The BQ27520-G4 measures the voltage, temperature, and current to determine battery capacity and state-of-charge (SOC) based on the patented Impedance Track™ algorithm (refer to the Theory and Implementation of Impedance Track Battery Fuel-Gauging Algorithm Application Report [SLUA450] for more information). The BQ27520-G4 monitors charge and discharge activity by sensing the voltage across a small-value resistor (5 mΩ to 20 mΩ typical) between the SRP and SRN pins and in series with the battery. By integrating charge passing through the battery, the battery’s SOC is adjusted during battery charge or discharge. Battery capacity is found by comparing states of charge before and after applying the load with the amount of charge passed. When a system load is applied, the impedance of the battery is measured by comparing the open circuit voltage (OCV) obtained from a predefined function for present SOC with the measured voltage under load. Measurements of OCV and charge integration determine chemical state-of-charge and chemical capacity (Qmax). The initial Qmax values are taken from a cell manufacturers' data sheet multiplied by the number of parallel cells. It is also used for the value in Design Capacity. The BQ27520-G4 acquires and updates the battery-impedance profile during normal battery usage. It uses this profile, along with SOC and the Qmax value, to determine FullChargeCapacity( ) and StateOfCharge( ), specifically for the present load and temperature. FullChargeCapacity() is reported as capacity available from a fully charged battery under the present load and temperature until Voltage( ) reaches the Terminate Voltage. NominalAvailableCapacity() and FullAvailableCapacity() are the uncompensated (no or light load) versions of RemainingCapacity() and FullChargeCapacity(), respectively. The BQ27520-G4 has two Flags( ) bits and two pins to warn the host if the battery’s SOC has fallen to critical levels. If RemainingCapacity() falls below the first capacity threshold specified by SOC1 Set Threshold, the Flags() [SOC1] bit is set and is cleared if RemainingCapacity() rises above the SOC1 Clear Threshold. If enabled via OpConfig C [BATLSPUEN], the BAT_LOW pin reflects the status of the [SOC1] flag bit. If enabled by OpConfig B [BL_INT], the SOC_INT will toggle upon a state change of the [SOC1] flag bit. As Voltage() falls below the SysDown Set Volt Threshold, the Flags() [SYSDOWN] bit is set and SOC_INT will toggle once to provide a final warning to shut down the system. As Voltage() rises above SysDown Clear Voltage the [SYSDOWN] bit is cleared and SOC_INT toggles once to signal the status change. Additional details are found in the BQ27520-G4 Technical Reference Manual (SLUUA35).