Rechargeable battery and heat treatment device

A heat treatment device for a case including a side wall forming an opening to receive an electrode assembly of a rechargeable battery and a bottom sealing the opposite side of the opening, the heat treatment device comprising:

• a cooling portion maintaining a first elongation ratio by partially cooling first portions formed at both sides of a first length direction of the bottom and
• a heating portion provided between the cooling portions in the first length direction to apply a second elongation ratio that is higher than the first elongation ratio by partially heating a second portion set between the first portions.

A battery case comprising:

• a plurality of side walls and a bottom wall that are continuously attached to each other, wherein the plurality of side walls define an internal space that is sized to receive an electrode assembly and an opening that provides access to the internal space, wherein the opening is sized to receive a cap assembly;
• wherein the bottom surface of the battery case includes a first portion that has a first bending parameter and two second portions located outward from the first portion that has a second bending parameter wherein the first portion bends at a lower application of force than the second portions wherein the first and second portions of the bottom surface extend into the side walls.