BQ27500 Datasheet, PDF (22/39 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500
bq27501
System-Side Impedance Track™ Fuel Gauge
SLUS785 – SEPTEMBER 2007
www.ti.com
Dsg Current Threshold
This register is used as a threshold by many functions in the bq27350 to determine if actual discharge current is flowing into or
out of the cell. The default for this register is 100mA 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.
Chg Current Threshold
This register is used as a threshold by many functions in the bq27500/1 to determine if actual charge current is flowing into or
out of the cell. The default for this register is 50mA 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.
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 of 10mA and should be above the standby current of the system.
Either of the following criteria must be met to enter relaxation mode:
1. | AverageCurrent( ) | < | Quit Current | for Dsg Relax Time.
2. | AverageCurrent( ) | < | Quit Current | for Chg Relax Time.
After about 30 minutes in relaxation mode, the bq27500 attempts to take accurate OCV readings. An additional requirement of
dV/dt < 4 μV/sec is required for the bq27500/1 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.
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 bq27500/1 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.
Update Status 0 and Update Status 1
Bit 1 (0x02) of the Update Status n registers indicates that the bq27500/1 has learned new Qmax parameters and is accurate.
The remaining bits are reserved. Bits 1 is user-configurable; however, it is also a status flag that can be set by the bq27500/1.
Bit 1 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” (see SLUA334.pdf). Bit 1 is updated as needed by the bq27500/1.
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 never need to be modified. It is only updated by the
bq27500/1 when required.
Avg P Last Run
The bq27500/1 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 bq27500/1 continuously multiplies
instantaneous current times Voltage( ) to get power. It then logs this data to derive the average power. This register should
never need to be modified. It is only updated by the bq27500/1 when required.
Delta Voltage
The bq27500/1 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.
OCV, Default Ra, Rb, and Ra Tables
These tables contain encoded data and, with the exception of the Default Ra Tables, are automatically updated during device
operation. No user changes should be made except for reading/writing the values from a pre-learned pack (part of the process
for creating golden image files).
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FUNCTIONAL DESCRIPTION
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