Li-ion battery charging circuit
First of all, never connect a power supply directly to a Li battery. You have two option to charge your battery. The fastest way is to find and purchase a Li-charger &quot;module&quot; or a &quot;power-bank module&quot; which is designed exclusively for charging Li batteries. The second way is to use ICs designed to this purpose (like TP4054) and build your circuit (PCB) based on this IC.
Generally, a Lithium battery voltage ranges from 3.5v to 4.2v (Max). The charger's max voltage should be limited to 4.2volt and its current should be limited(depends on the charger and the battery). So in the middle of charging process, the charger's voltage might be lower than 4.2v to prevent from rushing current. Another feature of such a module or IC is that they stop charging if the battery is full, so it prevents from over/extra charging.