BATTERY PACK

There are provided a battery module assembly, a battery pack, and a device using a battery as a power source. The battery module assembly includes: at least two battery modules (20) arranged along a first direction (Z), each of the battery modules (20) including a plurality of battery cells (21), and a shell side wall (211) of the battery cell (21) including two oppositely arranged first side walls (211A) and two oppositely arranged second side walls (211B), an area of the first side wall (211A) being larger than that of the second side wall (211B), and the first side wall (211A) being perpendicular to the first direction (Z); and a cooling plate (31) arranged between two adjacent battery modules (20), and configured to perform heat exchange with the first side walls (211A) of the battery cells (21) of the two adjacent battery modules (20). The battery pack includes the battery module assembly. The device using a battery as a power source includes the battery pack.