A porous polymer battery separator includes variable porosity along its length and can increase the uniformity of the current density within electrochemical battery cells that may normally experience higher current density and higher temperatures near their terminal ends than they do near their opposite ends. By disposing a variable porosity separator between the electrodes of an electrochemical cell such that its terminal end has a lower porosity than its opposite end, the transport of ions through the separator can be more restricted in normally high current regions and less restricted in normally low current regions, thereby increasing the overall uniformity of current density within the cell. The separators may be produced by a dry-stretching process or by a wet process. These processes may include forming a polymer-containing film, producing a uniform distribution of pore sites within the film, and reforming the polymer-containing film to a uniform thickness.