8.3.1 Fuel Gauging

The bq27545-G1 measures the cell voltage, temperature, and current to determine battery SOC based on Impedance Track algorithm (see the Theory and Implementation of Impedance Track Battery Fuel-Gauging Algorithm Application Report [SLUA450] for more information). The bq27545-G1 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 cell. By integrating charge passing through the battery, the battery’s SOC is adjusted during battery charge or discharge.

The total battery capacity is found by comparing states of charge before and after applying the load with the amount of charge passed. When an application load is applied, the impedance of the cell is measured by comparing the 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 bq27545-G1 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 bq27545-G1 has two flags accessed by the Flags() function that warns when the battery’s SOC has fallen to critical levels. When RemainingCapacity() falls below the first capacity threshold, specified in SOC1 Set Threshold, the [SOC1] (State of Charge Initial) flag is set. The flag is cleared once RemainingCapacity() rises above SOC1 Clear Threshold. All units are in mAh.

When RemainingCapacity() falls below the second capacity threshold, SOCF Set Threshold, the [SOCF] (State of Charge Final) flag is set, serving as a final discharge warning. If SOCF Set Threshold = –1, the flag is inoperative during discharge. Similarly, when RemainingCapacity() rises above SOCF Clear Threshold and the [SOCF] flag has already been set, the [SOCF] flag is cleared. All units are in mAh.

The bq27545-G1 has two additional flags accessed by the Flags() function that warns of internal battery conditions. The fuel gauge monitors the cell voltage during relaxed conditions to determine if an internal short has been detected. When this condition occurs, [ISD] will be set. The bq27545-G1 also has the capability of detecting when a tab has been disconnected in a 2-cell parallel system by actively monitoring the SOH. When this conditions occurs, [TDD] will be set.