BQ27532-G1 Datasheet, PDF (11/35 Pages) Texas Instruments – Battery Management Unit Impedance Track Fuel Gauge

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bq27532-G1
SLUSBU6B – SEPTEMBER 2014 – REVISED JANUARY 2016
7.3 Feature Description
Information is accessed through a series of commands, called Standard Commands. Further capabilities are
provided by the additional Extended Commands set. Both sets of commands, indicated by the general format
Command( ), are used to read and write information contained within the control and status registers, as well as
its data flash locations. Commands are sent from system to gauge using the I²C serial communications engine,
and can be executed during application development, pack manufacture, or end-equipment operation.
Cell information is stored in non-volatile flash memory. Many of these data flash locations are accessible during
application development. They cannot, generally, be accessed directly during end-equipment operation. Access
to these locations is achieved by either use of the companion evaluation software, through individual commands,
or through a sequence of data-flash-access commands. To access a desired data flash location, the correct data
flash subclass and offset must be known.
The key to the high-accuracy gas gauging prediction is the TI proprietary Impedance Track™ algorithm. This
algorithm uses cell measurements, characteristics, and properties to create SOC predictions that can achieve
less than 1% error across a wide variety of operating conditions and over the lifetime of the battery.
The fuel gauge measures the charging and discharging of the battery by monitoring the voltage across a small-
value series sense resistor (5 to 20 mΩ, typical) located between the system V_SS and the battery PACK–
terminal. When a cell is attached to the fuel gauge, cell impedance is computed, based on cell current, cell open-
circuit voltage (OCV), and cell voltage under loading conditions.
The external temperature sensing is optimized with the use of a high-accuracy negative temperature coefficient
(NTC) thermistor with R25 = 10.0 kΩ ±1%, B25/85 = 3435 K ± 1% (such as Semitec NTC 103AT). The fuel
gauge can also be configured to use its internal temperature sensor. When an external thermistor is used, a
18.2-kΩ pullup resistor between the BI/TOUT and TS pins is also required. The fuel gauge uses temperature to
monitor the battery-pack environment, which is used for fuel gauging and cell protection functionality.
To minimize power consumption, the fuel gauge has different power modes: NORMAL, SLEEP, SLEEP+,
HIBERNATE, and BAT INSERT CHECK. The fuel gauge passes automatically between these modes, depending
upon the occurrence of specific events, though a system processor can initiate some of these modes directly.
For complete operational details, see bq27532-G1 Technical Reference Manual, SLUUB04.
7.3.1 Functional Description
The fuel gauge measures the cell voltage, temperature, and current to determine battery SOC. The fuel gauge
monitors the charging and discharging of the battery by sensing the voltage across a small-value resistor (5 mΩ
to 20 mΩ, typical) between the SRP and SRN pins and in series with the cell. By integrating charge passing
through the battery, the battery SOC is adjusted during battery charge or discharge.
The total battery capacity is found by comparing states of charge before and after applying the load with the
amount of charge passed. When an application load is applied, the impedance of the cell is measured by
comparing the OCV obtained from a predefined function for present SOC with the measured voltage under load.
Measurements of OCV and charge integration determine chemical SOC and chemical capacity (Qmax). The
initial Qmax values are taken from a cell manufacturers' data sheet multiplied by the number of parallel cells. It is
also used for the value in Design Capacity. The fuel gauge acquires and updates the battery-impedance profile
during normal battery usage. It uses this profile, along with SOC and the Qmax value, to determine
FullChargeCapacity( ) and StateOfCharge( ), specifically for the present load and temperature.
FullChargeCapacity( ) is reported as capacity available from a fully-charged battery under the present load and
temperature until Voltage( ) reaches the Terminate Voltage. NominalAvailableCapacity( ) and
FullAvailableCapacity( ) are the uncompensated (no or light load) versions of RemainingCapacity( ) and
FullChargeCapacity( ), respectively.
The fuel gauge has two flags accessed by the Flags( ) function that warn when the battery SOC has fallen to
critical levels. When RemainingCapacity( ) falls below the first capacity threshold as specified in SOC1 Set
Threshold, the [SOC1] (State of Charge Initial) flag is set. The flag is cleared once RemainingCapacity( ) rises
above SOC1 Clear Threshold.
When the voltage is discharged to Terminate Voltage, the SOC will be set to 0.
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