Nowadays, Lithium batteries are used extensively in portable devices, such as cellphones, laptop computers, electronic gadgets, … etc. There is a standard industry-defined procedure (cycle) for charging the lithium-ion/lithium-polymer batteries, otherwise, the lifetime of the batteries is reduced significantly or even they might explode and catch fire. As a basic rule of thumb, a lithium battery should be charged at the rate of 0.5C to 1C. In this article/video, I have introduced a universal double lithium battery charger that the charging current (C rate) can be adjusted simply by changing a resistor value. You just need a 5V power source (such as a mobile charger) and a USB Type-C cable.

More details check here:

https://www.pcbway.com/blog/technology/Double_Lithium_Ion_Lithium_Polymer_USB_Type_C_Charger_863d1ae1.html