NOVELTY - The method involves pressing a thin metal plate (14) to a positive terminal (11) of a battery system to form a mounting hole (15) in the thin plate. A thick metal plate formed to a negative terminal (12) is pressed to form a metallic part (17) that is smaller in diameter than the mounting hole. The metallic part is inserted and flattened in the mounting hole to widen the mounting hole and join the thin plate and the metallic part, where the pressing, insertion and flattening processes are continuously performed by a pressing device.    USE - Method for producing an electrode terminal connector to electrically connect positive and negative terminals of non-aqueous electrolyte secondary batteries i.e. lithium ion secondary batteries, in a power storage system. Uses include but are not limited to mobile communication device, laptop, electric vehicle, hybrid vehicle and solar battery.    ADVANTAGE - The method allows forming a positive terminal fixing hole using the thin metal plate which is the similar metal to the positive terminal and covering an inner surface of a negative terminal fixing hole with the metallic part which is the similar metal to the negative terminal, so that the similar metals are joined together so as to prevent corrosion and increase resistance the terminals due to local battery effect. The method allows formation of the mounting hole and joining of the metallic part by a small-scale pressing equipment, so that use of large-scale equipment for hydrostatic extrusion can be avoided, thus minimizing time to perform formation of the mounting hole and joining of the metallic part, and hence increasing working speed while suppressing oxide film growth during operation. The method allows flattening the metallic part in the mounting hole to widen the mounting hole and join the thin plate and the metallic part together, so that an oxide film formed around the inner surface of the mounting hole during flattening process can be destroyed immediately before joining the thin plate and the metallic part, and aluminum/aluminum-alloy of the mounting hole and copper/copper-alloy of the metallic part are bonded together by diffusion, thus resulting in a sufficient joint strength between the dissimilar metals, aluminum or aluminum alloy of the thin plate and copper or copper alloy of the metallic part.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a battery system comprising a set of non-aqueous electrolyte secondary batteries connected in series via an electrode terminal connector.    Positive terminal (11)    Negative terminal (12)    Electrode terminal connector (13)    Thin metal plate (14)    Mounting hole (15)    Metallic part (17)