A battery assembly is constructed of an elongated plastic shell irregularly shaped to fit optimally in anatomical cavities, particularly ear canals. The battery assembly relies structurally on a thin plastic shell which is designed to be inexpensively formed in a variety of complex shapes, thus offering superior volumetric utilization and energy capacity when compared with conventional metal button cell designs. In preferred embodiments, the shell is domed to distribute axial stresses so that it can be thin and still provide sufficient strength to avoid breakage in an extended wear environment despite its plastic composition. A preferred embodiment of a hearing device incorporating the battery assembly is disposable, to be discarded upon depletion of the electrical energy of the battery assembly.