A battery pack containing a plurality of battery modules, these modules comprising a plurality of individual battery units, these battery units comprising a plurality of bicells with opposing terminals. The battery modules are connected in series by use of a flexible circuit and by opposing positive and negative terminals of each battery unit. The terminals of each battery unit contain a region of apertures which allow the adhesive of the envelope-like packaging material to seal more effectively by flowing through the apertures and sealing to itself, thereby eliminating or reducing the amount of leakage from an individual battery unit. The terminals pass through overlapped edges of the packaging material and include tangs for electrical connection to flaps formed in the flexible circuit.