A plug assembly includes a first plug and a second plug. The first plug includes a main body whereon a first positive electrode, a first negative electrode and a first engaging portion are formed. The second plug has a chamber structure formed inside for containing the main body. A second positive electrode, a second negative electrode and a second engaging portion are formed on the chamber structure and located in positions corresponding to the first positive electrode, the first negative electrode and the first engaging structure, respectively. As the chamber structure completely contains the main body, the second positive electrode and the second negative electrode are respectively coupled to the first positive electrode and the first negative electrode, and the second engaging portion engages the first engaging portion, such that the main body is hold inside the chamber structure.