Pinomelean shows you how to build good Li-ion Battery Charger. He helps you understanding the theory of Li-ion Battery and design its charger circuit using Transistor TIP122 and Op-amp LM324.
Lithium based batteries are a versatile way of storing energy; they have one of the highest energy density and specific energy(360 to 900 kJ/kg) among rechargeable batteries. They're really useful in electronics due to their relatively high cell voltage; high energy density; their shape, size and capacity variety, and their charge/discharge efficiency.
The downside is that, unlike capacitors or other kinds of batteries, they cannot be charged by a regular power supply. They need to be charged up to a specific voltage and with limited current, otherwise they turn into potential incendiary bombs. It's no joke, storing such a high amount of energy in a small and normally tight packaged device can be really dangerous.