Secondary battery and its method of manufacture

A secondary battery, comprising:

• an electrode assembly;
• a can to contain the electrode assembly;
• a cap assembly including a cap plate attached to an open upper side of the can;
• a depression, arranged on either the cap plate or the can, the depression having a thickness less than a thickness of other portions of the cap plate or the can where the depression is arranged;
• and
• the depression further comprising:
• a flat section positioned within the center of the depression, the flat section being entirely flat and lying in a geometric plane of the cap plate;
• a rupture section entirely contained within the flat section having a thickness less than a thickness of other portions of the depression;
• and
• wrinkles entirely surrounding the flat section and arranged between and coupled to the flat section and a periphery of the depression, said flat section being of greater width than said wrinkles and said wrinkle having a height greater than a height of the flat section,
• wherein the rupture section has a “>--<” shape along an inner side of the wrinkles;
• and
• wherein the cap plate or the can has an upper and a lower side, the lower side being closer to an inner portion of the battery, wherein the depression is formed at the lower side;
• and
• wherein the wrinkles have a convex shape only, protruding outward from the cap plate or the can where the depression is arranged.