NOVELTY - The battery (8) has an anode (11) comprising alkali metal and alkali metal alloy. An alkali metal conductive membrane (10) separates the anode from a cathode (14). A catholyte (2) comprises cations of the alkali metal, where the catholyte is in fluid communication with the cathode. Hydrogen produced at the cathode is vent out of the catholyte via a hydrogen-egress zone (16). Water ingresses into the catholyte via a water-ingress zone (21), where the water-ingress zone restricts the ingress of cations dissolved in the water into the catholyte.    USE - Battery i.e. lithium battery for use as a source of power in underwater/undersea applications i.e. underwater vessels e.g. unmanned underwater vehicles, floating beacons in water, and offshore oil rigs, used for data gathering, and taking measurements under ocean.    ADVANTAGE - The battery protects the membrane, and prevents sodium ions from contacting the membrane, even if seawater is used as the water- replenishing source, so that the battery can be used freely with seawater without having the seawater foul the membrane within the battery. The battery can operate in a system that moves from salt water to freshwater or vice versa such that the battery can be used in an undersea vehicle in freshwater and moves into seawater, so that the battery can be used in both freshwater and seawater to provide flexibility of the battery.    DETAILED DESCRIPTION - The alkali metal and the alkali metal alloy comprise lithium and lithium alloy and sodium and sodium alloy. The alkali metal conductive membrane comprises Lithium Superionic Conductor (LiSICON) (RTM: Chemicals Used In the Electronics Industry and Sodium Superionic Conductor (NaSICON)). The anode is made of lithium metal. The cathode is made of nickel and platinized metal such as titanium and nickel. An INDEPENDENT CLAIM is also included for a method for protecting an alkali metal conductive membrane in a battery from fouling due to incompatible ions in a water supply.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a battery.    Catholyte (2)    Battery (8)    Alkali metal conductive membrane (10)    Anode (11)    Cathode (14)    Hydrogen-egress zone (16)    Water-ingress zone (21)