The bq26501, the first in the bqJUNIOR™ family of advanced gas gauge device for handhald applications, is a highly accurate standalone single-cell Li-Ion and Li-Pol battery capacity monitoring and reporting device targeted at space limited portable applications. The device monitors a voltage drop across a small current sense resistor connected in series with the battery to determine charge and discharge activity of the battery. Compensations for battery temperature, self-discharge, and rate of discharge are applied to the charge counter to provide available capacity across a wide range of operating conditions. Battery capacity is automatically recalibrated, or learned, in the course of a discharge cycle from full to empty. Internal registers include available capacity, cell temperature and voltage, state-of-charge, and status and control registers. The bq26501 can operate directly from single-cell Li-Ion and Li-Pol batteries and communicates to the system over a simple one-wire bi-directional serial interface. The 5-kbits/s HDQ bus interface reduces communication overhead in the external microcontroller.