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.