A safety device for preventing overcharging of a battery includes: a battery stack including a plurality of cells; a safety circuit connected to two or more cells and provided with an electrical conduction control unit that controls electrical conduction; and a closed circuit disposed between the cells connected to the safety circuit and provided with a switching unit which is switched on or off upon cell swelling, such that when cell swelling occurs, the switching unit is switched on by a pressing force, disabling the electrical conduction control unit of the safety circuit so electrical conduction between the cells is established via the switching unit, and when cell swelling occurs again, the switching unit is switched off, cutting off the electrical conduction between the cells.