SLUUD50 October 2024 BQ27Z758
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 BQ27Z746-R1 and BQ27Z758 must be configured for whatever current (or power) will flow in the future. While it cannot be exactly known, the BQ27Z746-R1 and BQ27Z758 can use load history, such as the average current of the present discharge to make a sufficiently accurate prediction.
The BQ27Z746-R1 and BQ27Z758 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 assigned by the user, can result in a significant error if a fixed estimate is far from the actual load.
| 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() × Voltage() |
| 4 = | Design Capacity mAh/5 | Design Capacity cWh/5 |
| 5 = | AtRate() (mA) | N/A |
| 6 = | User Rate-mA | User Rate-cW |
| 7 = | Max Avg I Last Run | Max Avg P Last Run |
To take into account pulsed loads while calculating remaining capacity until Term Voltage threshold is reached, the BQ27Z746-R1 and BQ27Z758 monitor 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. Delta Voltage is added to Term Voltage to determine end-of-discharge voltage (EDV).
The BQ27Z746-R1 and BQ27Z758 allow 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 BQ27Z746-R1 and BQ27Z758 force RemainingCapacity() to 0 mAh when the battery voltage discharges to Term Voltage for a period of Term V Hold Time.
A portion of the configuration options for Impedance Track is in the following data flash. Additional configuration options are located in the sections in which they are discussed.
| Class | Subclass | Name | Type | Min | Max | Default | Unit | Description |
|---|---|---|---|---|---|---|---|---|
| Gas Gauging | IT Cfg | Pack Resistance | I2 | 0 | 32767 | 0 | mΩ | Value of the battery pack serial resistance that is NOT along the cell voltage measurement path of the gauge. This can include the resistance associated with FETs, traces, sense resistors, and any additional resistance in the path. |
| Gas Gauging | IT Cfg | System Resistance | I2 | 0 | 32767 | 0 | mΩ | Value of the system serial resistance that is along the path from the battery pack to the system power converter input. This can include FETs, traces, sense resistors and any additional resistance in the path. |
| Gas Gauging | IT Cfg | Cell 1 Interconnect Resistance | I2 | 0 | 32767 | 0 | 2–10 Ω | Value of the battery pack serial resistance that is along the cell voltage measurement path of the gauge; that is, the negative rail to the bottom of the cell and the top of the cell to the gauge. This can include the resistance associated with FETs, traces, sense resistors, and any additional resistance in the path. The gauge will offset the measured resistance with this value to improve Ra accuracy. |
| Gas Gauging | IT Cfg | Term Voltage | I2 | 0 | 32767 | 3000 | mV | Minimum pack voltage used in IT simulations. Set Term Voltage to the minimum allowable voltage at the system power converter input. If it is desired to report 0% RelativeStateOfCharge() before the pack voltage reaches Term Voltage, it is recommended to increase Reserve Cap-mAh and Reserve Cap-cWh. |
| Gas Gauging | IT Cfg | Term V Hold Time | U1 | 0 | 255 | 2 | s | When Term Voltage is met for Term V Hold Time, the RemainingCapacity() is set to 0 mAh. |
| Gas Gauging | IT Cfg | Max Simulation Iterations | U1 | 20 | 50 | 50 | — | Enables the user to set the max number of simulation iterations IT is allowed to do. If the user finds that the watchdog is tripping, this number can be lowered. |
| Gas Gauging | IT Cfg | Simulation Near Term Delta | I2 | 0 | 32767 | 250 | mV | Voltage delta from Term Voltage, which defines "near EDV" for IT simulations. If Term Voltage is increased, decrease Simulation Near Term Delta to keep Term Voltage + Simulation Near Term Delta around 3.2 V–3.5 V, the knee of the discharge curve. |
| Gas Gauging | IT Cfg | Min Delta Voltage | I2 | –32768 | 32767 | 0 | mV | The minimum Delta Voltage that is saved during discharge cycles |
| Gas Gauging | IT Cfg | Load Select | U1 | 0 | 7 | 1 | — | Defines load compensation mode used by the gauging algorithm |
| Gas Gauging | IT Cfg | Load Mode | U1 | 0 | 1 | 1 | — | Defines unit used by the gauging algorithm: 0 = Constant Current 1 = Constant Power |
| Gas Gauging | IT Cfg | User Rate-mA | I2 | –9000 | 0 | 0 | mA | Discharge rate used for capacity calculation selected by Load Select = 6 |
| Gas Gauging | IT Cfg | User Rate-cW | I2 | –32768 | 0 | 0 | cW | Discharge rate used for capacity calculation selected by Load Select = 6 |
| Gas Gauging | IT Cfg | Reserve Cap-mAh | I2 | 0 | 9000 | 0 | mAh | Capacity reserved available when the gauging algorithm reports 0% RelativeStateOfCharge() |
| Gas Gauging | IT Cfg | Reserve Cap-cWh | I2 | 0 | 32000 | 0 | cWh | Capacity reserved available when the gauging algorithm reports 0% RelativeStateOfCharge() |
| Gas Gauging | IT Cfg | Predict Ambient Time | U2 | 0 | 65535 | 2000 | s | Wait time before the gauging algorithm starts to predict the ambient temperature during CHARGE and DISCHARGE modes (if [AMB_PRED] = 1). It is recommended to be set to 2× the value of Temp a to represent system thermal equilibrium. |
The parameters updated by AND used for Impedance Track are in the following data flash.
| Class | Subclass | Name | Type | Min | Max | Default | Unit | Description |
|---|---|---|---|---|---|---|---|---|
| Gas Gauging | State | Update Status | H1 | 0x00 | 0x0E | 0x00 | — | Bit 1:0: UPDATE1, UPDATE0 Update Status 0,0 = QMax NOT updated, Ra NOT updated 0,1 = QMax updated, Ra NOT updated 1,0 = QMax updated, Ra updated 1,1 = Unused Bit 2: Enables—Impedance Track gauging is enabled. This bit can only be changed via the Gauging() command. 0 = Disabled 1 = Enabled Bit 3: is_QMax_Field_Updated—QMax has updated in the field. 0 = Not updated 1 = Updated Bit 7:4: Reserved |
| Gas Gauging | State | Avg I Last Run | I2 | –32768 | 32767 | –500 | mA | Average current last discharge cycle |
| Gas Gauging | State | Avg P Last Run | I2 | –32768 | 32767 | –192 | cW | Average power last discharge cycle |
| Gas Gauging | State | Delta Voltage | I2 | –32768 | 32767 | 0 | mV | Voltage() delta between normal and short load spikes to optimize runtime calculation |
| Gas Gauging | State | Temp k | I2 | 0 | 32767 | 200 | 0.1°C/ 256 cW | Thermal model temperature factor |
| Gas Gauging | State | Temp a | I2 | 0 | 32767 | 1000 | s | Thermal model temperature time constant |
| Gas Gauging | State | Max I Last Run | I2 | –32768 | 32767 | –500 | mA | Max current last discharge cycle |
| Gas Gauging | State | Max P Last Run | I2 | –32768 | 32767 | –192 | cW | Max power last discharge cycle |