PS402-01XX datasheet(11/46 Pages) MICROCHIP

11 / 46 page

 2003 Microchip Technology Inc.

DS21766A-page 11

PS402-01XX

4.0

CAPACITY MONITORING

The PS402 internal CPU uses the voltage, current and

temperature data from the A/D converter, along with

parameters and cell models from the EEPROM and

OTP EPROM, 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 PS402 creates an accurate fuel gauge

under all battery conditions.

4.1

Capacity Calculations

The PS402 calculates state-of-charge and fuel gauging

functions using a ‘coulomb counting’ method, with

additional inputs from battery voltage and temperature

measurements.

continuously

and

accurately

measuring all the current into and out of the battery

cells, along with accurate three dimensional cell

models, the PS402 is able to provide run-time accuracy

with less than 1% error.

The capacity calculations consider two separate states:

charge acceptance or capacity increasing (CI) and

discharge or capacity decreasing (CD). The CI state

only occurs when a charge current larger than OTP

EPROM 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 OTP

EPROM. The CurrError value is the inherent error in

current measurements and should be set based on the

selection 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 PS402 electronics also drain current from the

battery system, another OTP EPROM value allows

even this minor drain to be included in the capacity

calculations. 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 PS402's nominal

power consumption of 300

µA.

The total capacity added or subtracted from the battery

(change in charge) per measurement period is

expressed by the following formula:

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

- CurrError (Current Meas. Error)

- PwrConsumption *

∆t (PS402 IDD)

- % of Self-Discharge * FCC

- SelfDischrgErr (Self-Disch. Error)

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 PS402

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 at all times and for any current rate, in order

to calculate total charge removed from or added to the

battery. Based on lookup table access, the capacity is

adjusted with self-discharging rates depending on

actual capacity and temperature, residual capacity

corrections depending on the discharging current rate

and temperature, and charge acceptance depending

on SOC, charging current rate and temperature.

4.2

Discharge Termination and

Capacity Relearn

Discharge capacity is determined based on the End-

Of-Discharge (EOD) voltage point. This voltage can be

reached at different times based on the discharge rate.

The voltage level at which this point occurs will also

change depending on the temperature and discharge

rate, since these factors affect the voltage 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.

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