Separation method of secondary battery

A separation method of a secondary battery for disassembling and separating a used battery pack which is integrated by sandwiching a plurality of stacked secondary batteries by a pair of binding plates on both ends thereof and binding the whole used battery pack with a coupling member coupled to the binding plates, so as to re-form a new battery pack,

• the method comprising the steps of:
• providing a length dimension of an unused battery pack at a time of design of the unused battery pack;
• a first measuring step, comprising measuring an open circuit voltage of each of the secondary batteries while retaining a binding state by means of the pair of binding plates coupled to each other with the coupling member of the used battery pack that is expanded in a stacking direction with the elapse of time;
• after the first measuring step, a compressing step comprising compressing the used battery pack in the stacking direction to reduce a length dimension of the used battery pack to the length dimension of the unused battery pack at the time of design, while maintaining the pair of binding plates being parallel to each other;
• after the compressing step, a second measuring step comprising measuring the open circuit voltage of the secondary battery in a state in which the battery pack is compressed;
• and
• after the second measuring step, a separating step comprising separating, from the used battery back, a secondary battery which can be reused for re-forming a new battery pack, based on an amount of change of the open circuit voltage of the secondary battery before and after the compression of the used battery pack in the binding state.