NOVELTY - A water activated alkali metal battery cell (100) comprises: protected alkali metal anode (130) containing alkali metal electro-active component material hermetically sealed inside an anode enclosure having a wall component of alkali metal ion conductive protective membrane architecture; cathode (110) having an active surface containing solid phase electro-active component material; and inter-electrode region defined by an area contacting the membrane exterior surface and the cathode active surface and configured to receive an aqueous liquid upon activation.    USE - As a water activated alkali metal battery cell, used in multi-cell battery stack, and in bipolar battery (claimed). Also useful for powering underwater devices and systems such as unmanned underwater vehicles (UUVs), sonobuoys, and sensors.    ADVANTAGE - The water activated alkali metal battery cell exhibits very high energy density; is open to the seawater environment and therefore operable without a pressure housing provides tremendous benefit for underwater applications; and is highly compact and configured to operate at high rates of discharge over long periods of time and with an exceptionally low noise footprint, or none at all.    DETAILED DESCRIPTION - A water activated alkali metal battery cell (100) comprises: protected alkali metal anode (130) containing an alkali metal electro-active component material hermetically sealed inside an anode enclosure having a wall component of alkali metal ion conductive protective membrane architecture, where the membrane architecture has an interior surface opposing the alkali metal electro-active component material and an exterior surface opposing the exterior environment about the anode enclosure; cathode (110) having an active surface containing solid phase electro-active component material, where the cathode is configured such that the cathode active surface opposes the exterior surface of the membrane architecture; and an inter-electrode region defined by an area contacting the membrane exterior surface and the cathode active surface and configured to receive an aqueous liquid upon activation, such that the aqueous liquid in the inter-electrode region directly contacts the membrane exterior surface and the cathode active surface once the cell is activated. The cell is devoid of aqueous liquid prior to activation. The protected anode and cathode are disposed in a spaced apart relationship with it defining the inter-electrode region, and the protective membrane architecture exterior surface does not come into direct contact with the cathode active surface. The protected anode and cathode are positively separated from each other by a spacer component. The spacer component is selected from frame-like structure having porous wall component, material layer, and an arrangement of discrete material elements. The spacer component is a material layer flow retardant membrane selected from hydrogel, water swellable polymer, micro-porous polymer membrane, and cellulosic paper. The spacer component is selected from frame-like structure having a porous wall component, an arrangement of discrete material elements, and a seawater flow retardant gasket; and seawater flow retardant gasket selected from hydrogel, water swellable polymer, a micro-porous polymer, and a cellulosic paper. INDEPENDENT CLAIMS are included for the following:    (1) a multi-cell battery stack comprising a first battery cell electronically connected to a second battery cell, where the second battery cell is identical to the first battery cell, and the electronic connection between the first and second battery is selected from series electrical connection, parallel electrical connection and series-parallel electrical connection;    (2) a bipolar protected electrode having two opposing active surfaces comprising: single sided protected anode having an active surface and second opposing surface; and single sided cathode having an active surface and a second opposing surface, where the protected anode active surface provides the bipolar electrode first active surface and the cathode active surface provides the bipolar electrode second active surface; and    (3) a bipolar battery comprising at least two bi-polar electrodes, where a first and a second bipolar electrode are configured such that the protected anode active surface of the first bipolar electrode is adjacent to and opposes the cathode active surface of the second bipolar electrode.    DESCRIPTION OF DRAWING(S) - The figure shows a cross sectional depiction and perspective view of a battery cell.    Water activated alkali metal battery cell (100)    Cathode (110)    Electro-active component layer (112)    Current collector (114)    Protected alkali metal anode (130)    Lithium electro-active component (132)