An boost battery charger is proposed to operate on 0V battery voltage in wireless power transfer (WPT) system for bio-medical application. The proposed battery charger for bio-medical application should be operated on low power. Thus, our target input power is very low such as 5mW∼10mW and target output power is 2mW∼5mW. Because the proposed battery charger starts up using rectifier output voltage, it doesn't need to consider battery voltage at start-up situation. Also our structure supply power to battery on constant current (CC) Mode. And we proposed burst mode (BM) protection to protect circuit and battery. The proposed battery charger is implemented using 0.18um CMOS technology. As a result, it achieves 83% efficiency and batteryless start-up and CC Mode operation.