A fitting set, including a battery assembly and a receiver having a recess for receiving the battery assembly are disclosed. The battery assembly may include a casing having a top wall, a bottom wall, two first sidewalls, and two second sidewalls. The battery assembly may further include a resilient member and a self-locking block. The first sidewalls may include notches extending to the bottom wall. The recess may include lugs on one or more of the inners walls, each of which may be engaged with a corresponding notch. The second sidewalls may each include a first stopper and one or more second stoppers, the first and second stoppers having a difference in height. The resilient member at both ends may abut against the first stopper and self-locking block, respectively, and may also press the self-locking block against an edge of the recess.