An alkaline cell having a flat casing, preferably of cuboid shape. The cell can have an anode comprising zinc and a cathode comprising nickel oxyhydroxide. The casing can have a relatively small overall thickness, typically between about 5 and 10 mm, but may be larger. Cell contents can be supplied through an open end in the casing and an end cap assembly inserted therein to seal the cell. The end cap assembly includes a vent mechanism, preferably a grooved vent, which can activate, when gas pressure within the cell reaches a threshold level typically between about 250 and 800 psig (1724.times.10.sup.3 and 5515.times.10.sup.3 pascal gage). The cell can have a supplemental vent mechanism such as a laser welded region on the surface of the casing which may activate at higher pressure levels.