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 one microchip MCP73831, available in SOT-23-5 package. MCP73831 is a highly advanced linear charge management controller for use in space-limited, cost-sensitive applications. This IC employs a constant current/constant voltage charge algorithm with selectable preconditioning and charge termination.
Lithium-ion batteries have become popular for portable electronics because they boast the highest energy density of any commercial battery technology. Benefits include thousands of recharges and no occurrence of the “memory effect” that infested early NiCd rechargeable cells. However, Li-ion batteries need to be charged following a carefully controlled constant current/constant voltage (CV-CC) pattern that is unique to this cell chemistry. Overcharging and careless handling of a Li-ion cell can cause permanent damage, or instability and potential danger!
The SMD Soldering!
Construction of this charger circuit is undoubtedly a difficult exercise for many beginners and hobbysists because the task calls for perfect skill in SMD soldering/prototyping. Fortunately, readymade Li-Ion Battery Charger Modules are now available at many online stores. In case of any difficulty with the task at hand, try to procure one from your favourite online store (recently I bought one – based on IC TP4056 – from here: http://dx.com/p/1a-lithium-battery-chargingmodule-blue-205188)!