SLUUCH2 User guide

SLUUCH2 March 2021 BQ40Z50-R2

6.4.2 QMax Update Conditions

A QMax update is enabled when gauging is enabled. This is indicated by the GaugingStatus()[QEN] flag. The BQ40Z50-R4 updates the no-load full capacity (QMax) when two open circuit voltage (OCV) readings are taken. These OCV readings are taken when the battery is in a relaxed state before and after charge or discharge activity. A relaxed state is achieved if the battery voltage has a dV/dt of < 4 µV/s. Typically, it takes two hours in a charged state and five hours in a discharged state to ensure that the dV/dt condition is satisfied. If five hours is exceeded, a reading is taken even if the dV/dt condition was not satisfied. The GaugingStatus()[REST] flag is set when a valid OCV reading occurs. If a valid DOD0 (taken at the previous QMax update) is available, then QMax will also be updated when a valid charge termination is detected.

The flag is cleared at the exit of a relaxed state. A QMax update is disqualified under the following conditions:

Temperature

If Temperature() is outside of the range 10°C to 40°C.

Delta Capacity

If the capacity change between suitable battery rest periods is less than 37%.

Voltage

If CellVoltage4..1() is inside a flat voltage region. (See the Support of Multiple Li-Ion Chemistries with Impedance Track Gas Gauges Application Report (SLUA372) for the voltage ranges of other chemistries.) This flat region is different with different chemistry. The GaugingStatus()[OCVFR] flag indicates if the cell voltage is inside this flat region.

Offset Error

If offset error accumulated during time passed from previous OCV reading exceeds 1% of Design Capacity, update is disqualified. Offset error current is calculated as CC Deadband / sense resistor value.

Several flags in GaugingStatus() are helpful to track for QMax update conditions. The [REST] flag indicates an OCV is taken in RELAX mode. The [VOK] flag indicates the last OCV reading is qualified for the QMax update. The [VOK] is set when charge or discharge starts. It clears when the QMax update occurs, when the offset error for a QMax disqualification is met, or when there is a full reset. The [QMax] flag will be toggled when the QMax update occurs. GaugeStatus3() returns the QMax and DOD (depth of discharge, corresponding to the OCV reading) data.

The BQ40Z50-R4 device includes a check in which, during discharge, there must be a minimum change in Voltage() programmed in Min Delta Voltage. There is also a maximum change set in Max Delta Voltage.

Table 6-3 Min DeltaV

Class	Subclass	Name	Format	Size in Bytes	Min Value	Max Value	Default Value	Unit
Gas Gauging	IT Cfg	Min Delta Voltage	Int	2	–32768	32767	0	mV

Table 6-4 Max DeltaV

Class	Subclass	Name	Format	Min Value	Max Value	Default Value	Unit
Gas Gauging	IT Cfg	Max Delta Voltage	I2	–32768	32767	200	mV