A method of blocking a flow of a short circuit current in a battery unit comprising: a plurality of cells (10a, 10b) connected in series and one being stacked on another through a connecting terminal, each cell (10a, 10b) including a plurality of power generating elements and an outer package configured for packaging the plurality of power generating elements while the connecting terminal being disposed outside of the outer package, each power generating element comprising a plurality of unit cell layers each having a first collector (11, 14) and a second collector (11, 14), wherein: the first collector (11, 14) and the second collector (11, 14) each has a predetermined thickness and is formed of a predetermined material wherein both the thickness and materials are different from each other, wherein a melting temperature of the first collector (11, 14) is lower than a melting temperature of the second collector (11, 14) and the first collector (11, 14) is thicker than the second collector, the method comprising the steps of: when a conductor from outside penetrates at least two unit cell layers and forms a first short circuit between the at least two unit cell layers fusing each shorted portion of the first or second collectors (11, 14) from heat generated by the first short circuit before a temperature of the unit cell layer reaches a predetermined value so that the first short circuit is blocked, and when a conductor from outside penetrates at least two of the plurality of cells (10a, 10b) connected in series and forms a second short circuit between unfused first or second collectors (11, 14) from each of the cells (10a, 10b) through the connecting terminal, fusing at least one of the unfused collectors (11, 14) from heat generated by the second short circuit before a temperature of the cells (10a, 10b) reaches a predetermined value so that the second short circuit is blocked, wherein the amount of heat generated at the shorted portions due to the current flow is larger than the amount of heat of fusion required to fuse the collector (11, 14). a plurality of cells (10a, 10b) connected in series and one being stacked on another through a connecting terminal, each cell (10a, 10b) including a plurality of power generating elements and an outer package configured for packaging the plurality of power generating elements while the connecting terminal being disposed outside of the outer package, each power generating element comprising a plurality of unit cell layers each having a first collector (11, 14) and a second collector (11, 14), wherein: the first collector (11, 14) and the second collector (11, 14) each has a predetermined thickness and is formed of a predetermined material wherein both the thickness and materials are different from each other, wherein a melting temperature of the first collector (11, 14) is lower than a melting temperature of the second collector (11, 14) and the first collector (11, 14) is thicker than the second collector, the first collector (11, 14) and the second collector (11, 14) each has a predetermined thickness and is formed of a predetermined material wherein both the thickness and materials are different from each other, wherein a melting temperature of the first collector (11, 14) is lower than a melting temperature of the second collector (11, 14) and the first collector (11, 14) is thicker than the second collector, when a conductor from outside penetrates at least two unit cell layers and forms a first short circuit between the at least two unit cell layers fusing each shorted portion of the first or second collectors (11, 14) from heat generated by the first short circuit before a temperature of the unit cell layer reaches a predetermined value so that the first short circuit is blocked, and when a conductor from outside penetrates at least two of the plurality of cells (10a, 10b) connected in series and forms a second short circuit between unfused first or second collectors (11, 14) from each of the cells (10a, 10b) through the connecting terminal, fusing at least one of the unfused collectors (11, 14) from heat generated by the second short circuit before a temperature of the cells (10a, 10b) reaches a predetermined value so that the second short circuit is blocked, wherein the amount of heat generated at the shorted portions due to the current flow is larger than the amount of heat of fusion required to fuse the collector (11, 14).