14 the two wings 22 can be rectangular the inner wall 122 defines an arcuate slot 124 deflecting away from the receiving groove 14 a connecting plate 126 extends vertically from a bottom of the arcuate slot 124 and the inner wall 122 the fixing post 24 extends upwardly from the support plate 126 the fixing post 24 extends in parallel to the inner wall 122 the housing 10 has a stage 16 adjacent the first side portion 162 referring also to fig4 the elastic latching piece 26 extends from the stage 16 toward the battery compartment 12 the elastic latching piece 26 has an arcuate protrusion 262 extending downwardly from a bottom thereof the latch plate 28 is substantially strip shaped and includes a first end portion 282 and a second end portion 284 the first end portion 282 is substantially cylindrical the first end portion 282 defines a through hole 286 extending axially therethrough the through hole 286 receives the fixing post 24 so that the latch plate 28 is rotatably connected with the housing 10 the latch plate 28 can be rotated to cover the open end 142 of the receiving groove 142 the second end portion 284 can be releasably locked by the elastic latching piece 26.