Abstract: A portable battery charger includes a rechargeable battery, a control board electrically connected with the rechargeable battery so as to control charge and discharge of the rechargeable battery, a middle case including an accommodation portion for accommodating the rechargeable battery and the control board mounted therein, a first case and a second case, which are coupled to the middle case and define the outer appearance of the portable battery charger, coupling hooks provided at a first edge and a second edge of the first case and including hook projections provided at respective ends thereof, coupling grooves formed in a first edge and a second edge of the second case so that the coupling hooks are inserted into the coupling grooves, and overflow holes formed in the coupling grooves so as to be open toward the middle case so that the hook projections are inserted into the overflow holes.