BQ27505-J4 Datasheet, PDF (32/38 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27505-J4
SLVSA40 – DECEMBER 2009
www.ti.com
5.9 AUTOCALIBRATION
The bq27505-J4 provides an autocalibration feature that measures the voltage offset error across SRP
and SRN as operating conditions change. It subtracts the resulting offset error from normal sense resistor
voltage, V_SR, for maximum measurement accuracy.
Autocalibration of the coulomb counter begins on entry to SLEEP mode, except if Temperature( ) is ≤ 5°C
or Temperature( ) ≥ 45°C.
The fuel gauge also performs a single offset when (1) the condition of AverageCurrent( ) ≤ 100 mA and (2)
{voltage change since last offset calibration ≥ 256 mV} or {temperature change since last offset calibration
is greater than 80°C for ≥ 60s}.
Capacity and current measurements continue at the last measured rate during the offset calibration when
these measurements cannot be performed. If the battery voltage drops more than 32mV during the offset
calibration, the load current has likely increased; hence, the offset calibration is aborted.
6 APPLICATION-SPECIFIC INFORMATION
6.1 BATTERY PROFILE STORAGE AND SELECTION
6.1.1 Common Profile Aspects
When a battery pack is removed from system equipment that implements the bq27505, the fuel gauge will
maintain some of the battery information, if it is re-inserted. This way the Impedance Track™ algorithm will
often have 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. 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 bq27505-J4 maintains two abbreviated profiles: Cell0 and Cell1.
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 bq27505-J4 can administrate 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 system might not obtain power from the battery (this
depends on actual implementation). In this state, the battery should be put in a condition with load current
less than C/20. Next, the bq27505-J4 measures its first open-circuit voltage (OCV) via the BAT pin. The
[OCVCMDCOMP] bit will set once the OCV measurement is completed. Depending on the load current,
the [OCVFAIL] bit indicates whether the OCV reading is valid. 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).The [INITCOMP] bit will be set afterwards and the
OCV command could be issued.
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APPLICATION-SPECIFIC INFORMATION
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