BQ27500-V120 Datasheet, PDF (20/36 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500-V120
System-Side Impedance Track™ Fuel Gauge
SLUS880 – OCTOBER 2008
Class
Calibration
Subclass
ID
107
Table 4-7. Data Flash Summary (continued)
Subclass
Current
Offset
Name
1
Deadband
Data
Type
U1
Min
Value
0
Security
112
Codes
Security
112
Codes
Security
112
Codes
Security
112
Codes
0
Unseal Key 0
2
Unseal Key 1
4
Full-Access Key 0
6
Full-Access Key 1
H2
0x0000
H2
0x0000
H2
0x0000
H2
0x0000
5 FUNCTIONAL DESCRIPTION
Max
Value
255
0xffff
0xffff
0xffff
0xffff
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Default
Value
5
0x3672
0x0414
0xffff
0xffff
Units
mA
–
–
–
–
5.1 FUEL GAUGING
The bq27500 measures the cell voltage, temperature, and current to determine battery SOC. The
bq27500 monitors charge and discharge activity by sensing the voltage across a small-value resistor (5
mΩ to 20 mΩ typ.) between the SRP and SRN pins and in series with the cell. By integrating charge
passing through a 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 a cell manufacturers' data sheet
multiplied by the number of parallel cells. It is also used for the value in Design Capacity. The bq27500
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 Term Voltage.
NominalAvailableCapacity( ) and FullAvailableCapacity( ) are the uncompensated (less than C/20)
versions of RemainingCapacity( ) and FullChargeCapacity( ) respectively.
The bq27500 has two flags accessed by the Flags( ) function that warn when a battery's SOC has fallen
to critical levels. When RemainingCapacity( ) falls below the first capacity threshold, 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. The bq27500's BAT_LOW pin automatically
reflects the status of the [SOC1] flag. All units are in mAh.
When RemainingCapacity( ) falls below the second capacity threshold, SOCF Set Threshold, the [SOCF]
(State of Charge Final) flag is set, serving as a final discharge warning. Set SOCF Set Threshold = 0 to
deactivate the feature. Similarly, when RemainingCapacity( ) rises above SOCF Clear Threshold and the
[SOCF] flag has already been set, the [SOCF] flag is cleared. All units are in mAh.
5.2 IMPEDANCE TRACK™ VARIABLES
The bq27500 has several data flash variables that permit the user to customize the Impedance Track™
algorithm for optimized performance. These variables are dependent upon the power characteristics of the
application as well as the cell itself.
5.2.1 Load Mode
Load Mode is used to select either the constant-current or constant-power model for the Impedance
Track™ algorithm as used in Load Select (see Load Select). When Load Mode is 0, the Constant
Current model is used (default). When 1, the Constant Power model is used. The [LDMD] bit of
CONTROL_STATUS reflects the status of Load Mode.
20
FUNCTIONAL DESCRIPTION
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