Here is a tried and tested sample circuit of a Li-Ion battery charger that can be used to charge any 3.7V Li-Ion battery using a 5VDC (USB, Solar Panel…) power supply. At the heart of the circuit is ...

Lithium cells can produce voltages from around 1.5V (comparable to a zinc-carbon or alkaline battery) to around 3.7V, depending on the design and chemical compounds that were used in the battery.