The bq26231 is a low-cost charge/discharge counter peripheral in an 8-pin TSSOP. It works with an intelligent host controller, providing state-of-charge information for rechargeable Li-Ion, Li-Pol, or NiMH batteries. The bq26231 measures the voltage drop across a low-value series sense resistor between the negative terminal of the battery and the battery pack ground contact. By using the accumulated counts in the charge, discharge, and self-discharge registers, an intelligent host controller can determine battery state-of-charge information. To improve accuracy, an offset count register is available. The system host controller is responsible for the register maintenance by resetting the charge in/out and self-discharge registers as needed. The bq26231 features 13 bytes of registers, which contain the capacity monitoring and status information. The RBI input operates from an external power storage source such as a capacitor or a series cell in the battery pack, providing register nonvolatility for periods when the battery is shorted to ground or when the battery charge state is not sufficient to operate the bq26231. During this mode, the register backup current is less than 100 nA. Packaged in an 8-pin TSSOP, the bq26231 is small enough to fit in the crevice between two A-size cells or within the width of a prismatic cell.