NOVELTY - A pressure vessel comprises outer housing, end cap assembly, and vent mechanism. The housing has closed end and opposing open end. The end cap assembly seals the open end of the housing. The outer housing and end cap assembly form a pressure boundary surface around the vessel interior. The vent mechanism is on the pressure boundary surface. The vent mechanism activates to release gas pressure from the inside of vessel.    USE - For use in primary alkaline cell used as battery for power small electronic devices, e.g. mini disc or media player 3 (MP3). It is also used as replacement for same size flat nickel metal hydride rechargeable cell.    ADVANTAGE - The invention does not bulge or swell during normal cell usage. It improves overall cell performance. It reduces deleterious effects of potassium hydroxide crystalline buildup between plug and inside surface of vent cap.    DETAILED DESCRIPTION - A pressure vessel comprises outer housing, end cap assembly (12), and vent mechanism. The housing has closed end (104) and opposing open end. The end cap assembly seals the open end of the housing. The outer housing and end cap assembly form a pressure boundary surface around the vessel interior. The vent mechanism is on the pressure boundary surface. The vent mechanism activates to release gas pressure from the inside of vessel. The vent mechanism comprises first and second ruptures. The first rupture forms a groove on the boundary surface. The groove defines material region thinner than the average thickness of the boundary. The second rupture zone ruptures when gas pressure in the cell rises to a second pressure level higher than the first pressure level. INDEPENDENT CLAIMS are included for:    (1) primary alkaline cell (10) comprises negative (290) and positive terminals, outer housing, anode containing zinc, and cathode (110) containing manganese dioxide, separator (140, 240), vent mechanism, and end cap assembly. The cathode comprises rectangular shaped cathode slabs (110a); and    (2) closing outer housing of alkaline cell comprising inserting metal cover into the housing in proximity to the open end so that the edge of the metal cover abuts the housing and portion(s) of the metal cover is exposed, cooling the metal cover to less than ambient room temperature, and laser welding the edge of the cover to the housing to close the open end.    DESCRIPTION OF DRAWING(S) - The figure shows a cross sectional view of the cell.    Alkaline cell (10)    End cap assembly (12)    Casing (100)    Closed end (104)    Wide side (106a-b)    Cathode (110)    Cathode slabs (110a)    Separator (140, 240)    Insulating sealing mechanism (220)    Metal cover (230)    Separator (240)    Plastic spacer (250, 280)    Rubber plug (260)    Negative (290)