Abstract: Plug for cells of electrical storage batteries comprising a cylindrical body (4), fastened to the vent stack (2) of the cover, capable of being connected by two pipes (7) to the immediately preceding and to the immediately following plug, and provided with two parallel cylindrical cavities (14 and 16): one central cavity (14), in the interior of which a toothed hollow stem (10) of a float (11) slides; and one lateral cavity (16), in the interior of which a smaller rod (17), which is also toothed, slides, on which a heavy metal sphere (18) rests. The stem (10) and the rod (17) are constrained to the two arms of a rocker lever (23) hinged on a pin (22) in the lower hollow portion (20) of the body (4). When the float (11) and its hollow stem (10) move upwardly, the rod (17) and the sphere (18) move downwardly. When the electrolyte is at its right level, the sphere (18) rests on a ring (19) and closes the access of filling-up water to the cavity (16) housing the rod (17) .