Note: Descriptions are shown in the official language in which they were submitted.
~.i47~:~
, . ._ ~ . . . . . ....
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FIELD OF THE INVENTION
This invention pertains~to battery chargers in general and
~pecifically to a method and apparatus for charging batteries
which permits any battery to be brought to it~ full state of
charge at a very rapid rate and also at maximum effioiency with-
out danger of damage to the battery or to the charger. This
invention will be described with particular reference to nickel-
cadmium batteries but it i8 also capable of charging many other
types of batteries in the optimum manner for each of those
particular batteries.
PRIOR ART
Battery usage in various products, particularly for the
retail con~umer, has increased tremendouqly in recent years.
However, batteries are still looked upon with substantial dis-
favor by many consumers because 80 much of their experience has
been with primary cells which are wasteful, which must be
~147~02
frequently replaced and which can cause serious damage if
leakage occurs. Rechargeable batteries have recently become
more popular in various devices, but problems are still encoun-
tered by the consumer. Frequently, he di~covers that his
batteries have self-discharged and need recharging at exactly
the moment when he would like to use the device, and recharging
in most instances takes an inconveniently long period of time.
One ~olution to this is to provide maintenance charging
systems in which the battery can be left on constant charge
between uses. Even this system is of no value if the consumer
fails to put the battery back on charqe after use; in addition,
most maintenance charging systems actually cause slow deteriora-
tion of the battery with time.
The solution to all of the above problems would be the
provision of an adequate fast charging system which would
reliably bring the battery up to its full state of charge in the
shortest possible time and without risk of damage. While the
prior art is replete with attempts to provide good fast charging
systems, no ~atisfactory system has yet been developed. Most
fast charging ~ystems today require very special conditions,
such as unusually expensive batteries which can accept the
; output of the fast charge system. Even under these special
conditions, there remains a ri~k of serious damage to either the
battery or to the charger. In addition, the present fast charge
techniques do not properly charge the batteries. Depending on
the termination mode used, all fast charge techniques of which
we are aware either overcharge or undex charge the battery,
,~either of which causes gradual deterioration of the battery and
premature failure.
In part, the failures of the prior art have been due to the
inability to accurately indicate full battery charge; this has
been due either to the failure of the prior art to select the
;,~ .
, , ~ .... .
~4~Z
proper mode of indication, or to the fact that, even if a
reasonably good indicator has been selected, the charging re-
quirements of a battery vary substantially with individual cell
chemistry, with individual cell history and with ambient temper-
ature. Thus, even an indication mode which is reasonably well
selected for a particular battery type may actually provide an
accurate indication only for a few cells having ideal charac-
teristics and only if the cells are charged under proper con-
ditions of ambient temperature.
For example, a major category of previous fast charging
systems has relied upon temperature cutoff to terminate the fast
charge mode. However, these systems are subject to several
difficulties: they may damage the batteries due to the constant
repetition of high temperature conditions, even in specially
manufactured (and expensive) cells which are theoretically
designed to accept high temperatures; such systems may not be
safe for use with defective cells; they actually do not charge a
battery to its full capacity, in high ambient temperature con-
ditions; the charge efficiency is low and the systems are there-
fore wasteful; and in low ambient temperature, the battery may
be driven to self-destruct by venting or possibly explosion.
Another major category of prior art fast charging systems
relies on voltage cutoff. However, in many types of battery
system~ including nickel-cadmium, this termination mode is
unreliable due to the large voltage variation which can occur
with temperature, or due to cell history or individual cell
characteristics. Thus, a voltage cutoff system can destroy a
battery through venting. Except in unusual ideal conditions, it
will never properly charge a battery to its full capacity.
A third major categroy of prior art battery charging
termination is based on simple passage of time. However, the
accuracy of this system depends on the battery, at the begin-
~47~02
ning of charge, having an assumed state of charge. There is a
very high likelihood that this will not be the case and that
the battery will be either over or under charged.
Most other charging methods which have been used to date
are based on combinations of one or more of the above techniques.
While some problems can be avoided by these combinations, at
least some of them still exist. Even the best fast charge
systems require expensive cell constructions; but the additional
cost only serves to delay the battery deterioration which is
caused by the charging system.
A more recent technique, illustrated by United States
Patent No. 4,052,656, seeks the point at which the slope of the
voltage-versus-time curve for a given battery is zero. However,
even this technique is subject to difficulties; it may detect
another point at which the voltage slope is zero but at which
the battery is only partially charged; in addition, even if it
properly locates the zero slope point which is close to full
charge, this lnherently overcharges the battery and will cause
battery deterioration due to heating.
All of the battery charging systems of which we are
presently aware embody one or another of the above techniques
and are subject to one or more of the above-listed defects.
This is true despite the fact that most currently known battery
chargers are designed to be used with only one type of battery
and, in general, with only one selected number of battery cells
of that particular type. The concept of a battery charger which
can accurately and rapidly deliver full charge to a variety of
different batteries including different number of cells or
different types of battery couples is totally beyond the present
state of the battery charging art.
OBJECTIVES
The overall object of the present invention is to overcome
~:~478~?2
the difficulties inherent in prior techniques of battery char~-
ing and to provide a new and improved method of and apparatus
for battery charging which fully charges batteries at a very
rapid rate and at maximum efficiency and without causing either
fast or slow deterioration of the battery.
A more specific object of this invention is the provision of
a method and apparatus for charging batteries which accurately
identifies the moment when the battery has reached full charge
and which terminates charging without either under or over-
charging the battery
A further object of this invention is the provision of amethod and apparatus for fully charging different batteries in-
cluding different numbers of cells at the maximum possible rate
and efficiency, from unknown starting conditions.
Another object of this invention is the provision of a
method and apparatus for fully charging different batteries
comprising different chemical couples at the maximum permissible
rate and efficiency, from unknown starting conditions.
Still another object of this invention is the provision
Of a method and apparatus for rapidly bringing a battery to
its full state of charge and terminating the fast rate charge
at that point, this being accomplished without regard to the
actual voltage o the battery, individual cell characteristics,
individual charging hi~tory of the particular battery, or the
actual ambient temperature.
In another aspect, it is an object of this invention to
provide a universal method for rapidly charging various types of
batteries and to further provide an apparatus which selects
the proper sub-method required to rapidly charge a battery of a
particular type.
In a further aspect, an object of this invention is the
provision of an apparatus for applying charge current to a
~147~02
battery and determining accurately the moment when a battery has
reached its full state of charge.
Still another object of this invention is the provision of
an improved method and apparatus for fast chaxging batteries which
recognizes accurately when a battery has reached a full state
of charge, which thereupon terminates the fast charge mode, and
which subsequently supplies a topping charge current to the
battery to compensate for batteries which, due to a particular
charging history, may produce a false indication of full state
of charge.
Still another object of this invention is the provision of
a method and apparatus for charging batteries which identifies
intermediate states in the charging cycle of a particular
battery and adjusts the rate of charging current applied so as
to maintain the applied current at the optimum level for rapid,
efflcient and non-destructive charging.
An additional object of this invention is the provision of
a method and apparatus for providing a non-destructive main-
tenance charge mode by which a battery can be kept at its full
8tate of charge without gradual battery deterioration.
It is an additional object of this invention to provide a
novel and unique method of evaluating the state of battery
charge and of controlling the applied charge current in re~ponse
to such evaluation so as to permit the battery to be brought to
its full charge state at the maximum possible rate and at
maximum efficiency without causing damage or deterioration o
the battery, such method also including safeguards to protect
against damage due to the introduction of a defective cell or
to the introduction of a cell which is already at full charge.
Further objects and advantages of this invention will
become apparent as the description and illustration thereof
proceed.
- ~1478~2
BRIEF DESCRIPTION OF TH~ INVENTION
In general, the present invention comprises a method of
applying a charge current to a battery, monitoring selected
battery parameters during the charging, inferring from changes
in these parameters an indication of the true charge condition
of the battery, and controlling the applied charging energy so
as to bring the battery to its full charge condition as quickly
as possible without damaging the battery. In addition, the
general method of this invention provides for the identification
of unu~ual conditions which may occur in some cases, and which
require charge termination to protect either the battery or the
charger; furthermore, this method provides for the application
of a topping charge in appropriate cases and for the application
of a maintenance charge to keep the battery at full charge, all
of the#e being accomplished without danger of damaging either
the battery or the charger. All of these ob~ectives are
accompli3hed regardless of the actual voltage of the battery;
despite wide variation in individual cell characteristics; de-
~pite previou~ harmful charging history in the case of a parti-
cular battery; and despite wide variations in the ambienttemperature to which the battery and/or the charger may be
exposed.
In particular, the present invention is based on the dis-
covery that the electrochemical potential of a battery exhibits
specific types of nonlinear changes of its value with respect to
time as the battery is charged~ The invention is further based
on the discovery that the true charge state of the battery
during charging may be analyzed by noting inflection points
which occur as the electrochemical potential changes with respect
to time.
In the case of specific batteries, proper charging may
involve determining the occurrence of either one or more of
47~02
such inflection points, or of determining a particular sequence
of ordered inflection points. Controlling the proper charge
mode may then involve simple conversion from a high rate fast
charge mode to a suitable maintenance mode which prevents or
compensates for self-discharge of the battery. In other cases,
proper control of the battery charging sequence may involve a
combination of inflection point determination with other
analyses of the variation of voltage with respect to time or of
the actual voltage at a particular time. In all of these
cases, a significant aspect of this invention is the determina-
tion of inflection points in the curve which represents the
electrochemical potential of the battery as a function of
time.
By way of illustration of the above general method, the
following specification describes appropriate variations on the
specific type of ana~ysis which may be performed to determine
the inflection points, and also describes vaxiations in the
analysis which may be necessary to accommodate differing modes
of battery charging such as constant voltage, constant current,
etc. Specific applications include techniques for charging
8uch batteries as nickel-cadmium, lead acid, and silver-cadmium.
In further accordance with the present invention, apparatus
is de8cribed for implementing these various methodq. In a
preferred embodiment, the apparatus includes a suitable source
of electrical energy, an analytical device for determining the
necessary controlling parameters, and means for controlling the
application of energy from the source to the battery.
In the particular example of a normal, discharged nickel-
cadmium battery, a useful charging pattern in accord with this
invention is to apply a fast-rate constant charge current to
the battery until two consecutive inflection points are passed,
~pecifically, a first one at which the sign of the slope of
--8--
~147~102
dV/dt (that is, the sign of d2V/dt2) changes from negative to
positive followed by a second one at which the sign changes
from positive to negative.
These analyses will be urther clarified with reference to
the voltage variation of a normal nickel-cadmium battery in the
detailed description hereinafter; for the present, it is
sufficient to note that one basic concept presented herein is
that of inflection point analysis. Specific techniques of
analysis and specific sequences adapted to accommodate different
battery couples may readily be developed within the context of
this general procedure.
DESCRIPTION OF THE FIGURES
Figure 1 is a graph illustrating the variation of
voltage as a function of time during the charge cycle of a nickel-
cadmium battery;
Flgure 2 is a block diagram illustrating the primary
element~ in a battery charger in accordance with this invention;
Figures 3 and 4 together comprise a schematic diagram il-
lustrating specific circuits which may be provided in accordance
with this invention to form the block diagram of Figure 2;
Figures 5 through 9 schematically illustrate the sequence
of operations performed by the microcomputer shown in Figure 4;
Figures 10-13 are graphs illustrating the variation of
voltage as a function of time during the charge cycle of several
different batteries; and
Figure 14 is a graph illustrating the variation of current
as a function of time during the charge cycle of a nickel-cadmium
battery.
SPECIFICATION
In the following specification, an explanation is given of
the battery charging process of nickel-cadmium batteries. The
inventive method for either monitoring or terminating the battery
478~2
charging process $~ next described, including several alternative
terminating modes u~ed for either protection or ~upplemental
termination. The apparatus of this invention is then presented,
including a preferred, detailed ~chematic circuit and a preferred
embodiment of the operational sequence performed by the micro-
computer. Finally, a general description of the application of
this invention to other types of batteries and to other charging
mode~ is provided.
BAT~ERY CHARGING PROCESS
In the course of recharging a nickel-cadmium battery, it has
been found that a very typical curve is produced if the increas-
ing battery voltage is plotted as a function of time. Figure 1
i~ 8 representation of a typical curve of this type, as taken
during a constant current charging cycle. A similarly typical
curve can be obtained by plotting current against time during a
constant voltage charging cycle, and a reproducible pattern
also occurs if neither voltage nor current are held constant.
Thi~ curVe may be divided into significant regions,
a~ indicated by the Roman numerals between the
vertical lines superimposed on the curve. While the curve
io ~ub~ect to variations in specific values of voltage or of
tlme, the general form is similar for all nickel-cadmium
batteries, including one or more cells, and the following dis-
cus~ion applies equally to all such batteries.
Region I of Figure 1 represents the initial stage of
voltage change which occurs when the charging cycle i8 first
~tarted. In this Region, the voltage is sub~ect to significant
varlation~ based on the initial charge level of the battery, lts
hi~tory of charge or discharge, etc. Since the shape of this
~ Region can vary, it is indicated in Figure 1 by a dotted line.
Becau~e the information in Region I varies, it is usually
preferable to ignore this segment of the curve. The battery
will generally traverse Region I completely within thé first
--10--
i'
- ~14'78CtZ
30 to 60 ~econds of charging and enter Region II; in general
the voltage in the Region I period incre~s relatively rapidly fro~
the initial ~helf voltage and the short peaks which may occur
in this Region are not harmful.
As the battery approaches a more stable charging regime,
it enters the portion of the curve designated Region II. Region
II may be of fairly long duration with little or no increase in
voltage. During this time, mo~t of the internal chemical con-
version of the charging process takes place. When significant
portions of the active material have been converted, the battery
begins to approach full charge and the voltage begins to increase
more rapidly. The inflection point A in the curve from a de-
crea~ing rate of increase to an increasing rate of increase is
~dentified as the transition from Region II to ~egion III.
Region III i~ characterized by a relatively rapid voltage
increase ag more and more of the active material is converted
to the charged 6tate. As the battery approaches full charge
more closely, that is, when perhaps 90 to 95% of its active
material has been converted chemically, oxygen beqins to evolve.
i 20 Thi~ produces an increase in the internal pressure and also an
increase in the temperature of the cell. Due to these effects,
the rapid increase in battery voltage begins to slow and another
inflection point occurs in the curve. This second inflection
point is identified as the transition point between Regions III
and IV, point B.
Within Region IV, the final portions of the active material
are being converted to the chemical composition of the fully
charged battery. At the same time, due to oxygen evolution
; from material already converted, the internal pressure increase
; 30 and the heating contribute to a slowing in the-rate of voltage
increase until th~e voltage stabilizes at some peak value for a
short period of time. This is designated as the transition
. . --11--
~47~:~
between Regions IV and V.
Within Region V, if charging is continued, the voltage of
the cell starts to decrease due to additional heating as
virtually all of the applied energy is converted into heat and
the negative temperature coefficient of the battery voltage
causes the voltage to aecrease. Continued application of charg-
ing energy in this Region would eventually cause damage to the
battery, either through venting or damage to the separator.
As previously noted, the relative time duration, slope
or value of any portion of this curve may be modified by such
factors as the initial temperature of the battery, the charge or
discharge history of the battery, the particular manufacturing
characteristics and the individual characteristics of the battery
cell. However, the major aspects of this curve and of each of
its Regions will be identifiable in any non-defective nickel-
cadmium battery which is brought from a substantially discharged
state to a fully charged state at a constant, relatively high
current.
In specific accordance with the present invention, the above
described curve and the information contained therein are utiliz-
ed in a novel manner to provide an improved battery charging
method. This method is much more accurate than those previously
used and is, in fact, so improved that it permits rapid charging
of any nickel-cadmium battery cell in a minimum time considering
reasonable system cost.
Up to the present time, rapid charging techniques for
batteries have carried the risk of serious damage to the battery.
To help in avoiding this problem, ordinary battery cells are
u~ually manufactured for use in conjunction only with so-called
"trickle chargers" which require some 16-24 hours to bring a
battery from a substantially discharged state to approximately
its fully charged state. Even when this time penalty is accepted,
-12-
i~78~2
such chargers can be harmful to the battery cells over a long
period of uqe.
- Rapid chargers are available for nickel-cadmium cells which
will bring a battery to approximately full charge within
approximately one hour. However, these chargers require the
use of high priced cells manufactured by special techniques 80
that the cells are capable of withstanding the poQsible harmful
- effects of rapid charging. This is due to the fact that the
chargers cut off by one or another of the methods de wribed
above with their attendant inaccuracies.
NFLECTION POINT ANALYSIS
In accordance with this invention, a new method of control-
ling the battery charge process is provided which identifies
exactly the conditions in the particular battery undergoing
charge and correspondingly controls the application of charge
current. Because of this new technique, a high rate charge
current can be applied to the battery ~o that the battery is
brought through its initial stages in the minimum po~sible time,
or example, as little as 15 minutes for a fully discharged
battery. AB the battery approaches full charge, its condition
i~ identified accurately and the charging current is reduced or
cut off at exactly the proper moment in the charge cycle.
Application of this new technique requires very sophisti-
cated proce~sing of the available information. In concise form,
; as applied specifically to nickel-cadmium batteries, the method
of this invention involves the identification of the inflection
point between Regions II and III and by the identification of the
~u~Yuent or fol~o~n~ inflection point between Regions III and IV. Once these
two inflection points have been identified and it has been
confirmed that their occurrence is in exactly that order, and
only then, the battery charging current can be discontinued
or reduced to a maintenance or topping mode if desired, with
.
~ -13-
~147802
absolute assurance that the battery has been brought to a full
state of charge regardless of its temperature, history, or
individual cell characteristics. secause of the accuracy of
this determination, this method can even be applied to batteries
which are constructed for use only with trickle chargers.
It shouid be noted that the exact sequence of occurrence
of these inflection points is critical to this invention. While
the preferred method of this invention involves ignoring the
voltage changes which occur within the first 30-60 seconds of
the charging cycle, the changes which occur in Region I may
overlap slightly into the time period within which the data
sampling apparatus of this invention is operative. In that
event, an inappropriate inflection point may occur near the
beginning of Region II. The apparatus of this invention is
designed so that it will ignore such inflection points until
those identified above occur in the proper sequence.
An alternative ~tatement of this technique can be made based
on the identification of changes of sign of the second derivative
of the voltage with respect to time. Specifically, Region II is
characterized by the gradual decrease of the slope or rate of
change of voltage versus time. For a fully discharged battery,
Region II constitutes the largest portion of the charging period
with voltage over most of this period increasing at a relatively
low rate. As the battery approaches full charge, the voltage
again starts to increase somewhat more rapidly. Thus, the slope
which had been becoming progressively smaller and smaller starts
to become larger again. This can be described as an inflection
point or a change in sign of the sacond derivative of voltage
with respect to time. Thus, we have a first such change in sign
giving indication that the battery is nearing the full charge
state.
During Region III the slope of the voltage-time curve in-
-14-
i~4'7802
creases further and further as the battery comes c~oser to full
charge. At or near the full charge point, there is the transi-
tion between Regions III and IV at which the slope of voltage
stops increasing and starts decreasing to smaller and smaller
values as Region IV progresses. Here again, a change in the
~ign of the second derivative of the voltage-time curve occurs.
This decreasing slope in Region IV indicates that virtually all
of the active material in the cell has been changed to the
charged state and that the energy going into the cell i8 begin-
ning to convert into heat rather than continuing the charging
process. Thus it is desirable to terminate charge during the
early or middle part of Region IV of the voltage time curve.
These two above described changes in sign of the second
derivative of the voltage-time curve are characteristic of
nickel-cadmium and other electrochemical cell~ during the
charging process. They provide a unique and reliable indication
of the state of charge of the battery. A particularly important
a~pect of ~he method of this invention is, accordingly, the use
of one or more of these observable changes of sign of the second
derivative of the voltage-time curve to determine when to
terminate battery charging.
The method of this invention of observing these inflection
points, or of changes in the sign of the second derivative of the
voltage-time curve of the battery charging process, can be
implemented in several ways including the apparatus hereinafter
described. For other types of electrochemical cells or different
- ~ types of charging sys~ems, other sequences of inflection points
may be required, but the detection of all of these types of
second derivative sign changes and specific sequences of them are
30 intended to be included within the scope of this general method.
One principal advantage of inflection point analysis
is that it does not depend on the actual value of the voltage of
-15-
~147802
the cell nor does it depend upon the value of the rate of change,
or slope, of voltage. It is an analysis of those points where
the rate of change of voltage (that is, the slope of voltage)
changes from decreasing to increasing or from increasing to
decreasing. In turn, these points are directly related to the
actual chemical occurrences within the battery being charged.
Thus, determination of state of charge and hence the most
appropriate time to terminate charge is dependent only upon very
universal characteristics of such batteries and not on the
particular cell characteristics or characteristics which might
be due to the history of use such as storage or very heavy use.
It is thus more reliable and a more valid indication of the most
appropriate time at which to terminate charge than previous
methods.
In some cases, the inflection point technique which is
appropriate for normal conditions may not be adequate, for
example, if a battery is damaged or defective or if a user in-
advertently places a fully charged battery on charge. In these
cases, the normal indicative points may not occur at all or they
may po5sibly occur within the first period of time in which the
apparatus is not sampling data. In order to protect against
these possibilities, the present invention further includes the
provision of qpecific controlling techniques or modes which may
be u~ed in combination with the basic method described above.
ABSOLUTE VOLTAG~ CHANGE ANALYSIS
A first of these techniques which can be incorporated is
that of terminating the application of charging current to the
battery immediateIy upon the occurrence of a negative change
of voltage. By reviewing the curve of Figure 1 it will be noted
that there is no point in the normal charge cycle when a negative
voltage change occurs. Thus, if a negative voltage change is
encounteredl it must mean that the battery is either defective
~1478~Z
or that it is already fully charged and that it has entered
Region V of the curve. Accordingly, provi~ion is preferably
included to terminate the high rate charge immediately upon the
occurrence of a negative voltage change. Preferably, the value
of this change should be large enough so that termination is not
inadvertently caused by inaccuracies in the monitoring equipment.
It is also noted that the absolute voltage change analysis
is utilized to prevent fast charging of a fully charged battery
which is inadvertently placed on fast charge by the operator.
Specifically, a fully charged battery to which a high current is
applied will traverse most, if not all of Regions I, II and III
very quickly. In many cases, this will occur in the time period
which a normally discharged battery would require to traverse
Region I. Since the system is instructed not to look for in-
flection points during the first 30 to 60 second portion of the
cycle, at least one and perhaps both of the significant inflec-
tion points, points A and B will pass before the system begins
to monitor for them.
There~ore, as monitoring of the fully charged battery begins,
2~ the battery will be passing through Region IV and entering
Region V. Within a fairly short time after it has been placed
on charge (e.g., 1-3 minutes) the battery will enter Region V
and its voltage will begin to decrease. As soon as the negative
voltage change is large enough to indicate to the apparatus that
the function of voltage with respect to time is no longer
monotonic, the apparatus will discontinue the fast charge rate.
Preferably, the charging mode then shifts into a maintenance mode
as will be hereinafter described. Since the high rate is only
maintained for a short period of time, the battery will not be
damaged by this sequence. It is also noted that even defective
batteries will not be driven into a hazardous condition by the
continuation of a maintenance charge mode after shut down of the
-17-
~147B~2
high rate due to a negative voltage change.
VOLT.~GE SLOPE ANAL~SIS
While the charge profile of nickel-cadmium batteries does
not lend itself to advantageous use of this technique, other
battery couples exhibit profiles wherein termination should be
predicated upon the occurrence of a particular voltage slope.
Thus, in a couple wherein Region v involves a slow downward drift
of voltage rather than a sharp decrease as in the nickel-cadmium
profile, the occurrence of a negative slope is useful in the
same manner as the absolute voltage change analysis just describ-
ed.
VOLTAGE LEVEL ANALYSIS
In some cases of dried or otherwise damaged nickel-cadmium
cells, application of a charging current can cause the voltage to
increase to a level significantly beyond the normal voltage of an
operative cell. Accordingly, the apparatus of this invention in-
clude~ the provision of a voltage level sensing means which
terminates charge if a predetermined level of voltage is encount-
ered. In other battery couples, this may serve as a primary
0 charge termination mode rather than as a secondary safeguard.
TIME ANAYLSIS
In other defective cells, the application of a high charge
current may simply be allowed to continue for an undue length of
time because the energy is being converted to heat or to oxygen
evolution, etc. In these instances, the defect in the cell may
prevent the inflection points from occurring and a maximum time
cutoff is provided.
In each of the above cases, the exact quantity chosen for
the negative voltage change, for the negative change in voltage
slope, for the absolute level of voltage reached, or for the
maximum time reached is, of course, a predetermined number based
on the type of cell for which the particular charger is intended.
-18-
~14'7802
MAINTENAN OE CHARGE MODE
After the main charge regime is terminated by one or more
of the above five methods of analysis, it is preferred to proceed
into two other charge regimes. The first of these is a programed
overcharge or surcharge to insure that all possible active
material in the cell is fully converted to the charged state and
and that all possible capacity in the cell will be available to
the user. The preferred method of overcharge or surcharge is to
charge at a relatively low charge rate for a fixed amount of time
depending on the type and size of the cell. This guarantees that
the cell is given a full amount of additional charge but at a low
enough rate to avoid damage. The fixed time also means that the
cell is not subject to long periods of time of overcharge which
would subject the cell to increased internal pressures and heat
which would eventually damage internal structures such as
separators.
At the end of the surcharge or overcharge period it i9 very
de~irable to provide only a maintenance charge which is used to
compensate for the internal self-discharge characteristics of
a}l electrochemical cells including nickel-cadmium cells. Nickel-
cadmium cells can self-discharge as much as 10% to 30% per month
depending on the storage temperature and the particular character-
istics of the cell. One method of maintenance charging is to
apply a low to medium charge current for a short period of time
one or more times per day. The preferred rate is a charging rate
of "C" ~a charge rate representing the same number of amperes of
charge as the ampere-hour rated capacity of the cell) for 15 to
30 seconds every 6 hours. This provides approximately twice the
typical loss rate in ampere hours of the cell without causing any
significant heating or pressure buildup in the ceIl. The
particular charge rate and particular choice of charged time to
resting time can be varied over a very wide range. The method is
-19-
" ~47802
merely to replace the calculated or measured energy lost to self-
discharge of the cell.
CHARGING APPARAlqJS
Figure 2 is a block diagram showing the major elements of
electronic circuitry which are used in accordance with this
invention to implement the above described charging method.
The flow of charging current in Figure 2 runs from an AC power
~nput pIug 8, connectable to an ordinary source of line current,
to a power ~upply 10 which converts the AC input to low voltage
DC. Next, the current passes through a resistor-controlled
current amplifier 12, and then through a charge/test switch 14
and finally to the o~ut bo~luJ~ 15 at which a ~ingle or multi-cell bat~ to
be charged iB connected. The power ~upply may, of cour~e, be
any alternative source of DC power such as a larger battery or
a converter operated from a DC ~ource. The amplifier i~
preferably a ~tandard serie~-pa~s current regulator although
other types of controllable current amplifiers could be u~ed.
The charge/test ~tch 14 normally connects the current amplifier
12 to the battery for the application of charging current;
thi~ ~witch al~o includes a test po~ition for use in a test
; mode which i~ described below.
The remainder of the block diagram illustrates a preferred
embodiment of the apparatus for performing the method of this
invention. In the illustrated embodiment, a start switch 16 is
provideds this comprises a momentary contact ~witch for inlti-
ating the sequence of operations. It is connected to one input
port of a microcomputer 18. In the preferred embodiment of thi~
invention, thi~ is an Intel type 8048 microcomputer. Thi~ ls a
self~xn ~ d oo~er including a p ~ ram memory for stDring L~structio~s,
a register m~y and a central proo~ssing unit (CPU) for oontrDlling the ex~
tion of the~sbored ~structio~s. The 8048 microcomputer ~s more com-
pletely described in the publication entitled "Microcomputer
-20-
~7~2
user's Manual No. 98-270A, published by the Intel Corporation of
Santa Clara, California.
When the tart switch 16 i8 actuated, which could be ac-
complished automatically on connection of a battery to the
output lines, the micKxrll *er 18 first allows the full charge
current to be applied to the battery through the amplifier 12
for a predetermined period of time, usually between 30 and 60
seconds, which allows the battery to be brought through the
segment of Figure 1 identified as Region I. For nickel-cadmium
batteries of the sub-C size, the preferred time is 40 seconds.
This application of power can be at full rated current since
even a defective battery or a fully charged battery will not be
seriously damaged by the application of this power for this short
an interval. The application of power is controlled by themicroxn~
puter 18 by its selection of the appropriate current control re-
sistor 20 through which to apply the input signal to the current amp-
liier 12.; After an appropriate period of time has passed as
described above, the microxnQuter 18 makes u~e of the analog-to-
digital oomR3ier ~4D) determine the battery voltage. The convert-
Z0 er 22 is preferably of the ~uccessive approximation type in whichsuccessive approximate digital values of battery voltage
; generated by the microcomputer are compared to the aatual battery
voltage until a close approximation is achieved. This information
li then fed back lnto the microcomputer. The microcomputer then
proceeds to execute its program so as to charge the battery ln
accordance with the method described above.
In addition to the basic elements of the block diagram
already mentioned, ~he circuit ~hould provide certain additional
features. If the battery charger is of a type adapted to handle
a variety of battery sizes and types, the battery type selection
circuit 24-is included which selects the specific program for
the given battery type from several stored in the camputer.
-21-
This may be done either by the operator or automatically by some
identification means such as particular terminal types provided
on the battery itself.
The system also preferably includes a temperature cutoff
circuit 26. The purpose of this circuit is to prevent charging
if the ambient temperature is either 80 low or so high as to
cause damage to the battery or to the charging circuit itself.
Reset circuit 28 is provided to reset the entire micro-
computer program to time zero as soon as power is supplied to
the system, or in the event of a power interruption. This is
done to prevent unpredictable charging effects which might occur
if the computer were to be initiated at an incorrect point in
its program.
Finally, the operator display circuit 30 provides for communi-
cating such information as may be appropriate to the operator.
In the case of a simple charger for use by a consumer, the ~is-
play 30 may consist only of a light to indicate that charging is
in proce~. In the case of a complex battery charger used by a
qualified technician, the display circuit may provide for the
display of a variety of different information which may be of
u~e to the technician in evaluating the condition of the battery.
Figures 3 and 4 together comprise a schematic diagram of
one suitable embodiment of Figure 2. The respective segments
of the circuit as identified in Figure 2 are enclosed in dotted
line boxes identified by corresponding numbers.
i In the specific embodiment of these figures, a o~m~ntional
line plug 8 is pr ~ ded for ocnnection to a soNroe of p~wer. The
po~ supply 10 includes a transformer Tl and a full wave rectifier bridge
made up of diodes Dl-D4 . The output from the bridge, which
30 may be approximately 20 volts D. C., is applied through amplifier
12 and-switch 14 to the battery (shown in dotted line illustra-
tion). A portion of the bridge output is also applied to a
filter made up on resistor ~1' diode D5 and capacitor Cl and to
~1 ' , .
~47~2
voltage regulator ICl. Regulated voltages of 25 volts and S
volts for use in the other portions of the circuit are taken at
the indicated output terminals.
The resistor-controlled current amplifier 12 operates
according to outputs taken from the microcomputer 18 through
current control resistors 20 shown in Figure 4. In accordance
with its internal program, the oJputer 18 selects a current level
by o~pleting a cLrcuit thrDugh one of the cun~nt oontrDl resisbors R29, R30,
or R31. This controls the input to operational amplifier IC2b
10 which iB taken at the midpoint of a voltage divider made up of
the parallel o~bination of resistors R18 and ~7,and the ~elected current
control resistor. The output from the amplifier IC2b is compared
to the voltage developed across a current shunt resistor R5.
Any error signal due to a difference is amplified by operational
amplifier IC2a and applied to driver transistor Q3. The output
of transistor Q3 is ~pplied to current oontrol transistors Ql and Q2 to
produce a very stable constant current which is applied to the
battery through Slc.
If the output current to the battery cannot reach the
selected current level, for example because there is no battery
connécted, transistor Q3 is turned fully on which, through the
comparison amplifier IC3a' supplies a signal to the computer
which turn~ the system off.
A5 shown in Fig. 3 a momentary oontact push button switch 16, which
may be operator-controiled or may be built into the battery
~ocket, supplies a signal to the battery to indicate that
the charging cycle ~hould be initiated. This could al~o be ac-
~compli~hed by monitoring for the presence of battery voltage orcurrent flow.;`; 30 Selection circuit ?4 ~ig 3) oompric~c a plurality of selectpr switchec S3, S~
which allow the operator to ~elect a particular computer program
appropriate to a particular battery. Diodes D7-Dlo are provided
-23-
.
..... . ... ... . .. .... .. .. ~. " ., ~
~478~2
to protect the computer. Alternatively, this selection could be
provided automatically by using different sets of unique termi-
nals to which different battery types are connected. Also, the
entire selection circuit 24 might be omitted if the charger is
intended for use with only a single battery type.
T~erature cut-off circuit 26 oomprises a safety ci~t bo prevent
cperation at t3~rat w outside a predeten~ned p~ssible range. In the
particular arrangement shown, the voltage at the midpoint of the
voltage divider comprising resistor R36 and thermistor THl con-
trols the input to both sides of the comparator amplifier IC3b.In the case of a high temperature (e.g., 125F), the resistance
of THl is low which reduces the voltage input to the positive
side of IC3b; in the case of a low temperature (e.g., 25F),
the resistance of THl is high which increases the voltage at the
negative side of IC3b. Either extreme produces a low output
~ignal from IC3b which instructs the computer to discontinue
charging.
In subcircuit 22, the battery develops an input signal
across the voltage divider R4/R67 which is amplified in operation-
al amplifier IC2d. The resistances R64, R65 and R66 and capaci-
tors Cg and C10 comprise a filter on the output of IC2d and this
signal is used as one input to comparator amplifier IC3C.
; At the same time, another input to comparator IC3 is
developed through operational amplifier IC2C from a voltage
divider comprising the parallel re~istors R52 and R53 and a
binary coded combination of the resistor ladder R43-R50 as
selected by the computer. Resistors R44-R50 each have values
which are twice the value of the preceding sequential resistor.
The computer, under the instruction of its program as will be
described hereinafter, selects an initial minimum value, for
example, by turning on only R43. This develops a voltage across
IC2C which is compared in IC3C to the signal received from the
-24-
, ' .
,.... ,. ~ ~.s - ~ '' '~ `~'' ''
11~78C~2
battery. If this minimum voltage suppliea from the computer is
not equal to or greater thsn the' battery voltage, then ~uccessive~
ly increased values are tried by the computer until a match is
reached. This information i8 communicated back to the computer
from the output of IC3C and the computer uses the last input to
the comparison circuit as the battery voltage.
A reset circuit 28 is provided. In this circuit, the
comparator IC3d amplifies a signal derived from the 25 volt sup-
ply and compares it to a S volt reference. If the 2S volt signal
goe~ below approximately 10 volts, as would occur upon the re-
moval of power from the ~ystem either due to a power failure or
due to the operator unplugging the charger, the output signal
from the comparator instructs the computer to return all of its
programming functions to the initial conditions; that is, those
which must be used when a new charge cycle i8 initiated. This
can oxur either during power~n as the pa~r is fall ~ from n~l input to
zero due to a power fail D or during p~-up as the p ~ r is bui~g from zero
to it~ normal level when the sy~tem is first connected to a power
~ource. In either case, thi~ system i~ useful to ensure that the
computer does not begin a cycle at ~ome indeterminate midpoint in
it~ cycle with inappropriate information stored in its memory.
Item 30 comprises the display ~ystem by which the computer
communicates appropriate information to an operator. As il-
lu~trated, the display preferably comprises two seven segment
display elements and transi~tors Q4 and Q5 which form a o~n~ntional
strobing control which enable~ eight output lines to control
both display~. Alternatively, the display might compri~e'simply
a single indicator lamp.
;-' Finally, el~t 14 ~ig 3) comprises a charge/test switch. In the
normal, charge position Sib and Slc connect the'current con-
trolled amplifiex 12 and resistor network (items 12 and 20 of
Figure 2) so that current from power supply 10 is supplied
-25-
11478G2
through Slb to transistors Ql and Q2 to switch Sl to the
battery with a return path through resistor R5. In the test .
position, the battery is connected through Slb to transistors
Ql and Q2 thro~lgh switch Slc to resistor R2 and returning
through resistor R5 to the battery. For example, this could
allow the system to be used to discharge the battery at a pre-
determined rate and, by means of appropriate programming, to
determine and display the ampere-hour capacity of the battery.
In addition, switch Sla provides an alternate signal to the
microcomputer to instruct it to enter the charge program or a
separate discharge program wherein it tests the condition of
the battery.
In one embodiment of Figures 3 and 4, the following circuit
elements were used:
Rl 10 ohm 1/4 watt Rlg lOOk ohm 1/4 watt
R2 '3 ohm 1 watt R20 22k ohm 1/4 watt
R3 lk ohm 1/4 watt R21 lOk ohm 1/4 watt
R4 lOOk ohm R22 220k ohm 1/4 watt
R5 .1 ohm 1 watt R23 lOk ohm 1/4 watt
20 R6 10 ohm 1/4 watt R24 lOk ohm 1/4 watt
R7 12 ohm 1/4 watt R25 lOk ohm 1/4 watt
R8 560 ohm 1/4 watt R26 lOk ohm 1/4 watt
Rg 560 ohm 1/4 watt R27 Trimpot 3k ohm
Rlo lOk ohm 1/4 watt R28 8. 2k ohm 1/4 watt
Rll Trimpot lOOk ohm R29 lOOk ohm 1/4 watt
R12 lk ohm 1/4 watt R30 12k ohm 1/4 watt
R13 lOk ohm 1/4 watt R31 4. 7k ohm 1/4 watt
R14 2.2k ohm 1/4 watt R32 lOk ohm 1/4 watt
R15 1 Megohm 1/4 watt R3 1 Megohm 1/4 watt
30 R16 8.2k ohm 1/4 watt R34 33k ohm 1/4 watt
R17 lOk ohm 1/4 watt R35 22k ohm 1/4 watt
R18 Trimpot lOOk ohm R36 4.7k ohm 1/4 watt
il
-26-
~,~478~2
R37 22k ohm 1/4 watt R67 lOOk ohm 1/4 watt
R38 33k ohm 1/4 watt Cl 1000 microfarads 35 volts
R39 680 ohm 1/4 watt C2 .1 microfarads 35 volts
R40 lk ohm 1/4 watt c3 10 microfarads 35 volts
R41 lk ohm 1/4 watt c4 1 microfarads 35 volts
R42 1.8k ohm 1/4 watt c5 .1 microfarads 35 volts
R43 5k ohm 1/4 watt C6 20 picafarads 35 volts
R44 lOk ohm 1/4 watt c7 10 microfarads 35 volts
R45 20k ohm 1~4 watt C8 .1 microfarads 35 volts
R46 40k ohm 1/4 watt Cg 10 microfarads 35 volts
R47 80k ohm 1/4 watt C10 10 microfarads 35 volts
R48 160k ohm 1/4 watt Dl 3 amp 50 volts
R49 320k ohm 1/4 watt D2 3 amp 50 volts
R50 640k ohm 1/4 watt D3 3 amp 50 volts
R5 270 ohm 1/4 watt D4 3 amp 50 volts
R53 Trimpot 3k ohm D5 1 amp 50 volts
R lOk ohm 1/4 watt D Zener diode
54 6 5.6 volts 1/2 watt
R55 lOOk ohm 1/4 watt D Type IN4148
7 .1 amp 50 volts
R 2.2k ohm 1/4 watt D8 Type IN4148
56 .1 amp 50 volts
... R57 lOk ohm 1/4 watt Dg ~ype IN4148
.1 amp 50 volts
R lOk ohrn 1/4 watt D Type IN4148
58 10 .1 amp 50 volts
R59 220k ohm 1/4 watt Dl Type IN4148
1 .1 amp 50 volts
R6 lOk ohm 1/4 watt D12 Type IN4l48
.1 amp 50 volts
R61 lOk ohm 1/4 watt
R62 1 Megohm 1/4 watt
R63 lOk ohm 1/4 watt
R64 47k ohm 1/4 watt
- R6 33k ohm 1/4 watt
R66 22k ohm 1/4 watt
J,1478~2
Ql PNP transistor 3 amp 40 .volt type TIP-30
Q2 NPN transiStOr 15 amp 40 volt type TIP-35
Q3 NPN transistor .5 amp 40 volt type MPS A05
Q4 NPN transistor .S amp 40 volt type MPS A05
Q5 NPN transistor .5 amp 40 volt type MPS A05
ICl Voltage regulator 5 volt .5 amp type 78MO5
IC2 Quad operational amplifier type LM 324
IC3 Quad comparator type MC 3302
IC4 Microcomputer type 8048
Tl Transformer 120/240 volt AC input
10-20 volt AC output 1-5 amps
LEDl 7 segment light-emitting diode display common cathode
LED2 7 segment light-emitting diode display common cathode
Fl Slow blow fuse, 1 amp
E2 Fuse, 5 amps
THl Thermistor RL28Fl
Sl Switch 3 pole double throw ~3 amp contacts)
S2 Swltch SPST N.O. momentary
s3 Swit~h SPST
S4 Switch SPST
S5 Switch SPST
-28-
li478~2
FLOW CH~R~ - MICROCOMPUTER OPERA~ION
Figures 5-8 comprise a flow chart of the basic operations
which are performed within the microcomputer. This flow chart
summarizes the programming steps which are presented in complete
detail in the program included herewith as Appendix B. The flow
chart illustrated in Figures 5-8 has been prepared at a level of
detail which would permit an experienced programmer to complete
the detailed implementation of this invention in a type 8048
microcomputer but which, at the same time, is not so detailed
as to require repetitious description of iterative steps. For a
complete description, reference may be had to the program set
forth in Appendix B.
As has previously been noted, when power is first applied
to the ~yctem, the reset circuit 26 automatically sets all
operations of the computer to an initial or ~reset~ mode. In
the flow chart, the "start" block 110 signifies the application
of the start slgnal to the computer due to the clo~ing of the
~tart ~witch 16 of Figure 2. Immediately, the internal tokal ~me
regi~ter is ~et at 0. This i~ indicated by block 112. The
further step~ of the process ~hown in Pigures 5-8 are then
performed by the microcomputer.
~ The next step in the process, identified as bloc} 114, is
; to increment the total time regi~ter. Then the program moves
to block 116 which does a comparison between a maximum allow-
able time a~ set for the particular battery and the time that
has elapsed. If the comparison shows that the maximum allow-
able total time has been reached, the sequence moves to block 118
which indicates the execution of the sequence of instructions
to ~top the charging cycle, including either turning off the
charging current or turning it to a lower value. This may also
- involve changing to a timed overcharge mode or to a surcharge
mode or to a maintenance mode if desired.
29
1147eO2
If the total time has not been reached, which it wi}l not
this first time through, the microcomputer goes on to block 120.
Here, the time register i8 again used to determine whether this
is the first time through this sequence of steps. If it i8, then
the program moves to the series of steps 122-128 which direct the
computer to set up certain registers within the computer 80 that
they are ready for use later in the program. First, as indicated
at biock 122, a flag identified as F0 is cleared. This flag will
later be ~et upon the occurrence of a first inflection point or
changé in sign of the second derivative. The program then
continues through block 124, 126 and 128. As indicated ln the
drawing, each of these steps controls the placement of an
initial value in particular registers, namely, "Minimum Slope",
~Maximum Slopen, and "Maximum Voltage Sum~ respectively. The
~Minimum Slope" register is set to a large number, while the
~Maximum Slope" and ~Maximum Voltage Sum" registers are each set
to a large negative number such as -10,000. The use of these
registers will be described below. ~hereafter, the program moves
to block 130 designated "loop 2". This i8 a common return
location to which the program is redirected after each of several
alternative sequences have been completed. In this instance,
after the three registers have been initialized as de~cribed
above, the program moves through block 130 to block 132.
In block 132, the stated interrogation i8 "has two ~econds
gone by~. Block 132 together with the closed loop 133 for a
negative response to this interrogation simply amount to a delay
circuit to prevent the program from proceeding until a period of
~' time, arbitrarily selected to be two seconds, has passed since
the la~t time that the time register was incremented in
accordance with block 114. After each such increment, a two
second timer is restarted and it runs while the computer program
proceeds through its next sequence of step~. At the end of the
-30-
, ~ .
47t~02
sequence, the program returns to block 130 and the computer is
held in the delay loop until two seconds have passed. The time
register is then incrementally increased and the computer
proceeds to its next sequence of steps.
The program then continues through the previously described
loop. The interrogation of block 116 is asked and answered in
the same manner as previously described and, since the maximum
allowable time has not yet been reached, the program moves
directly to block 120. When the interrogation of block 120 is
asked, the answer will be in the negative since this is the
second time through this sequence. At this point, the program
directs the computer through location 1 in Figure S to location 1
in Figure 6 and thus into block 134.
This instruction, namely, to read the voltage and put in
"Tempsum", operates the analog-to-digital converter as previously
de w ribed in connection with Figure 2 and stores the resultant
digital statement of the battery voltage in a storage register
in the microprocessor. This register is referred to as "Tempsum".
The program sequence next proceeds to block 136 where the
descriptive step is stated as "Calculate Difference = Tempsum-Kl".
This is followed immediately by block 138 which inquires
whether the difference is negative. If the difference is either
0 or greater than 0, the answer is no and the computer is direct-
ed by step 139 to stop charging. This represents a sequence of
steps which would be the same as that stated above with regard to
block 118. If the difference is negative, then the answer is yes
and the program proceeds to block 140.
In fact, the combination of steps 134, 136, and 138 is a
test for an excessively high level of battery voltage. Thus, K
is preset at a value which, for the particular battery being
charged, represents an excessiveIy high level of voltage, which
could only be reached by a defective battery. Accordingly, if
-31-
11478~2
the value of voltage in the register "Tempsum" equals or exceeds
Kl, the battery must necessarily be defective, or some portion of
the charger is defective, and the charging sequence must be
stopped immediately. Yor example, Kl may equal 2 volts per
cell for a nickel-cadmium battery. In normal charging, the
battery voltage will never equal Kl, and the answer to the
interrogation of step 138 will be affirmative so that the
program proceeds normally to step 140.
In step 128, the register "Max. Voltage Sum" was set to an
initial large negative number. In Step 140, the value in "Tempsum"
and the value in "Max. Voltage Sum" are compared. If the value
in "Tempsum" is greater than that in "Max. Voltage Sum", then
the value in "Tempsum" is placed in the "Max. Voltage Sum"
register by instruction 142 and the program proceeds to step 144.
If not, then the "Max. Voltage Sum" register value is left un-
changed and the program proceeds directly to step 144.
In step 144, the difference between the values used in
"Tempsum" and in "Max. Voltage Sum" are compared to another
constant, K2, which is preset according to the battery being
charged. In fact, the test being performed by the series of
program steps 140, 142 and 144 is that of checking to see if the
voltage has moved downwardly by more than a given minimum amount
from a previously achieved maximum value. As described above
in the section entitled Absolute Voltage Change Analysis, if
this has occurred, this must indicate that the battery has al-
ready passed its maximum charge level and is in the region
indicated as Region V in Figure 1, or that the battery is
defective. Accordingly, the program is instructed to move to
block 145 which stops the charging process in the same manner as
steps 118 and 139.
If this is not the case; that i5, if the latest value of
battery voltage present in "Tempsum" is either e~ual to or
-32-
114780X
greater than the largest value previously recorded, then it is
known that the battery is somewhere in Regions I-IV and
charging can safely continue.
It should be noted that the value K~ iS a small number.
Its purpose is to prevent spurious or transient errors caused
by drift in the electronic circuit values, or small negative
changes in the battery voltage, etc., from shutting down the
charging sequence. It is al~o noted that this test is preferably
performed even during the initial period identified as Region I
of Figure l wherein the battery voltage is varying in a somewhat
undetermined manner. This is because a negative change in
battery voltage which exceeds K2 even in this Region is also
indicative of a defective battery. K2 may equal 25 millivolts
per cell for nickel-cadmium batteries.
The next stage in the process, identified as b}ock 146,
interrogates the timing system to determine whether a slope
calculation should be done. This actually represents the begin-
ning of the inflection point analysis previously described; as
will be clear from the following description of ~igures 5 and 6,
the phrase "Slope Calculation" used in this program identifies
the series of steps which locate the inflection points in the
curve of Figure 1.
As indicated in step 146, the slope calculation is performed
every minute beginning at an arbitrary time identified as K3
seconds. K3 is the time interval chosen to allow the battery to
pass through the initial stage identified previously as Region I
and i3 uBually between 30 and 60 seconds. K3 is preferably 40
seconds in the case of nickel-cadmium batteries.
The first several times through the program, the interro-
gation of step 146 will be answered in the negative and, asindicated, the program returns to step 130. Thus, until the
total time registers equal the value K3, the program simply
-33-
li47802
directs the computer to monitor the time and voltage to make
sure that nelther assigned maximum has been exceeded, these
checks being performed at steps 116 and 134-138 respectively,
and also monitors the voltage for a negative drop in steps
140-144. Once the total time register reaches K3, the interro-
gation of step 146 is answered in the affirmative and the
program passes through connection point 2 and enters the
series of steps shown in Figure 7.
In Figure 7, the program continues with step 148 which
refers two additional register locations in the microcomputer.
One is called "Sum" and the other is "Oldsum". In step 148, the
contents of the register "Sum" are moved into the register loca-
tion "Oldsum" and the previous contents of the register "Oldsum"
are cancelled. In ~lock 150, the contents of the latest readings
in "Tempsum" are transferred into the register location "Sum".
The sequence then moves to block 152 where a test is made to see
if the time i9 equal to K3 seconds. If it is, the program
returns through Loop 2, step 130. Thus, the first entry into the
steps of Figure 7 at T=K3 simply sets a voltage reading in the
"Sum" register which will later be transferred into "Oldsum".
Calculation of a slope requires at least two points on the line
and therefore the first calculation can only be done when the time
equals 1 minute plu8 K3 when the previous voltage value is present
for comparison to the new value. Of course, this is really an
approximation of the slope rather than an accurate determination.
Accordingly, if the time elapsed equals X3 seconds, the
sequence goes back to loop 2, block 130 and continues for another
minute. Subsequently, when the time equals K3 plus any integral
number of minutes, the sequence goes on to block 154 where the
difference in value between the "Sum" register and the "Oldsum"
register is calculated and put into a register location called
"Slope". The sequence then continues to block 156.
-34-
~i47B(~2
In step 156, the register "Min. Slope" which was set to an
initial large value in step 124 is used. Specifically, the value
in "Slope" is subtracted from the value in ~'Min. Slope" and the
result tested to see if it is greater than or equal to 0. If the
"Slope" register is less than the previous "Minimum Slope"
register, which had been initialized to a very large number, the
"Slope" value is put into the "Minimum Slope" register. Thus,
once per minute, each time through this program sequence, a
slope is calculated and a check is done to see if the new value
of ~lope i9 less than the previous lowest slope reading. If it
is, this new slope is put into the "Minimum Slope" register in
block 158 and the sequence continues to block 160. If the
newest slope is not less than the minimum slope, the sequence
also goes to block 160.
Here, the slope reading just taken is subtracted from the
"Max. Slope" register which was initialized at block l26 to a
very small number. If this difference i8 less than 0, meaning
that the new value in the "Slope" register is greater than the
previous value in the "Max. Slope" register, then this slope
value is put into the "Max. Slope" register and replaces the old
contents. This is done in block 162.
Next, the sequence flows to block 164 where a test is
done to see if the flag F0, which was cleared in step 122, is
~et. Up to this time, it has not, so the sequence will proceed
through connection point 3 to block 166. At block 166, a test is
done to see if the latest slope value is greater than the minimum
slope by a preselected increment, K4. The value of X4 is select-
ed to define some minimum value of positive change which must
occur, to avoid transient effects, before the system is allowed
to recognize that the slope has stopped decreasing and is now
increasing. In the case of nickel-cadmium batteries, K4 may be
15 millivolts per minute per cell. Once this occurs, an
~147802
inflection point will have been identified by approximation.
If the slope value has not increased over the "Minimum
Slope" value by this necessary increment, the sequence returns to
block 130 which is the loop 2 return. This means that the slope
is either continuing to become less or if it is increasing, it
has not increased sufficiently. If the latest slope is greater
than "Min. Slope" by K4, meaning that inflection point has been
passed (or that the sign of the second derivative has changed),
the sequence flows to block 168 where flag F0 is complemented or
set. This means, referring to Figure 1, that the transition into
Region III has been made and that the charge cycle is well along
toward completion. From block 168 the sequence also continues
back to block 130 to continue the process as previously described.
At this point, although it is not shown in the flow chart,
lt is usually preferable to replace the value in the "Max. Slope"
register with the value in the "Slope" register. This insures
that additional slope values after the fir~t inflection point
will be compared to the actual slope at the first inflection
point and not to an earlier value which may have been carried
because it was slightly larger than the inflection point value.
Eventually, the process will continue through sufficient
cycles so that it will arrive again at step 164. Now, the
response to this interrogation will be "yes" and the program will
proceed through the connection point "FURTHR" into Figure 9.
There, the sequence continues to block 170 where the slope value
is tested to see if it is less than the value in register "Max.
Slope" by an increment K5 which may be approximately the same in
value as K4. This is the test for the Region III-to-Region IV
transition shown in Figure 1. If the slope is less than "Max.
Slope" by K5, then the charge cycle has reached this second
inflection point and the charge cycle is complete. The sequence
then goes to block 172 and, the charging process is terminated
-36-
~1478C~2
in the same manner as described in regard to step 118. If, how-
ever, the latest slope is not less than the "Max. Slope" by a
sufficient increment, then the sequence returns to block 130 and
continues until one of the four charge method analyses described
above causes the charging sequence to stop.
In this way, this flow of operations takes the apparatus
through the methods of analysis described above, testing at
appropriate time intervals for time analysis of excessive total
time elapsed, for excessively high voltage on the cell or battery,
indicating possible damage, for a drop in voltage from one period
to another of sufficient magnitude indicating that the cell or
battéry is in Region V or for the sequence of second derivative
tests indicating that the cell or battery has gone through the
transition from Regions III to IV, as described in Figure 1 in
the change of sign of second derivative test.
VOLTAGE PROFILE ANALYSIS
The present invention, as thus far described, has been
directed to the profile of voltage change with time which occurs
in a battery when the charging system used is of the type
génerally known as a "constant current" charger. This type of
voltage change is actually obtainable in several different ways.
First, it may be obtained by applying a steady unchanging charg-
ing current to the battery and measuring the change of voltage
with time. In this method, the charger power supply and current
amplifier may be chosen to provide a predetermined aurrent level
at any battery voltage between zero and a value slightly in
excess of the voltage of the battery at full charge. The
current level is chosen on the basis of factors such as the charge
efficiency, the cost of the power supply and amplifier, and the
desired time to fully charge a totally discharged battery. In
general, in nickel-cadmium batteries of the C size or sub-C size,
the current applied is about three times the C-rate of the
~1478CP2
battery. The C-rate of a battery is a current in amperes which
is numerically equal to its ampere-hour capacity. A "3C"
current would bring a battery to full charge in about 20 minutes.
In other cases, charging rates such as C or 5C may be
selected; these would fully charge a discharged battery in about
one hour or in about 12 minutes, respectively.
A second method of obtaining the voltage profile of Fig. 1
is by applying the charging current in pulses and measuring the
rest voltage of the battery when the current is zero. This is
known as trough voltage sensing. In a sense, the voltage measure-
ments are taken at a "constant" current level of zero amps. The
profile of voltage with time will correspond in form, although
not in scale, to that shown in Figure 1 and exactly the same
method of analysis as described above may be applied.
A third method of obtaining this same profile is to apply
a current which may vary cyclically but which has a constant
average value. If the measured voltage is averaged over a
similar time period, thus compensating for the cyclic variations
in çurrent, the voltage profile obtained is exactly the same in
form as that shown in Figure 1, and again, the same method of
analysis may be applied.
A fourth method of obtaining the same profile is to allow
the current to vary but to measure the voltage only at the
time when the current equals some preselected constant level;
again, this produces the same results as the other methods just
described.
In all of these instances, the voltage profile for a
given battery will assume the same general form. Since the novel
method of analysis described in this specification is a function
only ~f the form of the profile and not of its actual value,
this method may be applied to any o~ these charging techniques.
For convenience, all of these methods are commonly referred to
-38-
~47~02
by the term "voltage profile".
APPLICATI~N ~F VOLTAGE PROFILE AN~LYSIS TO
OTHER B;ATTERY COUPLES
Figures 10-13 illustrate a variety of voltage profiles for
particular examples of several different types of batteries, all
of which have been developed using the "constant current" method
referred to above. Specifically, Figure 10 is a representative
profile obtained in the case of a nickel-iron battery. It will
be noted that the general appearance of this curve is similar to
that of Figure 1 and in particular, similar inflection points
occur at A' and B' as the battery approaches full charge. Thus,
exactly the ~ame technique can be applied to the nickel-iron
battery as has been described for the nickel-cadmium. The only
differences are that the constants must be selected in accordance
with the needs of the particular battery, considering its internal
construction and the level of current which it can accept, the
number of cells and the corresponding maximum voltage; and the
maximum time or maximum voltage which can be accepted without
damage, Also, the small scale of the changes in the voltage
profile require the system of voltage measurement to havs a
higher resolution than is true in the case of a nickel-cadmium
battery. In principle, however, the method of charging is
substantially identical.
Figure 11 illustrates the charging curve of a representative
lead acid battery, Once again, it can be seen that the five
; Regions as described in connection with Figure 1 are repeated
in the case of the typical lead acid profile and similar in-
flection points A" and B" occur. The only differences are that
;~ the overall change of voltage is larger and the rate of change
in Region III is greater. However, since the Regions are the
same and the sequence of inflection points is the same, essential-
ly the same method as described in connection with nickel-cadmium
-39-
batteries and nickel iron batteries can again be used for lead
acid batteries.
However, it has been found that full (100%) charging of a
lead acid battery can be better obtained by the addltional appli-
cation of a surcharge after the second inflection point has been
reached. This is due to the internal chemistry of the lead acid
battery which causes the final addition of energy to occur at a
slower rate than in the case of a nickel-cadmium battery. There-
fore, the optimum charge method for lead acid batteries is to
apply the inflection point method of analysis as previoulsy
described, and, when the second inflection point between Regions
III and IV is identified, the microcomputer is instructed to
shift the charging rate to an intermediate level. This inter-
mediate rate is then applied for a fixed period of time.
In general, lead acid batteries have a structure which
permit the constant current to be about C or 2C in the fast
charge mode. The surcharge rate selected is generally about
one-half of the full charge rate. The fixed period of time is
calculated by determining how long it takes to add 25% of the
full battery capacity to the battery at the suxcharge rate. At
the end of that time, the battery charger automatically terminates
the full charge mode and begins a maintenance mode cycle which
simply compensates for self-discharge.
Figure 12 illustrates the charging curve for a lithium
battery having an iron sulfide electrode. In this case, the
inflection points occur much earlier in the charge cycle and
there are almost no distinguishing features of the voltage
profile after the second inflection point. Because of this
voltage profile, it would be extremely difficult to provide a
reliable fast charger for such a battery using only prior art
techniques. In accordance with the present invention, the
inflection points can be determined very precisely. This in-
-40-
~i~'7802
dicates that the battery is approximately at 45% of capacity.
Accordingly, a charging program for a lithium battery of this
type may use the same system for determining inflection points as
has been described above, coupled with a timing sequence. When a
battery is attached to the charger, a timer is started and it is
set to discontinue the full charge rate when enough time has pass-
ed to add approximately 55% of the total battery capacity to the
battery. If no inflection points are encountered during this
period, the timer alone shuts off the system at the end of the
period. This accommodates a battery which may be placed on charge
although it already has a reasonably full charge.
However, if the inflection points are encountered before the
time has expired, then the timer is simply restarted. This ensures
that a battery which was discharged, or only partially charged,
initially will receive its full charge.
Figure 13 illustrates still another variation of voltage
profile, namely, that for a silver cadmium battery. In this
instance, simple determination of two consecuti.ve inflection
points is not sufficient; addition of energy to a battery which
ic fully discharged should produce four consecutive inflection
points before full charge is reached.
In order to fully charge this battery, another combination
of the inflection point analysis method with the alternative
charge termination modes previously described will fully charge
this battery. Specifically, the charger is arranged to seek the
four consecutive inflection points which indicate that the
battery being charged has gone through its entire cycle from fully
diccharged to fully charged; if this occurs, the charger termi-
nates the application of the fast rate charge current. However,
30 this termination mode alone is not sufficient. In addition, the
system is instructed to compare the to-tal voltage to some pre-
selected value after each inflection point is measured. If the
-41-
~7~
voltage is above the preselected level when an inflection point
is reached, it will then be known that the battery was not fully
discharged when the charge program was started and that the
battery is now fully charged. Accordingly, the application of
the full rate current is discontinued. Thus, the system accommo- -
dates both batteries which are placed on charge while already
either fully or partially charged and also batteries which are
fully discharged; in both cases, the charger brings the battery
precisely to its full charge capacity without the harmful effects
of prior art charging techniques.
Of course, in devising the method and system for each of
the batteries mentioned in connection with Figures 10-13, the
additional safeguards to prevent serious overcharge and to shut
the system off in the event that either the battery or the
charger is defective are also included; thus, a maximum total
time limit, a maximum voltage limit, a negative change in volt-
age, and a negative slope limit may all be included as appropriate.
CURRENT PROFILE ANALYSIS
The description of this invention as set forth above has
been given in terms of the battery analysis method which applies
when the state of charge of the battery is measured under "con-
stant current" conditions. In addition, it is possible to charge
the battery in a "constant voltage" mode, to measure the change
in current with the passage of time, and to apply similar methods
of inflection point analysis to the resultant profile of changing
current with time. This technique involves the selection of a
constant voltage to be applied to the battery by the charger; the
voltage chosen is selected so that the current which it applies
to the battery during the bulk of the charge time is reasonable
on the basis of the same parameters as described in the case of
the constant current charger, namely, the charge efficiency, the
cost, and the time required to fully charge a discharged battery.
-42-
~7e~2
Once again, this application of constant voltage produces a
known and predictable form for the curve traced by the change
in current with time.
Actually, the term "constant voltage" is applied equally to
systems in which the actual applied voltage is constant through-
out the charge period, to systems in which the current is always
measured when the voltage is at a preselected value, or to
systems in which a pulsating applied voltage has a constant
average and in which the measured current is correspondingly
averaged. All of these systems produce a curve of current
against time which has the same general form and which may be
treated by mean~ of the same inflection point analysis; accord-
ingly, this profile is referred to herein as the "current profile".
In the particular case of a nickel-cadmium battery, the
current profile is illustrated in Figure 14. In fact, this
curve is exactly the same in form as that shown in Figure l ex-
cept that the entire curve is inverted. Thus, the method of in-
flection point analysi~ as applied to this profile is exactly the
~ame as has been described in connection with Figure 1 except
that all of the pertinent analyses regarding signs, direction of
change, etc. are reversed. Initially, the current decreases in
a manner corresponding to that in Region I in which the voltage
of Figure 1 increased. This is followed by an interval in which
the current decreases slowly; this is normally the longest time
interval and the one in which the major increase occurs in the
energy stored in the battery. This corresponds to the increasing
voltage of Region II of Figure 1.
The inflection point which must be identified between this
interval and the next Region of sharply decreasing current occurs
at the same point in time as point A in Figure 1. However, it
identifies a change in the sign of the second derivative of
current from positive to negative whereas Point A in Figure 1
-43-
-
ii4l7~2
identified a change in the sign of the second derivative of
voltage from negative to positive. Similarly, the inflection
point between Regions III and IV is now identified as that at
which the second derivative changes from negative to positive
whereas in Figure 1, the change was from positive to negative.
Thus, the entire description of the method of inflection
point analysis as applied in connection with Figures 1-9 can be
converted to a method of inflection point analysis for the
constant voltage case by changing the word "voltage" to "current"
and by reversing all words such as "increasing", "decreasing"~
"positive", "negative", etc.
Similarly, with regard to Figures 10-13, the particular
batteries identified there can be charged by the constant voltage
technique. In each case, the general method of inflection point
analy~is as set forth in the specification exactly corresponds to
that which has already been described.
R~TE OF CHARGING
A primary benefit of the present invention is that any
normal battery, that is, any battery which is not defective, can
be charged at a relatively high rate. In using previously known
battery charging methods, it has been necessary to limit the
application of high rate charging currents to batteries which
are especially adapted to accommodate the inadequate shut-off
modes in use. This is due to the fact that previous methods
cannot stop the fast charge current at the proper moment and the
various harmful effects previously noted can occur. Only
batteries designed to withstand these effects can be used and
even such batteries experience shortened lives, etc.
In contrast, the method of the present invention provides
such precise control over the application of energy to the
battery that it can be used to fast charge even those batteries
which were previously intended for charging only by slow rate
~47~2
methods.
The term "trickle charge" usually refers to a charge rate
such that the battery receives its full charge only over a period
of 12 to 24 hours. Thus, typical trickle chargers apply a
current of between 0.05C and O.lC. In accordance with previous
methods, the terms "fast charge"or ~'quick charge" are generally
applied to rates in excess of 0.2C; that is, charge rates which
would charge a battery in less than 5 hours.
All batteries accept currents of the "fast charge" level
for limited periods of time. The upper limit for a particular
battery is governed by the current-acceptance capability of the
battery; that is, of its internal and external connections, and
of its internal plates, and also by its internal ion transit time.
~his level is generally given by the manufacturer. For example,
sub-C size nickel-cadmium batteries available from General
Electric can accept fast charge current at the 4C rate; lead acid
batteries of the sealed type available from Gates Energy Products,
Inc. can accept fast charge current at the 0.3C rate.
Even though batteries could accept such fast charge currents,
pre9ently known chargers are not capable of shutting off the
fast charge current at the proper moment and even batteries
which structurally could accept fast rate currents can only be
charged at the trickle charge rate. In general, any charge rate
above the 5 hour rate (0.2C) has previously required a special
battery design.
Because of the accuracy with which the present invention
determines the full charge level, the present charging method
permits the use of fast charge currents with many batteries which
; could previously be charged only by slow, trickle charge rates.
This is particularly true in the categories of nickel-cadmium
batteries and lead acid batteries which predominate among the
rechargeable battery couples presently available.
Thus, the present method permits essentially all of nickel-
cadmium batteries presently in use by consumers to be recharged
in a time on the order of 1 hour. Lead acid batteries of the gel
type can be charged in a time on the order of 2 hours; those of
the liquid type can be even more quickly charged.
In general terms, the present invention permits the appli-
cation of a high rate; that is, a rate in excess of a . 2C and
up to the rated current acceptance level of the battery; normal
batteries so charged by the system of the present invention will
receive a full charge and will not be damaged.
FINISH MODE
In the case of nickel-cadmium batteries, the inflection
point analysis described above brings a battery to essentially
100% charge. Thus, when the second inflection point has been
reached, the charger can shift into a maintenance mode in which
short pulses of high rate charging current are applied periodi-
cally to compensate for self-discharge. For example, a lC current
may be applied for 15 seconds every 6 hours. Other maintenance
cycles might be used if desired.
In actual practice, repetitive charging of the battery to
exactly the second inflection point may cause minute reversible
degradation because this point occurs a small fraction of a per-
centage point below 100% charge. This degradation may be reversed
when the battery is left on maintenance or when the operator,
occasionally, places the battery on charge even though it is not
discharged. This drives the voltage slightly into Region V of
Figure 1 so that cut-off occurs in accordance with block 145 of
Figure 6 which reverses the degradation.
To completely prevent even the possibility of such degra-
dation, a surcharge current of 0.lC can be applied for a few
hours after the second inflection point has been reached. The
above-described maintenance cycle may then begin.
-46-
i:l47t~32
In the case of lead acid batteries, as has previously been
discussed, an interval of low rate charging may be useful to
completely charge the battery; thereafter, an appropriate main-
tenance mode is used to compensate for self-discharge. In other
battery couples, other finishing techniques may be utilized as
appropriate.
SUMMARY
The foregoing specification describes a battery charging
method which basically utilizes the inflection point analysis
method to identify very precisely significant points in the
variation of the electrochemical energy in a battery during its
charge cycle. Accordingly, the appended claims are broadly
directed to this method and are intended to include all vari-
ations of this method as may be obvious to those skilled in the
art.
Among the many possible variations, it should be noted that
the above apparatus particularly described has made use of an
approximation technique for determining the occurrence of an
inflection point. It is, of course, fully within the contempla-
tion of this invention to use this or other approximationtechniques for locating critical points in a profile, or to
provide a circuit which is capable of directly monitoring the
second derivative for a change in sign. Similar variations may
al~o be used with regard to other parametric profiles.
Another set of variations comprises the particular battery
characteristic selected for analysis. While the present de-
scription has been directed particularly to the voltage or
current, profiles of other characteristics, particularly electri-
cal characteristics might also be analyzed. It is noted that
~0 this profile may also vary with other battery conditions; in
fact, as previously described, the analysis of this invention
partially depends on the fact that other battery conditions
-47-
~1~7~2
affect the profile.
In addition to the extremely precise method of inflection
point analysis as hereinbefore described, the present invention
also encompasses the analysis for other critical points in the
profile of variation with time of a characteristic of the
battery which changes with the energy level stored in the battery.
In addition, therefore, to inflection point analysis, the present
invention is also in part directed to improvements in method and
apparatus for charging batteries which relate to detailed
analyses of the profile of battery characteristics, the analyses
involving combinations of such factors as limiting value, slope,
and passage of time. By analyzing the profile of the particular
characteristic for the battery under charge, particular combi-
nations of these events may be identified and used by those
familiar with batteries and the art of battery charging to
provide improved techniques of fast battery charging without
departing rom the spirit of the present invention.
In addition, the present invention presents numerous
subcombinations of this method which have not previously been
knowns the many variations of these combinations which will
readily occur to those familiar with the battery and battery
charging art are also intended to be included.
Particular emphasis has also been placed on the charging of
nickel-cadmium batteries and lead acid batteries in view of the
importance of these couples. The specific methods perfected for
charging such batteries are also fully within the contemplation
of the present invention.
Finally, a specific apparatus has been disclosed for
performing the method of this invention. A great many obvious
variations of this appaxatus will be readily apparent which
correspond generally to the alternative methods described. It
is fully intended that the apparatus claims in this application
-48-
~147t~Z
be extended to cover all such alternative embodiments of this
basic apparatus.
-49-
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