Method and an apparatus for charging a rechargeable battery

A rechargeable battery is charged by connecting the terminals of the battery to an electrical power source. In order to avoid overcharging and undue temperature increase in the battery cell, the course of a least one charging parameter, such as the increase rate of voltage, is surveyed during at least part of the process of charging the battery. This charging parameter surveyed is compared with a number of reference parameter courses representing idealistic or desireable processes of charging the battery for different starting states of charge of the battery. Such comparisons may for example be made by means of a microprocessor, which may also select the reference course with a starting state of charge similar to the actual starting state of charge of the battery. Thereafter the process of charging the battery may be controlled so as to approximate the course of said charging parameter to the selected reference course. The charging voltage may be limited to a maximum value (Vmax). When such value has been reached the charging process may be terminated after a certain predetermined time period being one of the reference values.

The invention concerns a method of charging a rechargeable battery, wherein 
an electrical source of energy is connected to the battery. One or more 
characteristic parameters of the charging process are currently measured 
and optionally calculated during charging, and these are compared with 
reference values. The invention also concerns an apparatus for performing 
the method. 
When charging a rechargeable battery, such as for example an NiCd battery, 
an electrical voltage greater than the terminal voltage of the battery is 
applied to the terminals of the battery, whereby a current will run 
through the battery. This current initiates a chemical process by which 
energy is stored in the battery. 
When the battery has reached a full charge condition, the chemical process 
stops, and the added energy will instead be converted into heat. Since the 
battery is constructed as a sealed container, the pressure in the battery 
increases, which causes chemical destruction. This means that the capacity 
of the battery is reduced, and the capacity may eventually have been 
reduced significantly after several such chargings. For the battery to be 
utilized in the best possible manner it is therefore important partly that 
the battery will be charged fully partly that charging is interrupted 
before the generation of heat becomes too great. The problem is thus to 
interrupt charging as precisely as possible at the proper time. 
Frequently, the charging period for a battery is desired to be as brief as 
possible, which has led to the use of greater and greater charging 
currents, and since this accelerates the heat generating process 
additionally, it is even more important to interrupt charging at the 
proper time. 
In a typical charging sequence the voltage across the battery increases 
evenly as the battery is charged. As the battery approaches its full state 
of charge, the voltage increases more steeply to a peak marking the full 
state of charge. The voltage then drops again owing to the increase in 
temperature because the temperature coefficient of the voltage is 
negative. Correspondingly, the charging current typically falls to a 
minimum at full charge and then increases. 
The art includes some methods which attempt to ensure optimum charging by 
cutting off charging at the proper time. However, they have been found to 
be lacking in precision. If charging is interrupted too late, the result 
will be heat generation and mechanical destruction, as mentioned, and if 
charging is cut-off too early, the battery will be undercharged. 
One of the known methods comprises measuring the temperature of the battery 
and then cutting-off charging when an increase in temperature is observed. 
However, it is frequently too late when the increase in temperature is so 
great that it can be measured, and it is moreover difficult to measure the 
temperature sufficiently accurately, one reason being the possible 
variations in ambient temperature. This will, for example, be the case if 
a battery from an automobile telephone is moved in winter from a cold car 
to a charger which is present at room temperature. 
Another know method comprises measuring the voltage across the battery and 
cutting off charging when the voltage assumes a determined level. However, 
this voltage often varies somewhat from battery to battery, even in case 
of batteries of the same type, and it is moreover temperature dependent so 
that it is very difficult to determine the voltage at which charging is to 
be cut-off. 
Similarly, it is possible to measure the charging current, and the same 
observations apply here as well. 
Many known chargers rely on fixed periods of time so that charging is 
simply cut-off after the elapse of this time. This, however, is a very 
inexpedient method because it is not known in advance whether the battery 
is completely or only partially discharged, and the necessary charging 
time depends strongly upon this. This might be solved by discharging the 
battery fully prior to charging; but in addition to the waste of energy 
involved, it takes a certain time, and there will still be a good deal of 
difference between the necessary charging time from battery to battery. 
A more advanced method is to measure the voltage change (or current change) 
as a function of time, i.e. the slope of a curve showing the voltage as a 
function of time. For example, U.S. Pat. No. 4,052,656 discloses a method 
which finds the point at which the slope is zero, corresponding to the 
peak where the battery is fully charged; however, it is difficult to 
determine the point accurately since the curve may be very flat here, and 
another drawback is that there may be other points on the curve where the 
slope is zero so that charging is cut-off too early. 
In U.S. Pat. No. 4,747,854 it is detected correspondingly when the voltage 
curve assumes a negative slope exceeding a reference value. The 
observations just made also apply here; however, already at this time the 
battery may have been overcharged to a certain degree, which can damage 
the battery. 
Also U.S. Pat. No. 4,388,582 measures the slope of the voltage curve to 
find the point where the slope of the curve changes from increase to 
decrease. This is a more reliable method since the battery will rarely be 
overcharged; on the other hand, however, the location of the point in 
question may vary greatly, and charging will typically be terminated much 
too early so that the battery will only be charged to part of its full 
capacity. Further, it involves a risk of wrong measurements if, for 
example, the charging current or the voltage supply is changed during 
charging. 
It is also known to use a combination of some of the above-mentioned 
methods. Thus, for example, U.S. Pat. No. 4,639,655 relies on four stop 
criteria, viz. a voltage limit, a predetermined time limit, a calculated 
increase on the voltage curve as well as the point where the slope of the 
voltage curve is zero. Charging is interrupted if just one of these 
criteria is satisfied. The mentioned time limit is selected after charging 
has been started, an initial voltage measurement being made, and on the 
basis of this a short or a long charge time is selected, for example 1 
hour or 1.75 hours. The advantage is that some regard can be had to the 
battery discharge state from the beginning as well as to the number of 
cells in the battery; but it is still a rather imprecise method which 
involves a risk of battery overcharging. 
The invention provides a method where charging of the battery can be 
terminated at the optimum time where the battery has been fully charged 
without any risk of overcharging and thereby mechanically destroying the 
battery. 
This is achieved according to the invention in that in response to 
comparison between the measured or calculated parameters and known 
reference values it is possible at any time to determine a remaining 
charge time and thereby a possible stop point of time for the charging 
process following which charging can be cut-off in response to these stop 
points of time. 
Characteristic parameters include, for example, the voltage across the 
battery or the charging current. Experience shows that with respect to 
curves showing these parameters as a function of time, there is great 
correlation between the momentary appearance of the curves and the 
distance to the point of time where it is optimum to terminate charging. 
Having measured the momentary appearance of the curve, it can thus be 
determined relatively easily by comparison with the reference values how 
long the battery is still to be charged. 
Typically, charging will be terminated when the first of the generated stop 
points of time occurs; however, also more sophisticated solutions are 
conceivable, for example where more importance is attached to the stop 
points of time last calculated. Thus, a stop point of time may optionally 
be ignored if later calculations show that it was wrong. 
It is particularly expedient to find the rate of change in the parameters 
as a function of time, corresponding to the slope of the mentioned curves, 
and this may be done by storing the measurement values so that at a given 
time the actual value may be compared with a previous measurement value, 
whereby the rate of change may be calculated. 
In a particular embodiment there is just a limited number of reference 
values, and a new stop point of time for the charging process is 
determined only when the parameter or parameters concerned assume one of 
the reference values. This results in a simpler procedure which can 
nevertheless normally determine the optimum stop point of time 
sufficiently accurately. 
When the parameter being measured is the voltage across the connection 
terminals of the battery, a more exact measurement is obtained if the 
charging current to the battery is cut-off for a short period before the 
voltage is measured. The reason is that the battery has an internal series 
resistance, and the charging current provides a voltage drop across this 
resistance which should not be included in the voltage measurement. 
In particular in case of fast charge mode using a high charging current it 
may be advantageous to reduce the charging current gradually as the stop 
point of time approaches, because it will then be easier to find the 
optimum stop point of time. Thus, charging may, for example, be performed 
with a constant high charging current until one of the measured parameters 
has reached a determined level, following which the current can be reduced 
gradually. 
An expedient manner of obtaining the desired charging current is to use a 
constant voltage source which is pulse width modulated in a manner 
providing the desired charging current. 
It may often be an advantage that the procedure of determining the possible 
stop point of time for the charging process is not initiated until the 
charging process approaches its termination. Thus, a simpler method may be 
used, such as simple measurement of current or voltage, for deciding when 
the more accurate procedure is to be initiated. 
In a particular embodiment the accuracy of the measurements is improved in 
that the measurement values of the characteristic parameters for each of 
the mentioned points of time are an average of a plurality of intermediate 
measurements. The advantage is that the measurements will be less 
sensitive to transients, for example. Of course, the same effect can be 
obtained by integrating the parameter in question over the period which 
has elapsed since the last measurement. 
It may be an advantage to adopt some of the stop criteria which are used in 
the prior art as an additional safeguard. Thus, for example, a maximum 
charge period may be fixed. Charging will then be interrupted at this 
point of time at the latest even though the other stop criteria have not 
yet occurred. It is also possible to fix limits for one or more of the 
measured parameters so that charging is terminated if one of the 
parameters exceeds or falls below specific values. 
After the termination of charging it may be expedient to maintain the 
charge state of the battery by means of a pulsating current. This ensures 
that the battery is constantly fully charged even if it is not removed 
from the charger till long after the termination of the charging. 
It may likewise be expedient to apply a voltage to the battery briefly 
before the actual charging is initiated. By thus measuring the 
characteristic parameters it may be decided whether a battery of correct 
type and without errors is fitted in the charger. If this is not the case 
no charging takes place. Thus, this obviates charging wrong battery types 
or defective batteries where, for example, a cell is turned wrongly. 
An apparatus for charging a rechargeable battery according to the described 
method may comprise an electrical source of energy, a measurement device 
capable of measuring one or more of the said characteristic parameters and 
supplying the measurement results via an analog/digital converter to a 
control unit capable of calculating other characteristic parameters and 
controlling the source of energy, as well as a storage circuit for storing 
measurement values, calculated values and reference values. Further, 
during charging the apparatus can compare the characteristic parameters 
with reference values by means of the control unit. 
The desired effect is obtained in that in response to said comparison the 
apparatus determines a possible charging process stop point of time for 
each or some of the said points of time, and that it can terminate 
charging in response to the stop points of time thus produced. 
A particularly expedient embodiment of the apparatus is adapted to compare 
actual measurement values for the characteristic parameters with previous 
measurement values of the same parameters for each of the said points of 
time to thereby determine the rate of change in the parameters as a 
function of time, corresponding to the slope on the curve showing the 
parameter concerned as a function of time. The said stop points of time 
are determined by comparing the rate of change with reference values. 
If the apparatus measures the voltage across the connection terminals of 
the battery, it will be an advantage that the apparatus can cut-off the 
charging current to the battery for a short period before the voltage is 
measured, it being thereby possible to disregard the voltage drop 
occurring across the internal resistance of the battery. 
The apparatus can advantageously be constructed such that the charging 
current to the battery can be controlled by pulse width modulating a 
constant voltage, the pulse width being controlled by the control unit of 
the apparatus in response to the measurement results received. 
As an additional safeguard the apparatus may be adapted to cut-off charging 
if other and more simple stop criteria occur. Charging may, for example, 
be cut-off if a determined maximum charge period is exceeded, or if one of 
the measured parameters exceeds or falls below some predetermined values. 
In a expedient embodiment the apparatus is moreover adapted to maintain the 
charge state of the battery after the termination of charging by means of 
a pulsating current. It is ensured hereby that the battery is still fully 
charged even though it is not removed from the apparatus till long after 
the termination of the charging proper. 
An additional embodiment is adapted to briefly apply a voltage to the 
battery prior to the commencement of the charging process and to measure 
the characteristic parameters. The charging process will then be initiated 
only if these measurements satisfy certain conditions. It is ensured 
hereby that charging will not be performed on wrong or defective 
batteries. 
According to another aspect the present invention provides a method of 
charging a rechargeable battery having a pair of terminals, said method 
comprising connecting an electrical power source to the terminals of the 
battery, surveying the course of at least one charging parameter during at 
least part of the process of charging the battery, comparing the course of 
said at least one charging parameter with a number of reference parameter 
courses representing idealistic or desireable processes of charging the 
battery for different starting states of charge of the battery, selecting 
the reference course with a starting state of charge similar to the actual 
starting state of charge of the battery, and controlling the process of 
charging the battery so as to approximate the course of said at least one 
parameter to the selected reference course. 
A smaller or greater number of empirically determined reference parameter 
courses may be stored (such as drafts where values of the reference 
parameter is plotted versus the period of time lapsed since starting of 
the charging process), for example by electronic storing means, such as a 
memory. When it is desired to rapidly charge the rechargeable battery 
without substantially deteriorating the same the idealistic or desirable 
process of charging mainly depends on the state of charge of the battery 
prior to starting the charging process. Therefore, the reference parameter 
courses stored represent idealistic or desireable processes of charging 
for different starting states of charge of the battery. If the state of 
charge of the battery to be recharged is known or may be determined, the 
reference course with the starting state of charge closest to the actual 
state of charge of the battery to be recharged may be selected, and the 
process of charging the battery may be controlled so as to approximate the 
course of said at least one parameter to the selected reference course, 
whereby it may be secured that the battery is not at any time exposed to 
unduly high voltage or charging current or to undue heating. 
In principle, the state of charge of the battery to be recharged may be 
determined by a special measuring step, and the corresponding reference 
parameter course adapted to the same or a similar starting state of charge 
may then be selected, for example by providing the relevant information to 
an electronic control unit by suitable keys. In the preferred embodiment, 
however, the relevant reference course is selected automatically by an 
electronic control circuit. 
The charging parameter may, for example, comprise the potential across the 
battery terminals, the electric charging current supplied to the battery, 
the temperature of the battery cell, the rate of change of any such 
parameter, and any combination of such parameters and/or rate of change. 
It should be understood that the charging process may be controlled in any 
suitable manner by which the course of the charging parameter may be 
approximated to the selected reference parameter course. In the preferred 
embodiment, however, the process of charging is controlled by controlling 
the voltage supplied to the terminals of the battery. The voltage is 
preferably controlled so that the charging current supplied to the battery 
is relatively low at the beginning of the charging process, while the 
charging current is preferably maintained at substantially the same 
maximum value during a subsequent major part of the charging process, so 
as to accelerate the same. 
Towards the end of the charging process the internal resistance of the 
battery cell may increase, whereby the charging voltage tends to increase 
when the charging current is to be maintained at said maximum value. A too 
high voltage may course a detrimental temperature increase within the 
battery cells. Therefore, the voltage supplied to the terminals of the 
battery is preferably limited to a predetermined maximum, the charging 
process being terminated at the expiration of a predetermined period of 
time starting when the voltage has reached said maximum. This means that 
the charging voltage is preferably kept at its maximum value during said 
predetermined period of time, and as the inner resistance of the battery 
cells is increasing the charging current will normally decrease gradually 
during this period of time, which is preferably selected so that the 
battery is substantially fully charged when the said period of time has 
expired. Preferably, the predetermined period of time is related to the 
reference course selected, which means that each reference parameter 
course includes information about not only the maximum charging voltage to 
be supplied to the battery, but also about the period of time in which 
such maximum voltage should be maintained at the end of the charging 
period. 
As mentioned above, the reference parameter courses to be compared with the 
actual parameter course may be curves or graphs, and the comparison 
process may be performed by a design recognition technique by means of 
design recognition circuitry. In the presently preferred embodiment, 
however, the charging parameter is currently measured at short time 
intervals during charging, the measured parameter values being compared 
with corresponding reference values of the reference parameter courses, 
and the relevant reference parameter course being selected on the basis of 
comparison of such measured values and reference values. The comparison 
process may be performed currently during the charging process so that the 
control circuit or control unit may shift from one reference parameter 
course to another when the continued comparison process reveals that the 
reference parameter course chosen first is not the one which is closest to 
the actual charging process. 
In comparing the charging parameter values with the reference values it may 
be advantageous to compare the rate of change of the parameter values as a 
function of the charging time lapsed with similar reference values. As an 
example, the rate of change of the charging voltage as a function of the 
charging time lapsed may be compared with the corresponding reference 
values. In order to permit the detection of the internal resistance free 
voltage of the battery the charging current may be cut off for a short 
period of time immediately prior to each measurement of the potential 
difference of the battery terminals. 
The parameter values may be measured and the rate of change of the 
parameter values may be determined at uniform first time intervals, each 
determination of the rate of change being based on parameter values 
measured at second time intervals, the second time interval being a 
multiple of the first time interval. The parameter values may be measured 
rather frequently, which means that the said first time interval may be 
relatively short, for example about 10 seconds. The rate of change is, 
however, preferably based on measurements with a time spacing being 
several times greater, for example 90 seconds. 
The determination of the rate of change may be initiated at the beginning 
of the charging process. However, the determination of the rate of change 
may advantageously be postponed until a measured value of the 
characteristic parameters exceed a predetermined value, when it is obvious 
that the best distinguishable rates of change are found after such 
predetermined value of the parameter. 
The reference parameter courses stored may comprise not only courses 
representing charging processes which are idealistic or desirable for one 
and the same type of battery, but even a plurality of reference parameter 
courses for each of two or more different types of battery. In such case 
the first process step may be to determine the type of the battery to be 
charged and to select the reference parameter courses related to that type 
of battery. Thereafter, the process may proceed as described above. 
The present invention also provides an apparatus for charging a 
rechargeable battery, said apparatus comprising connecting means for 
connecting the battery to an electrical power source, means for surveying 
the course of at least one charging parameter during at least part of the 
process of charging the battery, storing means for storing a plurality of 
reference parameter courses representing idealistic or desirable processes 
of charging the battery for different starting states of charge of the 
battery, means for comparing the course of said at least one charging 
parameter with the reference parameter courses stored by the storing means 
and for selecting the reference course with a starting state of charge 
similar to the starting state of charge of the battery, and means for 
controlling the process of charging the battery so as to approximate the 
course of said at least one parameter to the selected parameter course. 
The operation of such an apparatus may, for example, be controlled by a 
microprocessor or another electronic control circuit, which may also 
comprise a memory for storing the reference parameter courses.

FIG. 1 shows a typical charging sequence for an NiCd battery. The curve 
shows the battery voltage as a function of time with a constant charging 
current. The curve shape will be the same for all NiCd batteries but the 
specific voltage and time values may vary, for example with the actual 
charging current and from battery to battery. The curve may be divided 
into regions representing various stages in the charging process. The 
figure shows four regions which are marked A, B, C, and D, respectively. 
The region marked A constitutes the start of the charging process. When the 
charging process is initiated, the voltage may vary somewhat depending 
upon the state of charge of the battery prior to the initiation of 
charging. Since the voltage in this region is thus rather indefinite, no 
measurements proper are usually performed in this region. 
The letter B indicates the actual charging period where the charging 
current is converted into stored energy in the battery by the chemical 
process mentioned before. In this period the voltage of the battery 
increases only slowly. In the region C the battery now approaches its full 
state of charge, and the voltage begins to increase more rapidly. Oxygen 
begins to develop at the end of the period C, which results in a pressure 
increase and thereby a temperature increase in the battery. This means 
that the voltage now increases more slowly again because of its negative 
temperature coefficient. The battery voltage does not increase 
additionally at the transition between the regions C and D, and it has 
thus reached its highest value. 
If the charging process is continued in the region D, the battery voltage 
now drops because the electrical energy is now generally just converted to 
heat. The resulting increase in temperature and pressure will cause 
mechanical destruction in the battery whose capacity is thus reduced. The 
charging process should therefore be cut-off at the commencement of or in 
the beginning of the period D. 
The invention is based on the fact that it has been found by tests that, 
even though the curve may vary somewhat in response to the charging 
current used and the history of the battery in question, there is close 
correlation between various charging parameter values within the regions 
A, B and C, such as the slope of the curve at a given moment in the region 
C and the distance in time from the moment in question to the optimum stop 
point of time for the charging process. 
If the information on the correlation is stored in an electronic circuit, 
an example of which will be given below, it is thus relatively simple to 
calculate or determine how long charging should be continued on the 
battery and thereby the optimum stop point of time for the charging 
process, after having measured the slope of the curve at a given moment. 
If this calculation is performed at several consecutive moments, a 
corresponding number of proposals for the optimum stop point of time will 
thus be obtained. FIG. 2 shows an example where three measurements are 
performed. A remaining charging period of delta T1 is calculated at the 
point of time T1, a remaining charging period delta T2 is calculated at 
the point of time T2 and a remaining charging period delta T3 is 
calculated at the point of time T3. In the figure, the three calculated 
stop points of time occur at precisely the same moment. However, in 
practice the calculated stop points of time will usually be slightly 
different, with a consequent number of proposals for stop points of time. 
In the embodiment of the invention described here, it is decided to 
cut-off the charging process when the first one of the calculated stop 
points of time occurs. Since a microprocessor is incorporated in the 
apparatus described below, more sophisticated stop criteria are also 
conceivable. Thus, it will be possible, for example, to attach more 
importance to the stop points of time last calculated. It is thus 
possible, for example, to disregard some of the values calculated first if 
all the subsequent calculations gather around a specific value. 
As mentioned, FIGS. 1 and 2 show the voltage across the battery as a 
function of time when a constant charging current is used. A corresponding 
typical curve will result if the charging current is plotted as a function 
of time with a constant charging voltage, and reproducible curves showing 
the above-mentioned stages in the charging process will be obtained even 
if neither charging current nor charging voltage is kept constant. It will 
be appreciated that these curves can be used in a manner similar to what 
is described above. 
Corresponding curves of a different appearance will be obtained for other 
battery types. For some of them the correlation between the actual 
measurement point of time and the optimum remaining charging time will not 
necessarily be associated with the slope of the curve at the moment in 
question, but with other parameters for the curve, such as, for example, 
the absolute voltage at the moment concerned. 
An embodiment of the invention comprises measuring the slope of the voltage 
curve currently, for example every tenth second. For each measurement a 
remaining charging period and thereby a new proposal for a stop point of 
time are calculated. The processor can then either store this value 
together with the others, or it can incorporate it in a more sophisticated 
calculation of when the charging process is to be terminated. 
Another embodiment comprises prestoring a limited number of reference 
values for the slope of the curve. In each measurement the actual slope of 
the curve is compared with the reference values, and only when the slope 
passes one of the reference values does the processor calculate a new stop 
point of time. Calculating time for the processor is saved in this manner, 
and the result will be fully satisfactory in many situations. 
As mentioned, the curves in FIGS. 1 and 2 are provided with a constant 
charging current. However, an alternative possibility is to cut-off the 
charging current briefly each time a voltage measurement is performed. A 
quite corresponding curve is obtained in this manner, but the absolute 
voltage values will be slightly lower because the curve does not include 
the voltage drop involved by the charging current across the internal 
resistance of the battery. 
Since this internal resistance typically increases at the end of the 
charging sequence, a voltage measurement without this contribution will be 
a more accurate measure of the state of the battery. 
As mentioned before, reproducible curves will be obtained even if the 
charging current is not kept constant during the entire charging 
procedure. The principle of the invention can therefore very well be 
combined with a charging procedure where charging is initially performed 
with a constant, high current which is then reduced toward the end of the 
charging procedure. By using the lower charging current during the last 
portion of the charging process it will be possible to determine the 
optimum stop point of time more precisely, without the overall charging 
time being diminished noticeably. This may be combined with the 
performance of just a simple voltage measurement during the first portion 
of the charging process. When the voltage has reached a predetermined 
value, the charging current may be reduced, and the measurement of the 
slope of the curve may be initiated as described above. Of course, it is 
also possible to reduce the charging current at one voltage value and 
initiating the measurement of the slope of the curve at another voltage 
value. 
FIG. 3 shows typical charging curves obtained in accordance with an 
embodiment of the method according to the invention when charging a NiCd 
battery. The curve V shows the battery voltage as a function of time, when 
the voltage supplied to the battery is controlled in accordance with the 
invention in order to obtain an optimum charging current curve C and an 
optimum battery temperature curve T. The battery voltage curve V may be 
divided into regions representing various stages of the charging process 
similar to FIG. 1. FIG. 3 shows four regions which are marked A, B, C and 
D, respectively. 
The region marked A constitutes the start of the charging process. Here, 
the voltage supplied is controlled so that the charging current supplied 
to the battery is relatively low. 
The B region indicates the actual charging period where the charging 
current is converted to stored energy in the battery. Here, the voltage 
supplied is controlled so that the charging current i maintained at 
substantially the same maximum value, which is determined by the type of 
battery involved, and the voltage across the battery increases only 
slowly. 
In the C region the battery now approaches its state of full charge and in 
order to maintain the maximum charging current, the voltage across the 
battery begins to increase more rapidly until the voltage across the 
battery terminals reaches the predetermined maximum Vmax (which is given 
by the type of battery involved). 
In the D region the voltage supplied is controlled so that the measured 
voltage across the battery terminals is equal to the maximum limit, Vmax. 
In the regions C and D the internal resistance of the battery cell is 
increased, and for a constant battery voltage, as in the D region, the 
resulting charging current will decrease. Due to the fact that the battery 
voltage is kept at a constant value in the region D, the resulting 
temperature increase is relatively low, keeping the destructive effect 
caused by the charging current on the battery cells at a minimum. 
Not later than the time Tmax when Vmax is reached, the remaining charging 
period is determined. When such remaining charging period starting at Tmax 
has lapsed, the charging process is being terminated. 
The charging current fed to the battery is controlled by pulse width 
modulating a constant voltage source. 
The voltage curve V shown in FIG. 3 represents a process of charging a NiCd 
battery which is nearly unloaded. FIG. 4 shows six similar voltage curves 
V1-V6 representing different charging courses for the same battery with 
different starting charges. The curve V1 represents the charging process 
of the battery when nearly fully charged, and the curve V6 represents a 
charging process of the battery when almost fully discharged. FIG. 4 shows 
the charging period necessary for obtaining the maximum voltage Vmax 
increases when the starting state of charge of the battery decreases. It 
can also be seen from FIG. 4 that the "remaining charging time", which is 
the time period from the reach of Vmax till the charging process is 
terminated, increases when the starting state of charge of the battery 
decreases. 
Information about ideal or desired reference voltage curves for the type of 
battery in question for a plurality of different starting charge 
conditions of the battery may be stored in an electronic memory. By 
comparing the course of the actual voltage curve, such as the slope of the 
curve with the stored reference values, the relevant reference voltage 
curve and the "remaining charging time" associated therewith may be 
determined. 
The slope of the voltage curve may be measured currently, for example every 
tenth second during the charge process. For each measurement a comparison 
is made with the stored reference slopes, and a new proposal for a 
"remaining charging time" is determined. When the measured battery voltage 
reaches the stored maximum voltage Vmax, determination of the "remaining 
charging time" is cancelled, and the last determined "remaining charging 
time" value is used. 
Another embodiment of method according to the invention, which also results 
in charging curves of the type shown in FIGS. 3 and 4, comprises 
prestoring a limited number of reference values for the slope of the 
voltage curve. In each measurement the actual slope of the curve is 
compared with the reference values, and only when the slope passes one of 
the reference values a new "remaining charging time" value is determined. 
Curves corresponding to those shown in FIGS. 3 and 4 will also be obtained 
for other battery types. These curves might be of different appearance and 
for some of them the correlation between the time for reaching the voltage 
Vmax and the optimum remaining charging time will not necessarily be 
associated with the slope of the voltage curve in question, but with other 
parameters of the curve, such as, for example, the absolute voltage at the 
moment concerned. The more parameters measured and stored, the more 
sophisticated determinations can be made in order to determine the optimum 
remaining charging time. 
a further embodiment of the method according to the invention which 
resulting in charging curves of the type shown in FIGS. 3 and 4, comprises 
measuring the battery voltage at a fixed time together with measuring the 
slope of the voltage curve when the maximum voltage Vmax is reached. In 
this embodiment, the voltage together with the slope of the voltage curve 
can be incorporated in a more sophisticated determination of the optimum 
remaining charging time. 
The voltage curves shown in FIGS. 3 and 4 have been plotted by measuring 
the voltage across the battery terminals when the battery is being 
charged. However, an alternative possibility is to cut-off the charging 
current briefly each time a voltage measurement is performed. A quite 
similar curve is obtained in this manner, but the absolute voltage values 
will be slightly lower because the curve does not include the voltage drop 
involved by the charging current across the internal resistance of the 
battery. Since this internal resistance typically increases at the end of 
the charging sequence, a voltage measurement without this contribution 
will be a more accurate measure of the state of the battery. 
In the embodiments described above the measurement of the slope of the 
curve takes place in the following manner. At each measurement point of 
time, i.e. for example every tenth second, the voltage of the battery is 
measured, and the an electronic processor may store this voltage value in 
a storage circuit. The processor then calculates the difference between 
this value just measured and, for example, the value which has been 
measured 90 seconds ago, and this difference is used as a measure of the 
slope of the curve at the time in question. In this manner, a new value of 
the slope which has been measured over a period of for example 90 seconds 
is obtained every ten seconds. 
To prevent the voltage measurements from being affected by transients and 
the like, the voltage is preferably measured much more frequently, for 
example 100 times between each of the said measurement points of time. 
Each of these intermediate measurements is stored by the processor, and at 
the actual measurement points of time the processor calculates an average 
of the 100 intermediate measurements which have been performed since the 
last measurement point of time. 
When the charging process has been terminated as described above, 
maintenance charging of the battery may take place if the battery is left 
in the charger. This takes place by passing current pulses through the 
battery at intervals. 
These current pulses and the time between them are adapted such that they 
compensate for the self discharge of the battery which would otherwise 
take place. The pulses may, for example, have a duration of 15-30 seconds 
and the distance between them may be a few hours. 
FIG. 5 shows a block diagram of an embodiment of an apparatus according to 
the invention. 220 volts are applied to the apparatus by means of an 
ordinary plug 1, and the voltage is converted in the rectifier block 2 to 
a 9 volts direct voltage. 3 shows a current regulator which supplies 
current to the battery to be charged via the terminals 4, 5. The current 
from the battery runs via the terminal 5 and the resistor 6 via ground 
back to the rectifier circuit 2. The current regulator 3 is controlled via 
a control stage 8 from a processor 7. The processor 7 is capable of 
measuring current and voltage by means of an analog/digital converter 9. 
The charging current is measured by measuring the voltage drop across the 
resistor 6, while the voltage of the battery is obtained as the difference 
between the voltages measured on terminals 4 and 5, respectively. The 
processor 7 is moreover connected to a storage circuit 10, which is used 
i.a. for storing measured current and voltage values as well as the 
calculated stop points of time. A regulator circuit 11 generates a direct 
voltage of 5 volts from the voltage of 9 volts from the rectifier circuit 
2. The voltage of 5 volts is used for supplying the circuits 7, 9 and 10. 
The current regulator 3 is controlled by means of pulse width modulation, 
and the processor 7 regulates the pulse width in a manner such that the 
desired charging current constantly runs through the battery. The 
processor measures this, as mentioned, by measuring the voltage drop 
across the resistor 6. If desired, the processor may perform the voltage 
measurement across the battery in the intervals between the current 
pulses. The voltage measurement will thus not be affected by the voltage 
drop which the charging current causes across the internal resistance of 
the battery. 
FIG. 6 shows a circuit diagram of an embodiment of the apparatus from FIG. 
5. The blocks from FIG. 5 are shown in broken lines and with the same 
reference numerals as in FIG. 5. The rectifier block 2 comprises a 
transformer T1 as well as a rectifier coupling consisting of the four 
diodes D1, D2, D3, and D4. The output voltage from this is a direct 
voltage of 9 volts, which is passed partly to the current regulator 3 and 
partly to the regulator circuit 11. The current regulator 3 consists of a 
transistor Q4, and it is controlled via the control stage 8 from the 
processor IC1. The control stage 8 consists of the resistors R5, R6, R7 
and R8 as well as a transistor Q3. When the output terminal P1.1 of the 
processor has a high output signal, the transistor Q3 will be in 
conductive state via the voltage divider consisting of R7 and R8. Current 
will hereby run through the voltage divider R5 and R6, causing Q4 to 
assume a conductive state whereby current is supplied to the battery. When 
the output terminal P1.1 of the processor is low, both the transistor Q3 
and the transistor Q4 will be in a non-conductive state, and no charging 
current is fed to the battery. 
The analog/digital converter 9 consists of the integrated circuit IC2 as 
well as the resistors R2 and R3 and the smoothing capacitors C4, C7. The 
measured voltages which are indicative of the battery voltage and the 
charging current, respectively, are converted to digital information in 
the integrated circuit IC2, and this digital information is passed further 
on to the terminals P1.2 and P1.3 of the processor. 
In this embodiment the processor circuit IC1 comprises both the processor 7 
and the storage circuit 10. Further, the capacitors C1, C2 and C3 as well 
as a crystal X1 are connected to the processor. Otherwise, the mode of 
operation of this processor circuit is generally known. 
The regulator circuit 11 consists of the integrated voltage regulator IC3 
as well as the capacitors C5 and C6. This circuit applies a direct voltage 
of 5 volts which is used for supplying the circuits IC1 and IC2 with 
voltage. 
The circuit described is useful no matter whether it is decided to measure 
the voltage during charging of the battery with a constant current, or to 
measure the current during charging of the battery with a constant 
voltage, just as combinations of these two may be employed. 
Of course, details in the structure of the circuit may be modified within 
the scope of the invention. Thus, for example, other processor types than 
the one shown may be used. It is also possible to use other voltages than 
those indicated in FIGS. 5 and 6, since this may for example depend upon 
the number of cells in the battery to be charged.