Abstract: A sealed water supply chamber 18 for a battery vessel 1 is formed by thermally bonding together the vessel, a lid member 2 having a plurality of vertically disposed level defining tubes 11, water inlet and gas outlet ports 12, and water pools or reservoirs 14 at each end, and a cover member 3 having threaded openings 16 at each end and downwardly depending cups 13 disposed above the respective tubes 11. Tight bonding is facilitated by molding the vessel, the lid member, and the cover member from the same thermoplastic material. L-shaped water inlet and outlet pipes 4 are screwed into the openings 16 and sealed by rubber packings 15, with the lower end of each pipe disposed just above the bottom of a water reservoir 14. Surplus water remaining in the reservoirs thus provides a gas barrier to isolate the chambers 18 of a plurality of batteries connected in an array, whereby a gas explosion in one battery cannot flash through the water connecing pipes to ignite the other batteries.