bq20z80-V101
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SLUS625D–SEPTEMBER 2004–REVISED OCTOBER 2005
bq20z80 can be configured to use several methods of this prediction shown in Table 1, by setting the DF.Load
Select value. Because this estimate has only a second-order effect on remaining capacity accuracy, different
measurement based methods (0x00 to 0x03) result in only minor differences in accuracy. However, methods
0x04-0x06 where an estimate is arbitrarily assigned by the user can result in significant error if a fixed estimate is
far from the actual load.
Table 1. Impedance Track Load-Compensation Settings
DF:Load Select
Load Mode
Load Compensation
Previous Discharge Average Current in DF:Avg I Last Run
Previous Discharge Average Power in DF:Avg P Last Run
Present Discharge Average Current
Present Discharge Average Power
SBS.Current( )
0x00
Constant Current
Constant Power
Constant Current
Constant Power
Constant Current
Constant Power
Constant Current
Constant Power
Constant Current
Constant Power
Constant Current
Constant Power
Constant Current
Constant Power
0x01
0x02
0x03
0x04
0x05
0x06
RAM.InstantPower( ) = SBS.Current( ) x SBS.Voltage( )
SBS.AverageCurrent( )
SBS.AveragePower( ) = SBS.AverageCurrent( ) x SBS.Voltage( )
SBS.DesignCapacity( ) / 5
SBS.DesignEnergy( ) / 5
SBS.AtRate( ) (mAh)
SBS.AtRate( ) (10mWh)
DF:User Rate-mA
DF:User Rate-10mWh
NAME
CLASS / SUBCLASS
FORMAT
VALID RANGE
2000 to 9000
3000 to 14000
SIZE (BYTES)
UNITS
mAh
DEFAULT VALUE
User Rate-mAh
User Rate-10 mWh
Gas Gauging / IT Config (80)
Gas Gauging / IT Config (80)
2
2
0
0
Signed Int
10 mWh
Pulsed Load Compensation
In order to take into account pulsed loads, while calculating remaining capacity until DF:Term Voltage is reached,
bq20z80 monitors not only average load but also short load spikes. The maximum voltage deviation during a
load spike is continuously updated during discharge and stored in DF:Delta Voltage.
NAME
CLASS / SUBCLASS
FORMAT
VALID RANGE
SIZE (BYTES)
UNITS
DEFAULT VALUE
Delta Voltage
Gas Gauging / State (82)
Integer
-32767 to 32768
2
mV
0
Reserve Battery Capacity
The bq20z80 allows an amount of capacity to be reserved in either mAh (DF:Reserve Cap-mAh) or 10 mWh
(DF:Reserve Cap-10mWh) units between the point where SBS.RemainingCapacity( ) is reported as 0%, and the
absolute minimum voltage, DF:Term Voltage. This enables a system to report zero energy, but still have enough
reserve energy to perform a controlled shutdown, or to provide an extended sleep period for the host system.
Also, if DF:Operation Cfg B [ResCap] = 0, the reserve capacity is compensated at a no-load conditions.
However, if DF:Operation Cfg B [ResCap] = 1, then the reserve capacity is compensated at the present
discharge rate as selected by DF:Load Select.
NAME
CLASS / SUBCLASS FORMAT
VALID RANGE
0 to 9000
SIZE (BYTES)
UNITS
mAh
DEFAULT VALUE
Reserve Cap-mAh
Reserve Cap-10 mWh
2
2
0
0
Gas Gauging /
Signed Int
IT Config (80)
0 to 14000
10 mWh
QMAX Update Conditions
The bq20z80 updates the no-load full capacity (QMAX) when two open circuit voltage (OCV) readings are taken
when the battery is in a relaxed state before and after charge or discharge activity. A relaxed state is achieved if
the battery voltage has a dV/dt of < 4µV. Typically it takes 2 hrs in a charged state and 5 hrs in a discharged
state to ensure that the dV/dt condition is satisfied. If 5 hrs is exceeded, a reading will be taken even if the dV/dt
condition was not satisfied. This update is disqualified under the following conditions:
10
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