PS810 Просмотр технического описания (PDF) - Microchip Technology
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PS810 Datasheet PDF : 46 Pages
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4.0 CAPACITY MONITORING
The PS810 uses the voltage, current and temperature
data from the A/D converter, along with battery algo-
rithms and cell models, to determine the state of the
battery and to process the battery data instruction set.
By integrating measured current, monitoring voltages
and temperature, adjusting for self-discharge and
checking for End-Of-Charge and End-Of-Discharge
conditions, the PS810 creates an accurate fuel gauge
under all battery conditions.
4.1 Capacity Calculations
The PS810 calculates State-Of-Charge and fuel
gauging functions using a ‘coulomb counting’ method,
with additional inputs from battery voltage and temper-
ature measurements. By continuously and accurately
measuring the current into and out of the battery cells,
along with accurate three-dimensional cell models, the
PS810 is able to provide accurate predictions of SOC
and run time.
The capacity calculations consider two separate states:
charge or Capacity Increasing (CI) and discharge or
Capacity Decreasing (CD). The CI state only occurs
when a charge current larger than the parameter
NullCurr value is measured. Otherwise, while at rest
and/or while being discharged, the state is CD.
Conditions must persist for at least NChangeState
measurement periods for a valid state change between
CD and CI. A minimum value of 2 is suggested for
NChangeState.
Regardless of the CI or CD state, self-discharge is also
calculated and subtracted from the integrated capacity
values. Even when charging, there is still a self-
discharge occurring in the battery. Self-discharge is
modelled internally in the PS810 and is a factor of
temperature and State-Of-Charge.
Since the PS810 electronics also drain current from the
battery system, another parameter value allows even
this minor drain to be included in the capacity calcula-
tions. The PwrConsumption value represents the
drain of the IC and associated circuitry, including
additional safety monitoring electronics, if present. A
typical value of 31 represents the modules nominal
power consumption, including PS810 typical
consumption of 85 μA.
PS810
The total capacity added or subtracted from the battery
(change in charge) per measurement period is
expressed by the following formula:
EQUATION 4-1:
ΔCharge = ΣiΔt (the current integrated over time)
- PwrConsumption * Δt
- Self-discharge percentage * FCC
The battery current will be precisely measured and
integrated in order to calculate total charge removed
from or added to the battery. Based on look-up table
values, the capacity is adjusted relative to discharge
rate and temperature.
4.2 Discharge Termination
Remaining capacity is determined based on the End-
Of-Discharge (EOD) voltage point. The voltage level at
which this point occurs will change depending on the
temperature and discharge rate, since these factors
affect the voltage curve and total capacity of the battery.
The EOD voltage look-up table predicts the voltage
point at which this EOD will be reached based on
discharge rate and temperature.
The PS810 will monitor temperature and discharge rate
continuously and update the EOD voltage from the
look-up table in real time. When the voltage measured
on the cell is below the EOD voltage for the duration of
EODRecheck x periods (500 ms), a valid EOD has
occurred. When a valid EOD has been reached, the
FULLY_DISCHARGED bit in BatteryStatus will be set.
4.3 Capacity Relearn
To maintain accurate capacity prediction ability, the
FullCapacity value is relearned on each discharge,
which has reached a valid EOD after a previous valid
fully charged EOC. If a partial charge occurs before
reaching a valid EOD, then no relearn will occur. The
difference between the old FCC and the new learned
FCC will be limited to 256 mAh, except when cycles are
less than RLCycles.
© 2006 Microchip Technology Inc.
DS21904C-page 13
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