NOVELTY - A battery comprises an anode containing zinc, aluminum, or magnesium; a cathode containing at least one oxygen reduction catalyst on an electrically conducting porous substrate; and a dry separator disposed between the anode and the cathode, comprising a hydrophilic membrane; where the anode, cathode, and separator are joined together to form a layered battery, and the anode, cathode, and separator are not further enclosed within the layered battery.    USE - As battery (claimed), preferably metal-air battery, used as electrical energy source in a device and in disposable or single-use devices; in water detection, such as leak detection of water (e.g. water, an aqueous solution), or moisture detection of water vapors; and in consumer products (e.g. diapers and pregnancy tests).    ADVANTAGE - The battery have relatively high energy density; is disposable, non-toxic, and environmentally friendly; have a long shelf-life when dormant, and can be activated when necessary by contacting water, particularly is activated more than once. The battery when contacted with water, have simultaneous water detecting and battery-activation. The battery can be relatively thin, does not include an enclosure (e.g. a housing), and can be amenable to water detection, medical applications, and household applications. When the battery is constructed with non-toxic materials, it can be used in applications where contact with a live subject (e.g. a human subject, an animal subject) is necessary. Further, the battery can be relatively easily and/or inexpensively manufactured.    DETAILED DESCRIPTION - A battery comprises an anode containing zinc, aluminum, or magnesium; a cathode containing at least one oxygen reduction catalyst on an electrically conducting porous substrate; and a dry separator disposed between the anode and the cathode, comprising a hydrophilic membrane; where the anode, cathode, and separator are joined together to form a layered battery, and the anode, cathode, and separator are not further enclosed within the layered battery. The battery further comprises an adhesive disposed between the anode, the cathode, and the separator, preferably the adhesive comprises a cellulose based hydrophilic material. The battery is activated when wetted with water.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of a metal-air battery.    Anode (2)    Separator (4)    Cathode (6)    Apertures (8)    Metal-air battery (10)