Secondary battery with gas release valve

1. A secondary battery comprising a gas release valve located at a battery container housing a power generating element, wherein:

the gas release valve includes a valve disk caused to expand outward by internal pressure in the battery container; wherein

a plurality of grooves is formed at the valve disk in order that a plurality of valve element pieces becomes fractured along the grooves when the valve disk expands due to the internal pressure;

a scatter-deterring portion is formed at the valve disk in order to prevent the fractured valve element pieces along the grooves from scattering away from the valve disk; and

retarding areas that slow propagation of fracturing along the grooves while the valve disk is being deformed following a start of fracturing at the grooves are formed, wherein

the plurality of grooves includes a perimeter groove formed along a perimeter of the gas release valve and an inside groove formed in an inner area surrounded by the perimeter groove;

the perimeter groove includes a plurality of perimeter groove elements set discontinuously in a circular form;

the inside groove includes a plurality of inside groove elements radially extending from a central area of the gas release valve toward the perimeter, the inside groove elements dividing an inner area of the valve disk into the plurality of valve element pieces;

at least one pair of perimeter groove elements between first discontinuous areas is set along at least one perimeter side of the valve element pieces;

ends of the inside groove elements on a perimeter side each reach a second discontinuous area formed between the perimeter groove elements;

the scatter-deterring portion is constituted with the first discontinuous area set between the pair of perimeter groove elements along the perimeter side of the valve element;

the retarding area is constituted with the second discontinuous area assuming a position between a perimeter-side end of the inside groove element defining the valve element pieces and the perimeter groove element; and

the perimeter groove elements and the inside groove elements are grooves formed at a surface of the valve disk, with the perimeter groove elements assuming a groove depth smaller than a groove depth of the inside groove elements.

2. A secondary battery according to claim 1 , wherein:

the plurality of perimeter groove elements are set so as to form a substantially elliptical shape; and

the plurality of inside groove elements are set to achieve symmetry relative to a major axis and a minor axis of the elliptical shape.

3. A secondary battery according to claim 2 , wherein:

the inside groove elements are set so that the inner area is divided into two trapezoidal regions assuming a substantially trapezoidal shape and achieving symmetry relative to the major axis, and two triangular regions assuming a substantially triangular shape and set apart from each other along the major axis;

the inside groove elements include a first inside groove element and a second inside groove elements, the first inside groove constituting a common top side for the two trapezoidal regions and the second inside groove elements each constituting a common lateral side of a trapezoidal region and a triangular region adjacent to the trapezoidal region; and

perimeter-side ends of the second inside groove elements are each located in the discontinuous area formed between the perimeter groove elements.

4. A secondary battery according to claim 1 , wherein:

the power generating element includes a winding group formed by winding a positive plate and a negative plate via a separator; and

the battery container is filled with an electrolyte.

5. A secondary battery according to claim 4 , wherein:

the battery container comprises a narrow case and a lid covering an opening end of the case, with the gas release valve disposed at the lid.