It can be quite tricky to reuse lithium-ion batteries from discarded equipment since these cells are often charged inside the equipment. In other words, there is no separate charger that can be reused. Fortunately, it turns out to be fairly easy to build a charger for used (or new) lithium-ion cells.
Like most people, you probably have old devices lying around that use lithium-ion batteries for the supply. This type of battery has been used in most portable equipment produced in the last few years because it can be easily manufactured in various sizes and shapes, and it has a relatively large capacity (compared to NiMH and NiCd batteries).
What can you do with that old MP3 player or cell phone that’s been replaced by a newer, better version? Usually the electronics cannot be used for any other purpose, but the battery can still be used, even for toys. Since electronics hobbyists tend to be an inventive lot, they will usually find a way to incorporate and recycle a battery. The author, for example, replaced three penlight cells in a Lego® train with a lithium-ion battery (see Figure 1).
However, this still leaves the need to charge the battery. The original equipment usually contains a special charger circuit for the battery, most likely on a small part of the PCB. It is difficult to figure out which components are part of the charger circuit since no circuit diagram is normally made available for portable equipment. In that case, we will just have to build our own lithium-ion charger!
The charger circuit described in this article is built around a lithium-ion charger IC made by Maxim Integrated, the MAX8677A (see Figure 2). This IC works completely autonomously so there is no need for a microcontroller (and hence no software)! A number of LEDs are used by the MAX8677A to indicate the state of the charging process.
For more detail: Lithium-Ion Battery Recycling Made Easy