BQ27500-V120 Datasheet, PDF (29/36 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500-V120
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6 APPLICATION-SPECIFIC INFORMATION
System-Side Impedance Track™ Fuel Gauge
SLUS880 – OCTOBER 2008
6.1 BATTERY PROFILE STORAGE AND SELECTION
6.1.1 Common Profile Aspects
When a battery pack is removed from host equipment that implements the bq27500, the fuel gauge
maintains some of the battery information in case the battery is re-inserted. This way, the Impedance
Track™ algorithm has a means of recovering battery-status information, thereby maintaining good
state-of-charge (SOC) estimates.
Two default battery profiles are available to store battery information. They are used to provide the
Impedance Track™ algorithm with the default information on two possible battery types expected to be
used with the end-equipment. These default profiles can be used to support batteries of different
chemistry, same chemistry but different capacities, or same chemistry but different models. Default
profiles are programmed by the end-equipment manufacturer. However, only one of the default profiles
can be selected, and this selection cannot be changed during end-equipment operation.
In addition to the default profiles, the bq27500 maintains two profiles PACK0 and PACK1. These tables
hold dynamic battery data, and keep track of the status for up to two of the most recent batteries used. In
most cases, the bq27500 can manage the information on two removable battery packs.
6.1.2 Activities Upon Pack Insertion
6.1.2.1 First OCV and Impedance Measurement
At power up, the BAT_GD pin is inactive, so that the host cannot obtain power from the battery (this
depends on the actual implementation). In this state, the battery is put in an open-circuit condition. Next,
the bq27500 measures its first open-circuit voltage (OCV) via the BAT pin. From the OCV(SOC) table, the
SOC of the inserted battery is found. Then the BAT_GD pin is made active, and the impedance of the
inserted battery is calculated from the measured voltage and the load current: Z(SOC) = [OCV(SOC) – V]
/ I. This impedance is compared with the impedance of the dynamic profiles, Packn, and the default
profiles, Defn, for the same SOC (the letter n depicts either a 0 or 1).
6.1.3 Reading Application Status
The Application Status data flash location contains cell profile status information, and can be read using
the ApplicationStatus( ) extended command (0x6a). The bit configuration of this function/location is shown
in Table 6-1.
Table 6-1. ApplicationStatus( ) Bit Definitions.
Application
bit7
bit6
bit5
bit4
bit3
bit2
bit1
Configuration
Byte
—
—
—
—
—
—
—
LU_PROF = Last profile used by fuel gauge. Pack0 last used when cleared. Pack1 last used when set. Default is 0.
bit0
LU_ PROF
6.2 APPLICATION-SPECIFIC FLOW AND CONTROL
6.2.1 Simple Battery
The bq27500 supports only one type of battery profile. This profile is stored in both the Def0 and Def1
profiles. The Defn and Packn profiles are the same on the first gauge start-up. Then the Impedance
Track™ algorithm begins fuel gauging, regularly updating Packn as the battery is used.
When an existing pack is removed from the bq27500 and a different (or same) pack is inserted, cell
impedance is measured after battery detection (see Section 6.1.2.1, First OCV and Impedance
Measurement). The bq27500 chooses the profile which is closest to the measured impedance, starting
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APPLICATION-SPECIFIC INFORMATION
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