ATmega8 Based Lithium Ion Battery Charger

Overview

The project illustrates a universal charger for Li-Ion battery using a Smart Battery interface based on Atmel ATmega8 microcontroller with GCC software.

Explanation

As compared to regular NiMH or NiCd, there are several advantages that are being provided by Lithium ion batteries such as high capacity by weight and volume, no memory effect, fast charging, lower self-discharge, and many more. However, special charging algorithm is required by these batteries. It need non-constant current and voltage during charging cycle and they cannot be plugged to some voltage and expect them to charge.

During the charging, the current control at the maximum charge current is used until the battery voltage reaches the voltage threshold with 4.1 or 4.2V per series cell normally. At this point, the battery is charged at about 70%. The voltage can be controlled very accurately at the threshold voltage while the current drops off naturally. Charging is stopped after a top-off time delay once the current reaches the lower cutoff.

The current/voltage control is managed by a PWM output into a P-channel MOSFET and the circuit is being stabilized by a large inductor/capacitor combination while providing continuous data stream output from PC.