PS501 datasheet(10/42 Pages) MICROCHIP

10 / 42 page

PS501

DS21818C-page 10

 2004 Microchip Technology Inc.

4.0

CAPACITY MONITORING

The PS501 internal CPU uses the voltage, current and

temperature data from the A/D converter, along with

parameters and cell models, to determine the state of the

battery and to process the SBData function 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 PS501 creates an accurate fuel gauge

under all battery conditions.

4.1

Capacity Calculations

The PS501 calculates State-Of-Charge and fuel gaug-

ing functions using a ‘coulomb counting’ method, with

additional inputs from battery voltage and temperature

measurements. By continuously and accurately mea-

suring all the current into and out of the battery cells,

along with accurate three-dimensional cell models, the

PS501 is able to provide accurate predictions of SOC

and run-time.

The capacity calculations consider two separate states:

charge acceptance or Capacity Increasing (CI) and dis-

charge or Capacity Decreasing (CD). The CI state only

occurs when a charge current larger than the EEPROM

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.

To compensate for known system errors in the capacity

calculations, a separate error term is also continuously

calculated. This term is the basis for the SBData value

of MaxError. Two error values are located in EEPROM.

The CurrError value is the inherent error in current

measurements and should be set based on the selec-

tion of a sense resistor and calibration results. The

SelfDischrgErr value is the error in the parameter

tables for self-discharge and depends on the accuracy

of the cell chemistry model for self-discharge.

Since the PS501 electronics also drain current from the

battery system, another EEPROM 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 77 represents the module’s nominal

power consumption, including the PS501 typical

consumption.

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 error terms are always subtracted, even though

they are +/- errors, so that the fuel gauge value will

never be overestimated. Current draw of the PS501

and

the

self-discharge

terms

are

also

always

subtracted. The SBData value, MaxError, is the total

accumulated error as the gas gauge is running.

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 for self-discharge

relative to current, temperature and SOC.

4.2

Discharge Termination

Discharge termination is determined based on the

End-Of-Discharge (EOD) voltage point. The voltage

level at which this point occurs can be chosen to be

constant, or to change depending on the temperature

and discharge rate, since these factors affect the volt-

age curve and total capacity of the battery. The EOD

voltage parameter table predicts the voltage point at

which this EOD will be reached, based on discharge

rate and temperature.

The PS501 will monitor temperature and discharge rate

continuously and update the Veodx in real-time. When

the voltage measured on the cell is below EOD voltage

for the duration of EODRecheck x periods (500 ms), a

valid EOD has occurred.

When

valid

EOD

has

been

reached,

the

TERMINATE_DISCHARGE_ALARM

bit

(bit 11)

BatteryStatus

will

set.

This

will

cause

AlarmWarning condition with this bit set.

Additionally, the REMAINING_TIME_ALARM and/or

REMAINING_CAPACITY_ALARM bits can be set first to

give a user defined early warning prior to the

TERMINATE_DISCHARGE_ALARM. The remaining

time alarm will trigger in battery status when the remain-

ing time calculation falls below a threshold set by the

SMBus command. The remaining capacity alarm will be

set in battery status when the capacity falls below a

threshold set by the SMBus command. Use an SMBus

Write command to Remaining TimeAlarm (command

code 0x02) or RemainingCapacityAlarm (command

code 0x01) to set these values.

∆Charge = Σi∆t (the current integrated over time)

- CurrError (Current Measurement Error)

- PwrConsumption *

∆t (PS501 IDD)

- % of Self-Discharge * FCC

- SelfDischrgErr (Self-Discharge Error)

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