Laser welding method and monitoring method for secondary battery

A laser welding method for a secondary battery, the method comprising:

• performing laser welding on a positive electrode base having a thin-film shape, a negative electrode base having a thin-film shape, and a thin-film multi-tab, so that each of the positive electrode base and the negative electrode base is joined to the multi-tab,
• wherein respective welded portions in which the multi-tab is welded with the positive electrode base and the negative electrode base are melting-joined by using a laser such that a plurality of welding spots is formed on each of the welded portions, and
• determining whether a defect of the welded portions exists,
• wherein the determining whether a defect of the welded portions exists comprises
• irradiating the welded portions with light in a direction from a first surface toward a second surface opposite the first surface of the welded portions, collecting light transmitted through the welded portions to form a first inspection image, and determining whether a perforated defect of the welded portions exists by analyzing the first inspection image, and
• irradiating the welded portions with light in a direction from the second surface toward the first surface, collecting light reflected by the welded portions to form a second inspection image, and determining whether a back bead defect of the welded portions exists by analyzing the second inspection image.