A microporous polyolefin battery separator membrane is extremely high in porosity, high in puncture strength, very low in shrinkage and with shutdown temperature of 130-140 degrees C. and melt integrity greater than 165 degrees C. It consists of 20-30% by weight of a UHMW polyethylene having Mw of 1.times.10.sup.6 or higher, 60-70% by weight of a HDPE having average Mw of between 200,000-500,000 and 5-15% by weight of TiO.sub.2 with an average particle size of less than 0.2 microns.