A temperature sensor for a battery cell of a rechargeable battery is described, and includes a resistive sensing element, a first electrode, and a second electrode. The resistive sensing element, the first electrode, and the second electrode are affixed to a porous separator. The porous separator is interposed between an anode and a cathode of the battery cell. The resistive sensing element is electrically connected in series between the first electrode and the second electrode, and the resistive sensing element, the first electrode and the second electrode are affixed onto the separator as film layers, and are porous.