NOVELTY - The battery cell (1) has an anti-explosion mechanism that is arranged between an electrode group (10) and a top cover (3). The anti-explosion mechanism deforms or cleaves due to elevation of internal pressure in battery cell. A connection plate (35) which is welded to a diaphragm (37) has a thinner portion. A spot at which connection lead (23) is connected is formed as thin, and a thicker portion where thinner portion (36) is not formed. A lower surface of thinner portion coincides with, or as hollowed out from lower surface of thicker portion.    USE - Cylindrical sealed battery cell such as lithium ion secondary battery cell for use in plug-in hybrid vehicle or electric vehicles.    ADVANTAGE - Since the junction portion of the connection plate is made in advance as a thinner portion that is hollowed out and thus lower than the region around it, it is made possible to prevent connection plate coming into contact with the diaphragm, even if the connection plate becomes deformed due to welding of the connection lead. The one end portion of a flexible electrically conducting positive electrode lead is made by laminating together a large number of layers of aluminum foil is joined to the upper surface of the base portion of the positive electrode current collecting portion by welding. Since this conducting positive electrode lead is made by laminating together and integrating several layers of aluminum foil, accordingly it is capable of carrying a large electrical current, and is endowed with flexibility. The upper surface of the connection plate is formed as the hollowed out thinner portion, it becomes possible to use laser welding for welding the connection portion to the connection plate that is integrated as a portion of the anti-explosion mechanism, so that it is possible to enhance the efficiency of production. Since the area of the thinned down portion of the connection plate is larger than the area irradiated by the beam of the laser, it is possible reliably to prevent the thinner portion from bulging upwards towards the diaphragm due to thermal deformation and the connection plate coming into contact with the diaphragm.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for manufacturing method for sealed battery cell.    DESCRIPTION OF DRAWING(S) - The drawing shows the enlarged sectional view of the sealed battery cell.    Sealed battery cell (1)    Top cover (3)    Electrode group (10)    Connection lead (23)    Connection plate (35)    Thinner portion (36)    Diaphragm (37)