SLUUCO9A April 2023 – November 2023 BQ28Z620
During normal operation, the battery-impedance profile compensation of the Impedance Track algorithm can provide more accurate full-charge and remaining state-of-charge information if the typical load type is known. The two selectable options are constant current (Load Mode = 0) and constant power (Load Mode = 1).
To compensate for the I × R drop near the end of discharge, the BQ28Z620 must be configured for whatever current (or power) will flow in the future. While it cannot be exactly known, the BQ28Z620 can use load history, such as the average current of the present discharge to make a sufficiently accurate prediction.
The BQ28Z620 can be configured to use several methods of this prediction by setting the Load Select value. Because this estimate has only a second-order effect on remaining capacity accuracy, different measurement-based methods (methods 0 to 3, and method 7) result in only minor differences in accuracy. However, methods 4–6, where an estimate is arbitrarily assigned by the user, can result in a significant error if a fixed estimate is far from the actual load. For highly variable loads, selection 7 provides the most balanced estimate and is preferable.
| Constant Current (Load Mode = 0) | Constant Power (Load Mode = 1) | |
| 0 = | Avg I Last Run | Avg P Last Run |
| 1 = | Present average discharge current | Present average discharge power |
| 2 = | Current() | Current() × Voltage() |
| 3 = | AverageCurrent() | AverageCurrent() × average Voltage() |
| 4 = | Design Capacity/5 | Design Energy/5 |
| 5 = | AtRate() (mA) | AtRate() (10 mW) |
| 6 = | User Rate-mA | User Rate-mW |
| 7 = | Max Avg I (default) | Max Avg P |
To take into account pulsed loads while calculating remaining capacity until Term Voltage threshold is reached, the BQ28Z620 monitors not only average load but also short load spikes. The maximum voltage deviation during a load spike is continuously updated during discharge and stored in Delta Voltage, with the minimum value allowed set in Min Delta Voltage.
The BQ28Z620 allows an amount of capacity to be reserved in both mAh (Reserve Cap-mAh) and cWh (Reserve Cap-cWh) units between the point where the RemainingCapacity() function reports zero capacity and the absolute minimum pack voltage, Term Voltage. This enables a system to report zero capacity/energy, but still have enough reserve capacity/energy to perform a controlled shutdown or provide an extended sleep period for the host system.
The BQ28Z620 device forces RemainingCapacity() to 0 mAh when the battery stack voltage reaches Term Voltage. If Impedance Track GaugingConfiguration[CELL_TERM] = 1, the cell-based termination is used, and the Term Min Cell V threshold is checked for a termination condition. The cell-based termination provides an option to enable the gauge to reach 0 mAh before the device triggers CUV, especially for an imbalanced pack.