PS501 datasheet(11/42 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21818C-page 11

PS501

4.3

Capacity Relearn at Discharge

Termination

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 condition (EOC). If a partial charge occurs

before reaching a valid EOD, then no relearn will occur.

If the discharge rate at EOD is greater than the ‘C-rate’

adjusted value in RelearnCurrLim, then no relearn will

occur.

When a valid EOD has been reached, then the error cal-

culations represented by the SBData value of MaxError

will be cleared to zero. If appropriate, the relearned value

of FullCapacity (and FullChargeCapacity) will also be

updated at this time.

4.4

Discharge Termination Voltage

Look-up Table

4.4.1

NEAR EMPTY SHUTDOWN POINT

As the graph in Table 4-1 shows, available capacity in

the battery varies with temperature and discharge rate.

Since the remaining capacity will vary with temperature

and discharge rate, a near empty shutdown point will

also vary with temperature and discharge rate.

Knowing the discharge rate that occurs in the system

during the shutdown process and knowing the tempera-

ture can pinpoint the exact save to disk point that will

always leave the perfect shutdown capacity. The PS501

uses this information to tailor the gas gauge to the sys-

tem and the remaining capacity and RSOC fuel gauge

function will always go to zero at the efficient shutdown

point. The table will use the voltage points at which this

happens as the error correction and FullCapacity

relearn point. This will ensure a relearn point before

shutdown occurs and will correct any error in remaining

capacity, also to ensure proper shutdown reserve

energy.

The shutdown point has to equal the capacity required

to shut down the system under the conditions of the

shutdown. That is, looking at the curve that represents

the actual discharge C-rate that occurs during the sys-

tem shutdown function, we must stop discharge and

initiate shutdown when the system has used capacity

equal to that point on the shutdown C-rate curve. This

is because no matter what the C-rate is when the shut-

down point is reached, the system will automatically

switch to the C-rate curve that represents the actual

current draw of the shutdown function. So it doesn't

matter if the system is in high discharge or low dis-

charge, it will be in “shutdown” discharge conditions

when shutdown begins and there must be enough

capacity left. An example is a computer’s save to disk

function.

Table 4-1 shows that the system will always shutdown

at the same capacity point regardless of C-rate condi-

tions (since the C-rate of the save to disk procedure is

a constant). Thus, we can automatically have an RSOC

that is compensated for C-rate; it will go to zero when

the capacity used is equal to the point at which

shutdown occurs.

Ignoring the effects of temperature, we could mark the

capacity used up to the shutdown point of the shutdown

curve. All the shutdown voltage would then represent

the same capacity and RSOC would always become

zero at this capacity and FCC would always equal this

capacity, plus the residual capacity of the save to disk

curve.

To compensate for temperature, we can look at the

series of curves that represent the shutdown C-rate at

different temperatures. The PS501 implementation is

to measure the temperature and choose a scaled

RSOC value that will go to zero at the save to disk point

at this temperature, assuming the temperature does

not change. If it does change, then an adjustment to

RSOC will be needed to make it go to zero at the

shutdown point.

Taking temperature into consideration, the amount of

capacity that can be used before shutdown is a

constant as C-rate changes, but not constant as tem-

perature changes. Thus, in the Look-up Table (LUT),

the individual temperature columns will have voltage

points that all represent the same capacity used, but

the rows across temperature points (C-rate rows) will

represent the different capacity used.

To compensate RSOC and RM, interpolation will be

used and the compensation adjustment will happen in

real-time to avoid sudden drops or jumps. Every time

the temperature decreases by one degree, a new inter-

polated value will be subtracted from RSOC and RM.

Every time the temperature increases by one degree,

RSOC and RM will be held constant until discharged

capacity equals the interpolated value that should have

been added to RSOC and RM (to avoid capacity

increases during discharge). With this interpolation

happening in real-time, there will be no big jumps or

extended flat periods as we cross over boundaries in

the LUT. This compensation will not begin until after the

fully charged status is reset, allowing RSOC to be

100% always when the battery is full.

4.5

Age Compensation

The voltage EOD points will be compensated due to the

age of the cells. A linear factor, AgeFactor, will be

applied to the voltage points as a function of

CycleCount. The voltage levels will decrease as the

battery pack ages to model the flattening of the voltage

vs. capacity curve that naturally happens to battery

cells.

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