BQ27425-G1 Datasheet, PDF (19/32 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge With Integrated Sense Resistor

bq27425-G1
www.ti.com
SLUSAI6 – NOVEMBER 2011
Class
Calibration
[NVM]
Security [RAM]
SubClass
ID
104
105
112
Table 7. Block Data Summary (continued)
SubClass
Data [NVM]
Offset
Name
0 CC Delta
Data
Type
Min
F4 2.98E+04
CC Cal [NVM]
4 CC Offset
6 Board Offset
7 Int Temp Offset
8 Pack V Offset
0 CC Gain
U2 0
I1 –128
I1 –128
I1 –128
F4 1.00E-01
Codes [RAM]
4 CC Cal Temp
I2 0
0 Sealed to Unsealed H4 0x0
Max Default
1.19E+06 567744.5
68
65535 –1200
127
0
127
0
127
0
4.00E+01 0.4768
32767
0xffffffff
2982
x367204
14
Unit
(EVSW Unit)
num
(2^–10Ω)
num (mV)
num (uV)
num (°C)
num (mV)
Num
(2^–10Ω)
0.1K
-
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 a small-value resistor (5 mΩ to 20 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.
In addition, a FACTORY_RESTORE subcommand is provided to restore default resistance and Qmax to factory
condition.
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 with it’s 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 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.
Copyright © 2011, Texas Instruments Incorporated
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