Technologies are described herein for implementing a space-efficient internal energy storage apparatus in a data storage device or other electronic device have a metallic or otherwise electrically-conductive housing or case structure. The energy storage apparatus comprises an interior surface of the metallic housing, a conductive layer disposed parallel to the interior surface of the metallic housing, and a separator disposed between the interior surface and the conductive layer. The metallic housing is configured to act as a first electrode of the energy storage apparatus and the conductive layer is configured to act as an opposing electrode to the first electrode.