Improved middle or large-sized battery pack of increased safety

Provided is a large-sized battery pack which is compact, is mechanically stable and is improved in the safety of battery by disconnecting the electrical connection in case of the expansion of unit cells. The large-sized battery pack(100) comprises a rectangular lower case(300) which comprises a upper accommodation part where chargeable unit cells(400) are layered in turn; a rectangular upper case(200) which comprises a lower accommodation part capable of covering the upper part of the layered unit cells; a first circuit part(500) which electrically connects the layered unit cells and contains a sensing board assembly for detecting the voltage and/or current of a battery; a second circuit part(600) which is electrically connected with the first circuit part and contains a main board assembly for controlling the battery pack wholly; and a third circuit part(700) which is electrically connected with the second circuit part and is connected with an external output terminal with preventing the overcharge, overdischarge and/or overcurrent.