The bq2040 Gas Gauge IC With SMBus Interface is intended for battery-pack or in-system installation to maintain an accurate record of available battery charge. The bq2040 directly supports capacity monitoring for NiCd, NiMH, and Li-Ion battery chemistries. The bq2040 uses the System Management Bus v1.0 (SMBus) protocol and supports the Smart Battery Data (SBData) commands. The bq2040 also supports the SBData charge control functions. Battery state-of-charge, remaining capacity, remaining time, and chemistry are available over the serial link. Battery-charge state can be directly indicated using a four-segment LED display to graphically depict battery full-to-empty in 25% increments. The bq2040 estimates battery self-discharge based on an internal timer and temperature sensor and user-programmable rate information stored in external EEPROM. The bq2040 also automatically recalibrates or ?learns? battery capacity in the full course of a discharge cycle from full to empty. The bq2040 may operate directly from three nickel chemistry cells. With the REF output and an external transistor, a simple, inexpensive regulator can be built to provide VCC for other battery cell configurations. An external EEPROM is used to program initial values into the bq2040 and is necessary for proper operation.