A battery module assembly has unit modules, each unit module having unit cells loaded on a cartridge with the unit cells are electrically connected to each other via busbars. The battery module assembly includes two or more sub-modules arranged in a lateral direction while being spaced apart from each other, each of the sub-modules including two or more unit modules stacked in a height direction from the ground. A battery management system (BMS) mounted between the sub-modules, the BMS being provided at one side thereof with communication terminals, a base plate, side cover plates, a top cover plate and a front cover plate mounted at fronts of the sub-modules. The front cover is coupled to the sub-modules, the base plate, and the side cover plates, the front cover plate being provided with through holes, through which the communication terminals are exposed.