MP4, MP3 Player, Mobile phones, various devices can be charged from the USB port of the computer, the charging adapters of these devices are generally designed according to the standard USB cable with a single usb connection cable can be charged from the PC and adapter. I’ve reviewed a charger used in mp4 players in China and have a very simple charging circuit.
The charging adapter is made up of two simple parts, which are roughly simple 5 sms section giving the output of the charge and the charge control section
The smps section of the circuit is 5 volts output is secondary if the control section is simple single transistor (S9015) circuit with 3.7 volt Li-ion Battery is being charged in the red LED is constantly on when the green led battery connection is lit up and after the battery goes out after a few models examined the control section is the same counts the difference according to the current of the charged battery smps power and R1 Resistance value 180..250ma li-ion batteries for the 5.6Ω 480..680 ma batteries 1.5Ω 2.7 için I saw that the smps part of the charge adapter with the outside of the control part from the outside with 5 volt welding i tested it worked just fine charging circuit for an mp4 player
If you are not going to charge the battery inside the player or another device, if you are connecting a direct battery, add D1 and C1 elements in the second circuit.
China Li-ion Charging Adapter Circuit