Patent ID: 7348098

Claim:
A flat prismatic battery comprising: a metal battery case formed as a polygonal half-shell body provided with a battery-case side-peripheral wall and defining a battery case opening; a metal seal case formed as a polygonal half-shell body provided with a seal-case bottom wall and a seal-case side-peripheral wall extending from said seal-case bottom wall to a seal-case peripheral wall top edge distal from said seal-case bottom wall, said seal-case side-peripheral wall and said seal-case bottom wall defining a seal case opening, said seal-case side-peripheral wall including a step having: a step riser narrowed portion at a position adjacent said seal-case bottom wall; a step riser widened portion at a position adjacent said seal-case peripheral wall top edge; and a step portion connecting said step riser narrowed portion and said step riser widened portion; said battery case opening and said seal case opening being disposed opposing to each other with said seal case inverted to define an inner space and having a gasket interposed therebetween; a power generation element disposed in said inner space; said step narrowed portion being separated from said battery-case side-peripheral wall by a predetermined interval; said gasket being pressed between an open end of said battery-case side-peripheral surface and said step for scaling; a part of said seal-case side-peripheral wall extending from a seal-case bottom inner surface of said seal-case bottom wall of said seal case to said step portion and forming said step riser narrowed portion, and said step portion are so formed as to have a thickness of 1.2 times or more of a thickness of said seal-case bottom wall; and said step riser widened portion of said seal-case side-peripheral wall having a thickness less than said step riser narrowed portion.