Disclosed herein is a battery module including a unit module stack configured to have a structure in which two or more unit modules, each of which includes two or more battery cells mounted at a cell cover in a state in which the battery cells are electrically connected to each other, and electrode terminal connection parts of which protrude outward from the cell cover in a state in which the electrode terminal connection parts are vertically bent, are stacked, a module case in which the unit modules of the unit module stack are arranged such that the unit modules are connected in series to each other, and electrode terminals of the unit modules are open, a bus bar assembly mounted at the electrode terminals of the unit modules in the module case, the bus bar assembly including one or more bus bars for connection in series between the unit modules, the bus bars being mounted in an exposed state such that welding is performed from outside, and an electrically insulative cover for covering the bus bar assembly.