NOVELTY - A separator (64) for an energy storage cell (50), comprises a microporous matrix including a reversible porosity-controlling agent, which changes size as a function of temperature and/or electrolyte concentration, to provide a change in an overall porosity of the separator.    USE - As a separator for an energy storage cell (e.g. primary and secondary energy-storage cells or batteries) (claimed) used in gas-operated vehicles and electric vehicles such as golf carts and fork trucks, marine applications, floor-sweeper scrubber and recreational vehicles.    ADVANTAGE - The use of porosity-controlling polymeric agent in the separator, protects the energy-storage cell/battery from over discharge, thus increasing the battery life. The separator including the reversible porosity-controlling agent, exhibits improved properties such as improved electrical resistance and improved overall porosity, as compared to conventional separators. In addition, the separators can take an active role instead of a passive role, in improving the performance of energy storage cells under variable conditions. The separators can be used to shut down an energy storage cell under high temperature conditions, and can enable the storage cell to be used again when conditions return to normal. Thus, the separators have reversible properties that enable the energy storage cells to be used continuously, yet providing protection during deep discharge cycles. The improved energy storage cell separators enhances performance and life of the energy storage cell under variable operating conditions. The separator includes the porosity-controlling polymeric agent, which increases the porosity of the separator under low temperature conditions, to obtain increased power from the energy storage cell.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method of improving an operating characteristic (e.g. electrical resistance of separator, and overall porosity of separator) of an energy storage cell, involving applying the reversible porosity controlling agent (0.05-20 wt.%) to a separator material; forming a separator from the separator material and the porosity-controlling agent; and operating the energy storage cell with the separator.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of an energy storage cell.    Energy storage cell (50)    Positive terminal of cell (52)    Negative terminal of cell (54)    Case (56)    Electrolyte (58)    Electrodes (60, 62)    Separators. (64)