In some examples, a method for forming an energy storage device assembly, e.g., for an implantable medical device, may comprise partially enclosing electrodes of an energy storage device within a foil pack, wherein the foil pack includes an unsealed portion and a sealed portion when partially enclosing the electrodes of the energy storage device. The method may further comprise forming a first heat seal at the unsealed portion of the foil pack, and subsequently forming a second heat seal that is redundant with the first heat seal of the foil pack.