    ADVANTAGE - The microporous membrane has well-balanced permeability, shutdown temperature and pin puncture strength, and improved heat shrinkage properties.    DETAILED DESCRIPTION - The microporous membrane comprises an ethylene-based polyolefin (5-15 wt.%) produced with single site catalyst, a second polyethylene having a low melting point (Tm) of Tm greater than or equal to 131 degrees C and a weight average molecular weight (Mw) of 1x 105 to 9x 105, and a third polyethylene having a Mw greater than or equal to 1.0x 106. The membrane has a normalized air permeability of less than or equal to 5x 102 seconds/100 cm3/20 mu m and a shutdown temperature of less than or equal to 130.5 degrees C. The ethylene-based polyolefin has a Tm of 120-128 degrees C, and a Mw of 1x 104 to 1x 105. The ethylene-based polyolefin is a copolymer of ethylene and a second alpha-olefin, where the copolymer has a Tm of 122-126 degrees C and is produced in a process utilizing a metallocene catalyst. INDEPENDENT CLAIMS are included for:    (1) a method for producing a microporous membrane; and    (2) a battery.    DESCRIPTION OF DRAWING(S) - The diagram shows a graphical representation of an effect of polyolefin melting peak and weight average molecular weight on membrane air permeability and shutdown temperature.