BQ27500 Datasheet, PDF (32/39 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500
bq27501
System-Side Impedance Track™ Fuel Gauge
SLUS785 – SEPTEMBER 2007
www.ti.com
impedance is measured immediately after battery detection. The bq27500 chooses the profile which is
closest to the measured impedance, starting with the Packn Ra profiles. That is, if the measured
impedance matches Pack0 Ra, then the Pack0 Ra profile is used. If the measured impedance matches
Pack1 Ra, then the Pack1 Ra profile is used. If the measured impedance does not match the impedance
stored in either Pack0 Ra or Pack1 Ra, the battery pack is deemed new (not any of the previously used
packs). Either the Def0 Ra or Def1 Ra profile is copied into either the Pack0 Ra or Pack1 Ra profile,
depending on which default impedance profile most closely matches. Care is taken not to over-write the
last used Packn Ra profile.
6.2.2 Battery With Resistor ID (bq27501 Only)
The bq27501 can administrate the information of up to two battery packs. For a given pack connected to
the fuel gauge, the identity of the battery is determined by a combination of (1) reading the pack ID
resistor and (2) measuring the impedance of the currently connected pack, and (3) remembering which
pack characteristics were most recently used by the gauge.
A battery pack’s ID resistor should connect to the RID pin of the fuel gauge. Either 'A' Ω or 'B' Ω resistor
values should be used, to indicate the battery type. If a battery connection is detected, bq27501 measures
the voltage developed at RID. If the voltage is Pack 0 Voltage, then it is identified as battery pack with 'A'
resistor and bq27501 will use the Pack0 Ra profile. If the voltage measured is Pack 1 Voltage then it is
identified as battery pack with 'B' resistor and the bq27501 will use Pack1 Ra profile. The measurement
window around each threshold is specified by Pack V% Range, which indicates the positive or negative
deviation around each level. Choosing RID values of 500Ω and 8kΩ for 'A' and 'B', correspond to Pack 0
Voltage and Pack 1 Voltage threshold levels of 110mV and 1070mV, respectively. These resistance
values assume a 300Ω resistance already exists in front of the RID pin for ESD protection.
If the bq27501 measures a voltage other than Pack 0 Voltage or Pack 1 Voltage, then it sets the
Application Configuration[UNSUPBAT] to ‘1’, alerting the system that the inserted battery is not
supported. The system can use this information to download the default profile for this battery if one
exists. The system should unseal the gauge, then download the new battery profile into the older Defn Ra
memory profile. The last-used profile is indicated by the Application Configuration[LU_PROF] bit.
Overwriting the older default profile allows the bq27501 to retain information stored regarding the most
recently used battery. After the new default profile is downloaded, the bq27501 clears the Application
Configuration[UNSUPBAT].
When the bq27501 starts operation for the first time, it copies the Def0 Ra profile into the Pack0 Ra profile
and the Def1 Ra profile into the Pack1 Ra profile. Then when a battery pack is inserted for the first time,
the bq27501 starts gauging using Pack0 Ra profile if the voltage measured on the RID pin is Pack 0
Voltage, or starts gauging using Pack1 Ra profile if the voltage measured on the RID pin is Pack 1
Voltage. The Impedance Track™ algorithm regularly updates the specific Packn Ra profile as the battery
is used.
If a pack is replaced with a second pack having the same resistor ID as the first, cell impedance is
measured after pack detection, as explained in Section 6.1.2.1 First OCV and Impedance Measurement.
This impedance is compared with the associated Packn Ra and Defn Ra profiles that correspond to the
current RID. If the impedance matches the Packn Ra impedance then the Packn Ra profile is used. If not,
the bq27501 resets the Packn Ra data, by copying the Defn Ra profile into the Packn Ra profile (this
operation overwrites the previously stored information). The Impedance Track™ algorithm begins
converging on the data for the new battery and storing it in the Packn Ra profile.
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APPLICATION-SPECIFIC INFORMATION
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