BQ27500-V120 Datasheet, PDF (22/36 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500-V120
System-Side Impedance Track™ Fuel Gauge
SLUS880 – OCTOBER 2008
www.ti.com
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 bq27500
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 bq27500 attempts to take accurate OCV readings. An
additional requirement of dV/dt < 4 µV/s is required for the bq27500 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 a very low rate of discharge,
such as the C/20 rate. For high accuracy, this value is periodically updated by the bq27500 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 updates these
values and maintains 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 bq27500 has learned new Qmax
parameters and is accurate. The remaining bits are reserved. Bits 0 is is a status flag set by the bq27500.
Bit 0 should not 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 bq27500.
5.2.10 Avg I Last Run
The bq27500 logs the current averaged from the beginning to the end of each discharge cycle. It stores
this average current from the previous discharge cycle in this register. This register should not be
modified. It is only updated by the bq27500 when required.
5.2.11 Avg P Last Run
The bq27500 logs the power averaged from the beginning to the end of each discharge cycle. It stores
this average power from the previous discharge cycle in this register. To get a correct average power
reading the bq27500 continuously multiplies instantaneous current times Voltage( ) to get power. It then
logs this data to derive the average power. This register should not be modified. It is only updated by the
bq27500 when required.
5.2.12 Delta Voltage
The bq27500 stores the maximum difference of Voltage( ) during short load spikes and normal load, so
the Impedance Track™ algorithm can calculate remaining capacity for pulsed loads. It is not
recommended to change this value.
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FUNCTIONAL DESCRIPTION
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