An alkaline cell having a flat housing, preferably of cuboid shape. The cell can have an anode comprising zinc and a cathode comprising MnO.sub.2. The housing can have a relatively small overall thickness, typically between about 5 and 10 mm. Cell contents can be supplied through an open end in the housing and an end cap assembly inserted therein to seal the cell. The end cap assembly includes a vent mechanism which can activate, when gas pressure within the cell reaches a threshold level typically between about 100 and 300 psig (6.89.times.10.sup.5 and 20.69.times.10.sup.5 pascal gage). The cell can have primary and supplemental vent mechanisms such as welded or thinned regions on the surface of the housing which may activate at different pressure levels. The cathode can be formed of a plurality of stacked slabs having aligned hollow centers with an elongated opening for anode material to be inserted therein.