A porous polymer battery separator is provided that includes variable porosity along its length. Such battery separators 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, such as lithium 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 battery cell. Variable porosity battery separators may be produced by a dry-stretching process or by a wet process.