PS810 datasheet(13/46 Pages) MICROCHIP

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DS21904C-page 13

PS810

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.

The total capacity added or subtracted from the battery

(change in charge) per measurement period is

expressed by the following formula:

EQUATION 4-1:

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.

ΔCharge =

ΣiΔt (the current integrated over time)

- PwrConsumption *

Δt

- Self-discharge percentage * FCC

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