Welded construction and resistance welding method

1. A welded construction in which a first metallic member, on one surface of which a projecting portion is formed and on the other surface of which a depressed portion corresponding to the projecting portion is formed, and a second metallic member are welded together at a welded portion which includes a summit portion of the projecting portion;

wherein, on at least one of the first metallic member and the second metallic member, on its opposite surface to its surface that faces the other one of the metallic members, a recessed portion is formed that is recessed from its peripheral region towards a region which corresponds to the welded portion; and

wherein the first metallic member is made from copper, and the second metallic member is made from nickel.

2. A welded construction according to claim 1 , wherein the first metallic member and the second metallic member have different resistance values, and a metallic member in which the recessed portion is formed has a larger resistance value than the other metallic member.

3. A welded construction according to claim 1 , wherein the recessed portion is formed around the depressed portion of the one metallic member in which the depressed portion is formed.

4. A welded construction according to claim 3 , wherein, on the other metallic member, a region that corresponds to the welded portion on its side opposite to its side that faces the one metallic region is planarized with its surrounding region.

5. A welded construction according to claim 3 , wherein, on the other metallic member, a region that corresponds to the welded portion on its side opposite to its side that faces the one metallic region is a recessed portion that is recessed from its surrounding region.

6. A welded construction according to claim 1 , wherein the recessed portion has a cone-shape.

7. A welded construction according to claim 1 , wherein the recessed portion has a hemispherical- or dome-shape.

8. A welded construction according to claim 1 , wherein the recessed portion has a groove-shape of constant width and predetermined length.

9. A welded construction according to claim 8 , wherein the recessed portion has a shallow groove-shape with a circular cross section.

10. A welded construction according to claim 8 , wherein the depressed portion has a groove-shape of constant width and predetermined length and of angle-shape in cross section.

11. A welded construction according to claim 1 , wherein the recessed portion has a rectangular-shape.

12. A welded construction in which a first metallic member, on one surface of which a projecting portion is formed and on the other surface of which a depressed portion corresponding to the projecting portion is formed, and a second metallic member are welded together at a welded portion which includes a summit portion of the projecting portion;

wherein, on at least one of the first metallic member and the second metallic member, on its opposite surface to its surface that faces the other one of the metallic members, a recessed portion is formed that is recessed from its peripheral region towards a region which corresponds to the welded portion; and

wherein the first metallic member is an electrode collector plate of a secondary cell, and the second metallic member is a power lead that is disposed between a casing of the secondary cell and the electrode collector plate.