BQ27425-G2_15 Datasheet, PDF (10/39 Pages) Texas Instruments – Fuel Gauge With Integrated Sense Resistor

bq27425-G2
SLUSB23B – OCTOBER 2012 – REVISED JUNE 2015
8.2 Functional Block Diagram
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CE
REGIN
VCC
2.5-V LDO
+
Power Mgt
SCL
SDA
Communications
HDQ/I²C
Oscillator
System Clock
Impedance
Track
Engine
ADC
Divider
Temp
Sensor
Co_- ulomb
Counter
BAT
SRX
Peripherals
BIN
Program Memory
Data Memory
VSS
8.3 Feature Description
8.3.1 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 fuel gauge monitors the charging and discharging of the battery by
sensing the voltage across an integrated small-value resistor (10 mΩ, typical) between the SRX and V_SS 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 the 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.
8.3.2 Fuel Gauging Configurations
The fuel gauge 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 Operation Configuration
(Op Config) Register 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 fuel gauge 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.
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