The bq2011 Gas Gauge IC is intended for battery-pack installation to maintain an accurate record of available battery charge. The IC monitors a voltage drop across a sense resistor connected in series between the negative battery terminal and ground to determine charge and discharge activity of the battery. The bq2011 is designed for systems such as power tools with very high discharge rates. Battery self-discharge is estimated based on an internal timer and temperature sensor. Compensations for battery temperature and rate of charge or discharge are applied to the charge, discharge, and selfdischarge calculations to provide available charge information across a wide range of operating conditions. Initial battery capacity is set using the PFC and MODE pins. Actual battery capacity is automatically "learned" in the course of a discharge cycle from full to empty and may be displayed depending on the display mode. Nominal available charge may be directly indicated using a five-segment LED display. These segments are used to indicate graphically the nominal available charge. The bq2011 supports a simple single-line bidirectional serial link to an external processor (common ground). The bq2011 outputs battery information in response to external commands over the serial link. To support subassembly testing, the outputs may also be controlled by command. The external processor may also overwrite some of the bq2011 gas gauge data registers. The bq2011 may operate directly from four cells. With the REF output and an external transistor, a simple, inexpensive regulator can be built to provide VCC from a greater number of cells. Internal registers include available charge, temperature, capacity, battery ID, and battery status.