Datasheet

5 FUNCTIONAL DESCRIPTION
5.1 FUEL GAUGING
5.2 IMPEDANCE TRACK™ VARIABLES
bq27500
bq27501
System-Side Impedance Track™ Fuel Gauge
SLUS785D – SEPTEMBER 2007 – REVISED APRIL 2008 www.ti.com
Table 4-7. Data Flash Summary (continued)
Subclass Data Min Max Default
Class Subclass Offset Name Units
ID Type Value Value Value
Calibration 104 Data 0 CC Gain F4 0.1 4 0.47095 –
Calibration 104 Data 4 CC Delta F4 29,826 1,193,046 559,538.8 –
Calibration 104 Data 8 CC Offset I2 –32,768 32,767 –1667 mV
Calibration 104 Data 10 Board Offset I1 –128 127 0 mV
Calibration 104 Data 11 Int Temp Offset I1 –128 127 0 0.1 ° C
Calibration 104 Data 12 Ext Temp Offset I1 –128 127 0 0.1 ° C
Calibration 104 Data 13 Pack V Offset I1 –128 127 0 0.1 ° C
Calibration 107 Current 1 Deadband U1 0 255 5 mA
Security 112 Codes 0 Unseal Key 0 H2 0x0000 0xffff 0x3672 –
Security 112 Codes 2 Unseal Key 1 H2 0x0000 0xffff 0x0414 –
Security 112 Codes 4 Full-Access Key 0 H2 0x0000 0xffff 0xffff –
Security 112 Codes 6 Full-Access Key 1 H2 0x0000 0xffff 0xffff –
The bq27500/1 measures the cell voltage, temperature, and current to determine battery SOC. The
bq27500/1 monitors charge and discharge activity by sensing the voltage across a small-value resistor (5
m Ω to 20 m Ω 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 bq27500/1
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 (no- or light- load)
versions of RemainingCapacity( ) and FullChargeCapacity( ) respectively.
The bq27500/1 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 bq27500/1'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.
The bq27500/1 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.
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