Increasing research attention has been attracted to automatic plug-in charging in an unmanned and dangerous environment. In this work, we develop an object detection solution based on deep learning on 3D point clouds using a mobile robot manipulator to provide mobility and manipulation. In this solution, the 3D point cloud technology is adopted to measure the shapes and depth information for plugin charging. Then the deep learning is employed to deal with the uncertainty in 3D detection, such as inconsistent light conditions, irregular distribution, and structural ambiguity of point clouds. We utilize a mobile robot manipulator carrying a 3D camera and a gripper to detect the targeted objects and automate plug-in charging operations. The proposed 3D object detection principle and procedure for the automatic plug-in charging are presented in detail. The automatic plug-in charging testing is conducted to validate the developed 3D object detection algorithm using a mobile robot manipulator.