a battery assembly and method for making the same comprising : a laminar battery , said laminar battery including : an anode layer ; an ionically conductive electrolyte layer ; a cathode layer ; said electrolyte layer being interposed between said anode layer and said cathode layer , and said layers being assembled to form an electrical cell ; a pair of electrically conductive terminals in electrical contact with said anode layer and said cathode layer ; and a protective sheet material enveloping said laminar battery ; said sheet material being heat sealed at the periphery of said laminar battery and about said terminals to exclude air and moisture and said terminals extending from or being accessible through said protective sheet material for connection to a device which is powered by said laminar battery is disclosed . the battery assembly is formed , and the method includes that the sealing occur under a vacuum .