When a continuous short-circuit occurs between both terminals of a battery pack, there is a possibility of a failure, destruction and rupture of the battery due to heating; thus, flowing of electricity needs to be immediately interrupted. Further, a problem lies in that when a momentary short-circuit occurs, a user continues to use, without knowing such a thermal and electrical damage to the batteries, storage batteries under a situation in which reliability for the batteries is impaired, To overcome the problem, in a battery pack configured by connecting a plurality of storage batteries in series, at least one storage battery is included which has a low capacity compared to the other batteries during high rate discharge, and at least one specific storage battery undergoes polarity inversion during external short-circuit, thereby preventing the other batteries from becoming damaged.