Batteries ---- The Power Source of MWD 1.Powering Downhole Measuring Instruments Driving the Operation of Sensors: Ensuring Data Acquisition and Processing: 2.Supporting Downhole Data Transmission Realizing Mud Pulse Transmission: Ensuring Electromagnetic Wave Transmission (if adopted): 3.Ensuring System Stability and Reliability Coping with the Complex Downhole Environment: Providing Emergency Power Support: