Without any USB to phone battery charger circuit we can charging phone battery using port on USB computer , but it will quickly damage the phone battery, and the battery will bulge. Because the voltage which was issued on usb is 5 volts , while the average-voltage phone battery 3.5 - 3.7 volts. That's why this USB to phone battery charger circuit is required , this USB to phone battery charger circuit reduce votlage to 3.7 volt usb, but will not reduce currents and will make a durable phone battery . USB to phone battery charger circuit diagrams List of components : R1 : 1 K R2 : 330 R R3 : 4K7 R4 : 300 R R5 : 27R D1 : 4.7 volt zener /1W C1 : 100uF/16V Q1 : BC548 Q2 : BC558A LED1 : Green Led X1-1 : Vcc USB X1-4 : Ground USB X2-1 , X2-2 : To phone battery See also this printed circuit board ( PCB ) of USB to phone battery charger : USB to phone battery charger printed circuit board