RECHARGEABLE BATTERY

1 . A rechargeable battery comprising:

an electrode assembly comprising a first electrode, a second electrode, and a separator between the first and second electrodes;

a case containing the electrode assembly; and

a cap plate sealing an opening of the case and comprising an electrode terminal connected to the electrode assembly,

wherein the cap plate further comprises a round unit arranged at an inner side of the case.

2 . The rechargeable battery of claim 1 , wherein the round unit comprises a concave surface facing an inner space of the case receiving the electrode assembly.

3 . The rechargeable battery of claim 2 , wherein the round unit has a curvature radius and is connected to the inner side of the case on a tangent line of the concave surface.

4 . The rechargeable battery of claim 3 ,

wherein the round unit is formed along a circumference of the opening, and

wherein the cap plate further comprises a planar unit surrounded by the round unit and connected to the round unit on another tangent line of the concave surface.

5 . The rechargeable battery of claim 4 ,

wherein the cap plate includes a vent opening for discharging gas from inside the case, and

wherein the vent opening is formed in the planar unit.

6 . The rechargeable battery of claim 4 ,

wherein the electrode terminal is connected to the electrode assembly through a lead tab,

wherein an insulator is arranged in a terminal opening of the cap plate,

wherein an insulating member is arranged between the lead tab and the cap plate, and

wherein the insulating member is adjacent the planar unit.

7 . The rechargeable battery of claim 6 , wherein a side of the insulating member and the round unit define a gas passage therebetween.

8 . The rechargeable battery of claim 2 , wherein the round unit is formed between opposite ends of the cap plate in a lengthwise direction connected to the case, the round unit having a curvature radius between the opposite ends.

9 . The rechargeable battery of claim 8 ,

wherein the cap plate includes a vent opening for discharging gas from inside the case,

wherein the cap plate has a minimum thickness at a first region corresponding to the vent opening, and

wherein the cap plate has a maximum thickness at a second region corresponding to an outermost part of the round unit.

10 . The rechargeable battery of claim 1 , wherein the round unit comprises:

a first round unit comprising a first concave surface facing an inner space of the case and having a first curvature radius, the first round unit being connected to the inner side of the case on a tangent line of the first concave surface; and

a second round unit comprising a second concave surface facing the inner space of the case and having a second curvature radius greater than the first curvature radius, the second round unit being connected to the first round unit.

11 . The rechargeable battery of claim 10 ,

wherein the cap plate substantially becomes thinner from the inner side of the case along an inward direction at the first round unit, and

wherein the cap plate gradually becomes thinner along the inward direction at the second round unit relative to the first round unit.

12 . A rechargeable battery comprising:

an electrode assembly;

a case having an inner space receiving the electrode assembly; and

a cap plate sealing an opening of the case and comprising a concave surface facing the inner space of the case.

13 . The rechargeable battery of claim 12 , wherein a first end of the concave surface is adjacent and tangent to an inner surface of the case proximate the opening.

14 . The rechargeable battery of claim 13 ,

wherein the cap plate further comprises a planar surface facing the inner space of the case, and

wherein a second end of the concave surface opposite the first end is connected and tangent to the planar surface.

15 . The rechargeable battery of claim 13 ,

wherein the cap plate further comprises another concave surface facing the inner space of the case, and

wherein a second end of the concave surface opposite the first end is connected and tangent to the another concave surface.

16 . The rechargeable battery of claim 15 , wherein the concave surface has a first curvature radius, and the another concave surface has a second curvature radius greater than the first curvature radius.

17 . The rechargeable battery of claim 12 , wherein the concave surface extends between opposite sides of the cap plate.