In this blog post I will show you an easy project that can charge Li-on batteries that have the built-in protection circuit. All you need is a 1A or higher rectifier diode, 1 amp linear adjustable voltage regulator(such as LM317), 240 and 680 ohm resistor(any wattage) and a 100 ohm trimmer(For adjusting the charging voltage).
Circuit Diagram
Circuit Simulator