BQ27505-J4 Datasheet, PDF (24/38 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27505-J4
SLVSA40 – DECEMBER 2009
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5.2.4 Reserve Cap-mWh
Reserve Cap-mWh determines how much actual remaining capacity exists after reaching 0
AvailableEnergy( ), before Terminate Voltage is reached. A no-load rate of compensation is applied to
this reserve capacity.
5.2.5 Dsg Current Threshold
This register is used as a threshold by many functions in the bq27505-J4 to determine if actual discharge
current is flowing into or out of the cell. The default for this register is in Table 4-7, which should be
sufficient for most applications. This threshold should be set low enough to be below any normal
application load current but high enough to prevent noise or drift from affecting the measurement.
5.2.6 Chg Current Threshold
This register is used as a threshold by many functions in the bq27505-J4 to determine if actual charge
current is flowing into or out of the cell. The default for this register is in Table 4-7, which should be
sufficient for most applications. This threshold should be set low enough to be below any normal charge
current but high enough to prevent noise or drift from affecting the measurement.
5.2.7 Quit Current, DSG Relax Time, CHG Relax Time, and Quit Relax Time
The Quit Current is used as part of the Impedance Track™ algorithm to determine when the bq27505-J4
enters relaxation mode from a current-flowing mode in either the charge direction or the discharge
direction. The value of Quit Current is set to a default value in Table 4-7 and should be above the standby
current of the system.
Either of the following criteria must be met to enter relaxation mode:
• | AverageCurrent( ) | < | Quit Current | for Dsg Relax Time
• | AverageCurrent( ) | > | Quit Current | for Chg Relax Time
After about 5 minutes in relaxation mode, the bq27505-J4 attempts to take accurate OCV readings. An
additional requirement of dV/dt < 4 μV/s is required for the bq27505-J4 to perform Qmax updates. These
updates are used in the Impedance Track™ algorithms. It is critical that the battery voltage be relaxed
during OCV readings to and that the current is not be higher than C/20 when attempting to go into
relaxation mode.
Quit Relax Time specifies the minimum time required for AverageCurrent( ) to remain above the
QuitCurrent threshold before exiting relaxation mode.
5.2.8 Qmax 0 and Qmax 1
Generically called Qmax, these dynamic variables contain the respective maximum chemical capacity of
the active cell profiles, and are determined by comparing states of charge before and after applying the
load with the amount of charge passed. They also correspond to capacity at very low rate of discharge,
such as C/20 rate. For high accuracy, this value is periodically updated by the bq27505-J4 during
operation. Based on the battery cell capacity information, the initial value of chemical capacity should be
entered in the Qmax n field for each default cell profile. The Impedance Track™ algorithm will update
these values and maintain them the associated actual cell profiles.
5.2.9 Update Status 0 and Update Status 1
Bit 0 (0x01) of the Update Status n registers indicates that the bq27505-J4 has learned new Qmax
parameters and is accurate. The remaining bits are reserved. Bits 0 is user-configurable; however, it is
also a status flag that can be set by the bq27505-J4. Bit 0 should never be modified except when creating
a golden image file as explained in the application note Preparing Optimized Default Flash Constants for
specific Battery Types (SLUA334). Bit 0 is updated as needed by the bq27505-J4.
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FUNCTIONAL DESCRIPTION
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