SLUUDA0 April 2026 BQ27Z855
During normal operation, the battery-impedance profile compensation of the Dynamic Z-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 device must be configured for the current (or power) that will flow in the future. While it cannot be exactly known, the device can use load history, such as the average current of the present discharge, to make a sufficiently accurate prediction.
The device 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 0–3 and method 7) result in only minor differences in accuracy. However, methods 4–6, where an estimate is arbitrarily user-assigned, 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 conservative 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 Capacity cWh/5 |
| 5 = | AtRate() (mA) | AtRate() (cW) |
| 6 = | User Rate-mA | User Rate-mW |
| 7 = | Max Avg I Last Run (default) | Max Avg P Last Run |
To take into account pulsed loads while calculating remaining capacity until Term Voltage threshold is reached, the device monitors not only the average load but also the short load spikes. The maximum voltage deviation during a load spike is continuously updated during discharge and stored in Delta Voltage. Delta Voltage is then added to Termination Voltage artificially raising it as part of the capacity simulation to account for the sudden voltage drop that potentially could be seen. With Delta Voltage being a learned parameter, to protect the gauge from over or under compensating, limits are put on Delta Voltage to cap the compensation. This range is defined by Min Delta Voltage and Max Delta Voltage. In addition, to avoid a rapid change in Delta Voltage the max it can change in any single step is limited to DeltaV Max Voltage Delta.
| Class | Subclass | Name | Format | Size in Bytes | Min Value | Max Value | Default Value | Unit |
|---|---|---|---|---|---|---|---|---|
| Gas Gauging | IT-DZT Cfg | Min Delta Voltage | Int | 2 | –32768 | 32767 | 0 | mV |
| Class | Subclass | Name | Format | Min Value | Max Value | Default Value | Unit |
|---|---|---|---|---|---|---|---|
| Gas Gauging | IT-DZT Cfg | Max Delta Voltage | I2 | –32768 | 32767 | 200 | mV |
The device allows an amount of capacity to be reserved in either mAh (Reserve Cap-mAh, Load Mode = 0) or cWh (Reserve Cap-cWh, Load Mode = 1) units between the point where the RemainingCapacity() function reports zero capacity and the absolute minimum battery stack voltage, Term Voltage. This enables a system to report zero energy, but still have enough reserve energy to perform a controlled shutdown or provide an extended sleep period for the host system.
The reserve capacity is compensated at the present discharge rate as selected by Load Select.
The device forces RemainingCapacity() to 0 mAh when the battery stack voltage reaches the Term Voltage for a period of Term V Hold Time.