BQ27425-G2_14 Datasheet, PDF (19/35 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge With Integrated Sense Resistor

bq27425-G2
www.ti.com
SLUSB23A – OCTOBER 2012 – REVISED FEBRUARY 2013
FUNCTIONAL DESCRIPTION
FUEL GAUGING
The bq27425 is an easy to configure fuel gauge that measures the cell voltage, temperature, and current to
determine battery state of charge (SOC). The bq27425 monitors charge and discharge activity by sensing the
voltage across an integrated small-value resistor (10 mΩ typ.) between the SRX and VSS pins and in series with
the cell. By integrating charge passing through the battery, the battery’s 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 state of charge and chemical capacity
(Qmax). The initial Qmax values are taken from the Design Capacity. The bq27425 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.
FUEL GAUGING CONFIGURATIONS
The bq27425 features easy to configure data NVM to speed-up fuel gauging design. Users are required to
configure Design Capacity, Termination Voltage, and Operation Configuration (see The Operation
Configuration Register section for details) to achieve optimal performance. The Impedance Track™ algorithm
uses these parameters along with built-in parameters to achieve accurate battery fuel gauging.
Several built-in parameters are used in the Impedance Track™ algorithm to identify different modes of battery:
• Charging : Chg Current Threshold (default = DesignCapacity /13.3 ),
• Discharging: Dsg Current Threshold (default = DesignCapacity /16.7 )
• Relax: Quit Current Threshold (default = DesignCapacity /25.0 )
To achieve accurate fuel gauging, the bq27425 uses a Constant Power Model for fuel gauging. This model uses
the average discharge power from the beginning of the discharge cycle until present time to compute load-
compensated capacity such as RemainingCapacity( ) and FullChargeCapacity( ) in the Impedance Track™
algorithm.
SOC Smoothing Feature
Rapid changes in operating conditions, such as temperature or discharge current, can lead to sudden changes in
the algorithm's immediate calculation of RemainingCapacity( ), FullChargeCapacity( ) and StateOfCharge( ).
SOC Smoothing provides filtered data to the host resulting in more gradual changes to SOC-related data when
conditions vary and can provide a better end-user experience. The feature is enabled via Op Config
[SMOOTHEN].
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Product Folder Links: bq27425-G2
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