BQ27500_08 Datasheet, PDF (32/39 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500
bq27501
System-Side Impedance Track™ Fuel Gauge
SLUS785D – SEPTEMBER 2007 – REVISED APRIL 2008
www.ti.com
Impedance Measurement). The bq27500 chooses the profile which is closest to the measured impedance,
starting with the Packn profiles. That is, if the measured impedance matches Pack0, then the Pack0
profile is used. If the measured impedance matches Pack1, then the Pack1 profile is used. If the
measured impedance does not match the impedance stored in either Pack0 or Pack1, the battery pack is
deemed new (none of the previously used packs). The Def0/Def1 profile is copied into either the Pack0 or
Pack1 profile, overwriting the oldest Packn profile used.
6.2.2 Battery With Resistor ID (bq27501 Only)
The bq27501 can manage 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,
(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 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, then 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 the bq27501 uses the Pack0 profile. If the voltage measured is Pack 1 Voltage,
then it is identified as battery pack with B resistor and the bq27501 uses Pack1 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 500 Ω and 8 kΩ for A and B,
respectively, correspond to Pack 0 Voltage and Pack 1 Voltage threshold levels of 110 mV and 1070
mV, respectively.
If the bq27501 measures a voltage other than Pack 0 Voltage or Pack 1 Voltage, then it sets
Application Configuration [UNSUPBAT] to 1, alerting the host system that the inserted battery is not
supported. The fuel gauge also writes the measured voltage into Pack 2 Voltage for this unsupported
battery. The host system can use this information to download the default profile for this battery if one
exists. The host system should unseal the gauge, then download the new battery profile into the older
Defn 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 sets Application
Configuration [EN_SENC] to 0.
6.2.2.1 Profile Selection
When a battery pack is inserted to the host for the first time, both the Packn profiles are empty. The
bq27501 copies the Def0 profile into Pack0 profile if the voltage measured on the RID pin is Pack 0
Voltage. The Impedance Track™ algorithm then begins fuel gauging, updating the Pack0 profile as the
battery is used. Similarly, it copies the Def1 profile into Pack1 profile if the voltage measured on the RID
pin is Pack 1 Voltage, and the Impedance Track™ algorithm then begins fuel gauging, updating the
Pack1 profile as the battery is used.
Assuming the bq27501 has copied the Def0 profile into Pack0 for the first pack used in the system, if the
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 Pack0 and Def0 profiles. If it matches the Pack0 impedance, then the
Pack0 profile is used (this situation indicates the last pack used was re-inserted). If not, a new pack is
assumed, so the bq27501 copies the Def0 profile into the empty Pack1 profile, and Pack1 is made the
active profile. This feature helps support accurate fuel gauging for two batteries with the same resistor ID.
The logic behind this profile selection process is also applied to the case where the first pack inserted
causes the bq27501 to copy the Def1 profile into Pack1.
If both the cell profiles are no longer empty (indicating two different battery packs have been used in the
system) and a battery is inserted, the bq27501 chooses the correct Packn profile to re-write. This decision
is based on the resistor ID, the impedance of the inserted battery, and the fact that the last-used battery
profile should not be overwritten.
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APPLICATION-SPECIFIC INFORMATION
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