bq20z80-V101
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SLUS625D–SEPTEMBER 2004–REVISED OCTOBER 2005
Gas Gauging
The bq20z80 measures individual cell voltages, pack voltage, temperature, and current using features of the
bq29312A AFE device. The bq20z80 determines battery state of charge by analyzing individual cell voltages
when a time exceeding 35 minutes has passed since the batteries last charge or discharge activity. The bq20z80
measures charge and discharge activity by monitoring the voltage across a small-value series sense resistor (5
mΩ to 20 mΩ typ.) between the cell stack negative terminal and the negative terminal of the battery pack. The
batteries State of charge is subsequently adjusted during load or charger application using the integrated charge
passed through the battery.
Resistance update takes place in discharge mode, while OCV and Qmax updates take place only in relaxation
mode. Entry and exit of each mode is controlled by ata flash (DF:) parameters in the subclass 'Gas Gauging:
Current Thresholds' section. Charge mode is exited and Relaxation mode is entered when SBS.Current( ) goes
below DF:Quit Current for a period of DF:Chg Relax Time. Discharge mode is entered when SBS.Current( ) goes
below DF:Dsg Current Threshold. Discharge mode is exited and Relaxation mode is entered when SBS.Current(
) goes above (–) DF:Quit Current threshold for a period of DF:Dsg Relax Time. Charge mode is entered when
SBS.Current( ) goes above DF:Chg Current Threshold.
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 applications load is applied, the impedance of each cell is measured by
comparing the open circuit voltage (OCV) obtained from a predefined function for present state of charge with the
measured voltage under load.
Measurements of OCV and charge integration determine chemical state of charge and Chemical Capacity
(Qmax). The initial DF:Qmax Pack, DF:Qmax Cell 1, DF:Qmax Cell 2, DF:Qmax Cell 3, and DF:Qmax Cell 4
values are taken from the cell manufacturers' data sheet multiplied by the number of parallel cells, and are also
used for the SBS.DesignCapacity( ) value in DF:Design Capacity. The bq20z80 acquires and updates the
battery-impedance profile during normal battery usage. It uses this profile, along with state-of-charge and the
Qmax values, to determine SBS.FullChargeCapacity( ) and SBS.RelativeStateOfCharge( ) specifically for the
present load and temperature. SBS.FullChargeCapacity( ) is reported as capacity or energy available from a fully
charged battery under the present load and temperature until SBS.Voltage( ) reaches the DF:Term Voltage.
NAME
Term Voltage
CLASS / SUBCLASS
FORMAT
VALID RANGE
-32768 to 32767
0 to 1000
SIZE (BYTES)
UNITS
mV
DEFAULT VALUE
Gas Gauging / IT Config (80)
Signed Int
2
2
1
1
2
2
2
2
2
2
2
2
12000
10
Quit Current
mA
Dsg Relax Time
Chg Relax Time
Chg Current Threshold
Dsg Current Threshold
QMax Pack
0 to 255
s
1
Gas Gauging /
Current Thresholds (81)
0 to 255
s
60
0 to 2000
mA
50
0 to 2000
mA
100
4400
4400
4400
4400
4400
4400
Integer
0 to 65535
0 to 65535
0 to 65535
0 to 65535
0 to 65535
0 to 65535
mAh
mAh
mAh
mAh
mAh
mAh
Qmax Cell 1
Qmax Cell 2
Gas Gauging / State (82)
Qmax Cell 3
Qmax Cell 4
Design Capacity
SBS Configuration / Data (48)
Impedance Track™ Load Model
During normal operation, the battery-impedance profile compensation of the Impedance Track algorithm can
provide more accurate full-charge and remaining state-of-charge information if the typical load type is known.
The two selectable options are constant current and constant power.
DF:Load Mode VALUE
COMPENSATION
Constant current
Constant power
0x00
0x01
Impedance Track Load Compensation
In order to compensate for the I x R drop near the end of discharge, the bq20z80 needs to be configured for
whatever current (or power) will flow in the future. While it can not be exactly known, the bq20z80 can use load
history such as the averaged current of the present discharge to make a sufficiently accurate prediction. The
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