Apparatus and methodology subject matters relate to an improved electrochemical single or multi-cell energy storage device. Also, an outer casing may be provided as a pair of U-shaped shells, a single foil piece, or a tube-shaped structure which encases the internal electrochemical cell stack. The energy storage device and such casing when used advantageously exhibit low internal resistance, low ESR, a high voltage/capacity, and a low contact resistance between the internal stack and the outer casing.