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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 Table4-1 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.
Table4-1 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.
