BATTERY CHARGER

Abstract: A charger for rechargeable cells or batteries utilizes a mechanism whereby resistance-free voltage is continually monitored during periodic current-off intervals in the charging process and compared to an independent reference voltage to prevent overcharge. After a certain degree of charge has been obtained, the current is gradually reduced to a finishing charge. Several means are provided for terminating the finishing charge while avoiding overcharge. Another embodiment of a charger provides for measurement of the resistance-free voltage during the current-off period after sufficient time is allowed for the recombination of any oxygen which may have been produced during charging, thus taking into account inherent variations in cells of the same type.

Description: BATTERY CHARGER FIELD OF THE INVENTION This invention relates to battery chargers, or more particularly to circuits for charging rechargeable batteries and cells. The rechargeable batteries and cells that can be recharged from circuits and methods of the present invention may range in value from very low voltage -- less than 1 or 1.5 volts -- with very low capacities - in the order of several hundred mAh — up to batteries in the range of from 12 or 24 volts up to several hundred volts, and with capacities in the range of hundreds to several thousand ampere-hours. The present invention provides circuits and methods consistent with the above, and provides circuits whereby the rate of charge current delivered to the rechargeable battery or cell being charged may be in the range of IOC to 15C, or more, and may be regulated down to a trickle charge. BACKGROUND OF THE INVENTION The applicant herein is the applicant in co-pending United States application Serial No. 07/253,703 filed October 6, 1988, and assigned to a common assignee herewith -- and corresponding to EPO Publication No. 0 311 460 published April 12, 1989. That previous application teaches a battery charger where a principal feature is the fact that the battery charger can deliver a current to a rechargeable battery or cell initially at a rate in amperes greater than the capacity in ampere- hours of the battery -- in other words, at a rate greater than C. Therefore, the rechargeable battery or cell being charged may be rapidly charged. Another feature of the previous invention is that means are provided for detecting the internal resistance free voltage of the rechargeable battery or cell being charged, and comparing it to a pre-selected reference voltage which is independent of die battery being charged. In other words, for a particular type and rating of rechargeable battery or cell to be charged, a reference voltage is pre- selected and is generated within the charger circuits. (It is possible that the reference voltage may be pre-selected by switch setting or the like, with prior knowledge of the condition, rating, and type of rechargeable battery or cell to be charged.) The resistance free voltage is compared to the internally generated reference voltage at an instant in time when the charging current being delivered to die rechargeable battery or cell has been interrupted. The prior invention provides that, as the internal resistance free voltage of the rechargeable battery or cell being charged exceeds the pre-selected reference voltage, means are provided to reduce the electrical charging current and thereby reduce d e rate of charging the rechargeable battery or cell, in order to maintain the internal resistance free voltage at a value equivalent to the pre-selected reference voltage. In other words, if it is noted that the internal resistance free voltage of the rechargeable battery or cell being charged marginally exceeds die reference voltage, then that is an indication that the rate of charging current delivered to die rechargeable battery or cell is too high, and the rate of delivery of the charging power - i.e., the charging current -- is reduced. The present invention provides circuits ' that are in some way similar to those described in the above referenced U.S. application and EPO published specification, in that it has been determined that conditions may exist when it is desirable to have better control over the charging process, or conditions may exist where it is important to have control over the reference voltage against which the resistance free terminal voltage is being compared, so as to prevent unwanted overcharging characteristics of any sort. Overcharging may occur in some circumstances, for example in the event that the internal temperature of the rechargeable battery or cell is or becomes high, or even in the event that the ambient temperature in which the charger is operating is or becomes high. Further, it is sometimes important to monitor not only the resistance free terminal voltage of the rechargeable battery or cell being recharged, but also die rate of charge, because the onset of certain charge current conditions may be indicative of an unwanted overcharge condition occurring. Recharging may occur in respect of a great many different kinds of rechargeable batteries or cells. Common conditions and types, however, particularly include nickel-cadmium d at may be used in household toys and appliances; and more particularly for such rechargeable batteries and cells - especially nickel cadmium -- which are used in significant quantities in products such as rechargeable hand tools and video camcorders. Other rechargeable batteries (or cells in some conditions) may be lead-acid systems: they may be found in very small sizes in portable audio tape/radio devices; and in much larger embodiments in forklift trucks, golf carts and the like, and electric vehicles. The voltage and capacity of such lead acid batteries may be from 2 volts (for a single cell) up to hundreds of volts and more, with capacities rated from fractions of an ampere-hour to thousands of ampere-hours. Obviously, particularly for large battery installations, it is desirable to provide charging currents consistent with the method of rapid charging, and if the rate of charging may be in the range of 5C to IOC or 15C, then the charging current may be in the range of several hundreds or thousands of amperes. It must be noted that battery charging occurs when there is a capability of the rechargeable battery or cell to accept charging current -- in other words, battery charging occurs as a function of the charging current and of die state of charge of the battery or cell being charged. In order for d ere to be a flow of current from the charging circuits to the rechargeable battery or cell to be charged, there is a terminal voltage for the charging circuit provided diat is higher than the rest voltage of the cell or battery to be charged. There is, therefore, by the difference between those two voltages, a driving voltage - often referred to as "overvoltage" or "polarization"; and d at voltage may, itself be controlled. But, it is also important to note that the cell voltage or battery voltage being spoken of is the resistance free terminal voltage thereof - that is, the terminal voltage of the rechargeable battery or cell being charged at a time during its charging sequence when flow of the charging current to the battery or cell has been interrupted. This eliminates all voltage losses due to resistances anywhere in the charging circuit or within the battery or cell being charged, and is therefore a true indication of d e electrochemical voltage of the battery or cell. It is also to be noted, however, that the determination of the resistance free terminal voltage is taken rather soon after the flow of charging current has been interrupted, so as to preclude internal changes occurring within the battery due to time dependent electrochemical effects. Thus, it is the steady state resistance free terminal voltage that is important to be detected. Those voltages differ, of course, for various kinds of cell or battery types: such as, for example, nickel cadmium (where the resistance free terminal voltage of a freshly charged cell may be in the order of about 1.38 volts, and of a substantially discharged cell at about 1.19 volts, about where the voltage for the most part remains at about 1.2 volts); or lead acid (where the resistance free terminal voltage may vary from about 1.90 volts to about 2.45 volts). It is one of the purposes of the present invention, as described in greater detail hereafter, to assure that any temperature rise within a battery or cell being charged comes as a consequence solely of the diermodynamics of the charging reactions and of die internal resistance of the battery or cell, and not as a consequence of die overcharge electrochemical processes occurring in the cell. As a consequence, it is a corollary of that purpose that battery chargers in keeping with the present invention provide higher efficiency when compared with conventional battery chargers. To achieve that purpose, the charging circuits must be capable of determining diat point during die charging cycle when overcharging of the battery or cell is about to occur. In other words, the battery charger must be capable of the determining the instantaneous capability of the battery or cell to accept charging current, and to adjust d e rate of delivery of the charging current accordingly. It happens that, by being able to demonstrate those characteristics, battery chargers according to die present invention have die effect of removing or eliminating the memory characteristic that is so prevalent with nickel cadmium batteries and cells - especially when the nickel cadmium battery or cell has been charged at a slow rate if it has not yet been fully discharged. It has been the practice, in the past, especially for persons using hand-held tools or camcorders, and die like, either to continue to operate the device until such time as it fails due to substantially complete discharge of the battery, or sometimes such as at the end of die day to remove die battery from the device and forcibly discharge it so as to assure that it has been fully discharged, before recharging it. Moreover, when batteries and cells such as nickel cadmium are charged at a relatively low rate, it is possible that short circuits can occur within the battery, and diat is much less likely to happen when the battery is charged at a high charging rate. Of course, in nearly every instance, battery chargers according to the present invention provide an initial high charging current if the battery or cell to be charged can accept such a current. As a consequence, it has been found diat the cell life - that is, the number of recharge cycles to which a battery or cell may be subjected -- may be increased by a factor of two or three in the case of nickel cadmium batteries or cells when they are consistently charged using battery chargers of the present invention. Thus, battery chargers of present invention are capable of providing just small quantities of recharging energy to partially discharged batteries or cells, without harming them. That, in turn, suggest that designers of devices using such batteries can ultimately design diem to use batteries having lesser capacity than at present, thereby resulting in those applications having lower capital cost of manufacture and of acquisition by die user. By being able to provide a "topping up" charge to such as lead-acid batteries or cells, deep discharge and therefore the adverse effects of deep discharge on battery life, is avoided. Still further, because the present invention provides battery charges that are capable of recharging batteries in a very short period of time, die necessity for duplicate or standby batteries, or the necessity for taking the battery operated device out of service for a significant period of time to recharge the battery, are eliminated or overcome. A typical example of the above might be a golf cart. Usually, golf carts have six 6-volt batteries each having a capacity of the about 134 ampere-hours. Such batteries have costs in the range of about $400.00, and the total weight is in the range of about 200 kilograms. If it were accepted that when the player using the golf cart returns the cart to be re-used by the next player, and that die next player will not use the cart for about 15 or 20 minutes, it is possible to provide the cart with three 12-volt batteries, each having a capacity of about 70 ampere-hours. That installation is capable of being recharged in about 15 or 20 minutes by battery chargers of the present invention; and such a battery installation may be obtained at a cost of approximately $200.00 and may have a weight of about 100 kilograms. Still further, a lighter golf cart can, itself, be designed, so diat its range may be extended or in any event its capital cost reduced due to die lighter battery weight that it might carry. Another typical example, is cordless - that is, hand-held — battery powered hand tools. It has been noted that manufacturers of such tools continually increase the size of the battery packs they require in order to provide diem with longer operating periods; and that by providing heavier and larger battery packs, the tool becomes bulkier and heavier. Since it was the intention of battery powered hand-held power tools to be small and easy to handle and manipulate, d e provision of heavier and bulkier battery packs is contrary to the initial purpose for which those tools were developed. On the od er hand, by providing battery chargers in keeping with the present invention, d e designer or manufacturer of the hand-held battery powered tools can bring to the public a tool with a much smaller battery, and which is therefore much easier to handle. The battery packs can be very quickly recharged, such as during a work break for refreshment, so that die capital cost of acquisition especially by professional tradesmen and die like can be reduced and convenience of use enhanced. Still other circumstances may be such as for hand-held portable telephones or portable dictating machines, such as the one on which the present application has been drafted. Such machines - and portable audio machines in general, especially those having recording capabilities -- may have various current demands placed on die batteries which power them, depending on whedier they are in a recording or playback mode, or if they are rapidly spooling tape from one reel to the other in the machine. invention, as discussed hereafter, means are provided for determining the internal pressure of the battery or cell being charged, and to alter or terminate the charging operation as a consequence of the sensed internal pressure. Applicant refers, in particular, to the following prior art as being of specific interest or note. The prior art comprises a number of patents and one publication, and is directed in one way or another to battery charging. However, the prior art is generally not directed towards battery charging where control is achieved by or is a function of die resistance free terminal voltage of the rechargeable battery or cell being charged. Reference is first made to a paper by Dr. Karl Kordesch et al entitled "Sine Wave Controlled Current Tester for Batteries", published at pages 480 to 483 of Journal of the Electrochemical Society for June, 1960. That paper is one of the first references to measurement of the resistance free terminal voltage of the battery being charged, and suggests the use of a portable instrument operated from a 60 Hz source to make direct meter readings of die resistance free terminal voltage, and in some way or other to make use of that reading for state-of-charge determination and charge control purposes. One of the first patents to teach resistance free charging is CHASE, United States Patent 3,576,487, April 27, 1971. That patent teaches the use of a multivibrator which turns on and off, thereby permitting pulsed charging current to be fed to die battery. During current interruptions, the battery voltage is sensed and compared against the reference. If the sensed battery voltage exceeds a predetermined value, the charging operation stops. There is no control other than that when the main charging operation terminated, a trickle charge continues to be fed to d e battery. Another early patent is MULLERSMAN, United States Patent 3,531,706, September 29, 1970, which teaches a charger that delivers pulsed D.C. charging current, and which senses temperature compensated resistance free terminal voltage of the sealed cell being charged. The purpose is that the flow of high charge rate current to the sealed cell may be terminated as the cell reaches nearly full voltage, and it is important for there to be a thermal integrator within the sealed cell unit if possible. A voltage responsive controller is provided, whose purpose is to terminate charging function when the voltage across the terminals of purpose is to terminate charging function when the voltage across the terminals of the sealed cell unit reaches a predetermined value. BROWN et al provide in their United States Patent 4,061,956 dated December 6, 1977, a D.C. battery charger which has a number of secondary functions whereby the status of the battery being charged is determined from signals that are indicative of d e battery terminal voltage and the temperature of the battery. Brown et al are particularly concerned with providing a boost signal to charge the battery in keeping with a pre-selected charging program which is related to the state of charge of the battery as determined by measurements of its voltage and temperature. The Brown et al patent contemplates a variety of charging programs, depending on die nature of the battery and the manner of its installation. Brown et al is also specifically concerned with the possibility of short circuited cells, and terminates or inhibits a charging operation if a short circuited cell is determined. MACHARG was granted United States Patent 3,886,428 on May 27, 1975, and a United States Patent 3,987,353 on October 19, 1976, each relating to a controlled system for battery chargers. Each battery charger is useful for a variety of batteries, but is particularly intended for use with lead acid batteries. In each Patent, Macharg derives a control signal by extracting the internal resistance voltage drop once the charging current has been switched off, and then differentiating the rate of decay of die open-circuit terminal voltage of the battery. A voltage is then derived from this differential to control the magnitude of d e charging current, in order to progressively reduce the charging current; and Macharg is particularly concerned with the phenomenon of gas generation, noting that gas generation has been detected as a result of a significant differential in the rate of decay of die open- circuit terminal voltage having occurred. SAAR et al have related United States Patents 4,388,582 of June 14, 1983 and 4,392,101 of July 5, 1983. Both patents are directed to fast charging circuits for nickel cadmium batteries, or others, and particularly of the sort that may be used in hand-held portable tools. What Saar et al are particularly concerned with, however, is to analyze the charging characteristic or profile of the battery and on die basis of pre-selected criteria adjust die charging characteristic when one or a particular series of values are determined. Override provisions may also be employed, in the event that the battery being charged fails to exhibit the charging characteristics that are expected of it. SUMMARY OF THE INVENTION The present invention provides circuits and mediods for charging rechargeable batteries and cells. The circuits have a variety of specific design criteria, so that the present invention can provide for temperature compensation, and it can produce variable reference voltages which are contingent upon a number of factors including the temperature of the battery or indeed its state of charge acceptance capability. Various and quite complex timing features are provided. Yet another feature of the present invention is diat, with certain design pre-conditions for the battery pack being charged, the precise characteristic of that battery pack and d erefore the number of cells and die charging voltage to be delivered to the battery pack, can be automatically determined. Yet odier features of the present invention provide for fine or detailed improvements to the charge cycle, whereby undesirable side effects such as thermal runaway - which may otherwise be undetectable using ordinary sensing operations -- may be precluded or inhibited. The present invention provides a circuit for charging rechargeable batteries and cells where a source of electrical charging energy is provided, and is delivered across an output of these circuits to which the rechargeable battery or cell may be connected. Between the source of electrical charging energy and die rechargeable battery or cell to be charged diere is a power control circuit, e.g. of the switching inverter type, and it is arranged so that the rate of the amount of charging energy to be delivered - and dierefore the charging current ~ may be regulated under the control of at least one sensing circuit and a control circuit. The sensing circuit includes means for detecting the terminal voltage of the rechargeable battery or cell, and a comparator means compares the detected terminal voltage with a reference voltage so that an output signal from the comparator is provided when a difference between those voltages exists. A pulsed timing signal which controls short interruption of the charging circuit is provided, and it also controls a latching means so that the latched output of the comparator means is delivered to a control circuit which controls the level of power delivered by die switching inverter under pulsed conditions as determined by die pulsed timing signal. When there is a predeteπnined relationship of the detected terminal voltage and the reference voltage, which is determined when the charging current has been interrupted -- in other words, a predetermined relationship of the resistance free terminal voltage of the rechargeable battery or cell being charged with respect to the reference voltage - - the operation of the controlled circuit is determined. The duty ratio of the switching sequence of the switching inverter is determined by die smoothed output of the latching means, so that die rate of the delivery of charging energy - the charging current - is controlled. In keeping with an important feature of the present invention, die reference voltage against which the resistance free terminal voltage is compared, may itself be altered at any instant in time as a consequence of the status of the rechargeable battery or cell being charged. Even more broadly stated dian above, it can be considered that die switching inverter device may be viewed as a power controller means which is in series with the source. The power controller means is arranged so that d e rate of the amount of energy being delivered to the rechargeable battery or cell -- the charging current - which is connected across the output of the charging circuits, may be regulated as stated above under the control of at least one sensing circuit and a control circuit. Briefly stated, typical power controllers apart from power transistors and MOSFETs described in greater detail hereafter, may be such items as silicon controlled rectifiers, linear regulators, switching regulators, and switching magnetic amplifiers. The status of the rechargeable battery or cell being charged « by reference to which the reference voltage may be altered - includes its temperature, or the temperature of the ambient in which the battery charging circuits are operating. To make that determination, temperature sensitive devices are provided, where the temperature sensitive device may be mounted so as to be affected by the internal temperature of the battery or cell, or by the ambient temperature; and in any event, so that the reference voltage against which the resistance free terminal voltage of the battery or cell is compared may be algebraically affected by the temperature of the temperature sensitive device. The present invention provides that when the temperature sensitive device becomes, for one reason or another, inoperative, the circuit arrangement is such that an inoperative temperature sensitive device will cause the charging circuit to be turned off, thus resulting in a failsafe operation. Still further, the present invention provides means whereby the value of the charging current being fed to die rechargeable battery or cell may affect the value of the reference voltage. Thus, the state of charge acceptance capability of the battery or cell being charged may, itself, further affect the manner in which the rechargeable battery or cell is being charged. Moreover, the present invention also provides means for sensing the internal pressure of the rechargeable battery or cell being charged. The pressure sensing means can be arranged to affect the reference voltage (or, as discussed generally hereafter, the sensed terminal voltage) so that under certain conditions the pressure status of the rechargeable battery or cell being charged controls the operation of the charger to either alter or terminate the charging operation. Alternatively, die circuits of the present invention may also be arranged so that the input to the comparator means which compares the detected terminal voltage with a reference voltage and which produces an output signal when a difference between those voltages exists, where the output signal from the comparator passes through the latching means to a control circuit so as to control the switching of the switching inverter or the operation of the power controller, may be slighdy differently connected. Thus, the present invention also contemplates that the value of the detected terminal voltage may be altered as a consequence of die status of the rechargeable battery or cell being charged, rather than the reference voltage itself being, altered as a consequence of die status of the rechargeable battery or cell being charged. Generally, the algebraic effect of the signal which comes as a consequence of the status of the rechargeable battery or cell being charged would be applied to d e detected terminal voltage signal in the opposite sense to the manner in which it would be applied in the more usual course to the reference voltage so as to alter that reference voltage as a consequence of the status of the rechargeable battery or cell being charged. Needless to say, the present invention provides for suitable visual and odier annunciator means to indicate die ongoing operation of die charger, its status, or whether the charging operation has been terminated. The present invention provides a mediod of recharging rechargeable batteries and cells which, in general, comprises the following steps: (a) providing an electrical charging current from a source thereof to an output across which a rechargeable battery or cell may be connected; (b) periodically interrupting the flow of electrical charging current to determine die resistance free terminal voltage of the battery or cell being charged, and comparing the sensed resistance free voltage with a reference voltage which is independent of the rechargeable battery or cell being charged; (c) delivering the electrical charging current to die output of the charger for a fixed and predetermined period of time at die lesser of a predetermined maximum current value or the ability of the rechargeable battery or cell to accept a charging current. In the event that die ability of the rechargeable battery or cell to accept a charging current is greater than a predetermined maximum current value, then the electrical charging current is delivered at that maximum current value; and after the first fixed period of time the electrical charging current continues to be delivered at the maximum value if it was being delivered at d e termination of the first fixed period of time, for so long as the sensed resistance free terminal voltage of the rechargeable battery or cell is less than the independent reference voltage. The second variable time period is terminated at the first instance when the sensed resistance free voltage reaches the same value as the independent reference voltage. At that time, the electrical charging current being fed to die rechargeable battery or cell is permitted to reduce, but die reduction of die electrical charging current is in such a manner that the sensed resistance free voltage and die independent reference voltage remain at the same value. In other words, the reduction of die electrical charging current follows the charge acceptance curve of the rechargeable battery or cell being charged; (d) a timer is operated from the beginning of the charge cycle so that, following a third predetermined period of time measured from the beginning of the charge cycle, the electrical charging current is reduced to a predetermined value of from zero to a predetermined low charging current in the event that it is still at the maximum value; and (e) also, a timer is operated so diat at the end of a fourth predetermined period of time which follows the instant when the charging current begins to be reduced, the electrical charging current is forcibly reduced to a predetermined value of from zero to a trickle current. In keeping with another feature of the present invention, the rechargeable battery or cell being charged is constandy monitored to determine if any one cell in the battery is faulty. In the event that a faulty cell is detected, the charging operation is instantaneously terminated and an alarm signal is given to that effect. As a further variation on die above constant monitoring activity for a faulty cell, the monitoring procedure is inhibited for a first predetermined period of time which is shorter than the first fixed period noted above, so diat die charging current may be delivered to die output at its maximum value. This permits a short period of time when an otherwise idle or deeply discharged battery is first cormected to die charger for there to be at least an initial setting up of electrochemical reaction within the cells of the battery before specific monitoring of the faulty cell within the battery is made. Generally, that first period of time when testing for a faulty cell is inhibited lasts for a period of time depending on die type and capacity of the battery, and die charging current used, and may typically be between 15 seconds and 3 minutes. In yet another variation of the methods of charging cells and in keeping with the present invention, after termination of step (c) as noted above, a different step (e) is initiated whereby die value of the electrical charging current is constandy sampled on a periodic basis and compared with the value of the electrical charging current at the prior sampling thereof. If an increase in the electrical charging current is sensed, or is sensed over a predetermined period of time depending on d e manner and storage of die periodic value of the sensed charging current, then another control is operated so as to force d e electrical charging current to continue to reduce. This procedure effectively precludes die possibility of thermal runaway. Still further, the electrical charging current may be forced to reduce in a controlled manner either to a trickle current, or even to zero. As yet a further fine variation, if a decrease of current of at least a predetermined amount within a predetermined period of time is sensed, then control circuit means are operated to force die electrical charging current to assume a different change of value over a further predetermined period of time. Thus, the slope of the charge current characteristic against time may, itself, be controlled. A still further variation of the above recognizes that, in some circumstances, a finishing charge which is greater than a trickle charge may be required to bring the state of die charge held by die rechargeable battery or cell fully to 100%. In that case, as the charging current is reducing, it may reach a level intended for a predetermined finishing charge current which may be at a value of about 0.5C to 3C. At that time, a further charge period of constant current at that predetermined finishing charge current value is initiated and die resistance free voltage of the battery or cell being charged is continued to be determined. Several methods for terminating the finishing charge are contemplated. In one method, die finishing charge current is terminated at die earlier of the first instance where a predetermined period of time measured from the beginning of the charge cycle expires, or in the event that prior to that time the sensed resistance free voltage has increased above die value of die independent reference voltage by a predetermined amount. Moreover, if an increase in the electrical charging current is sensed, and if at that instant in time the electrical charging current is higher than either the intended value of the finishing charge or even the intended value of trickle charge the charging current may be reduced to eidier of die finishing charge value or the trickle charge value. Another embodiment of die invention incorporates an ampere-hour counter. Such a device may be used in coordination with a microprocessor, and will record total charge delivered to the battery or cell during a selected period of time. The ampere-hour counter may be coordinated with the timing means via the microprocessor so as to terminate the finishing charge at a certain preselected value of total current delivered during a certain period of time. A further embodiment incorporates a means, usually a microprocessor, for detecting changes in the measured resistance free voltage with respect to time. The detection of certain characteristic change points may be used as a signal for termination of die finishing charge. In addition, die ampere-hour counter may be employed in conjunction with the change detecting means so as to terminate the finishing charge after a preselected value of total current has been delivered since the detection of a certain characteristic change point. Another embodiment sets the finishing charge time in relation to die time difference between the end of d e second variable period of time and die beginning of the fourth predetermined period of time. As a still further variation of the methods according to die present invention, d e internal pressure of the rechargeable battery or cell being charged may be sensed. That sensing may affect the reference voltage in the same way as the temperature sensing affects the reference voltage. The charging operation may be altered or terminated upon a predetermined change of internal pressure of the rechargeable battery or cell being sensed, or when a predetermined absolute internal pressure of the rechargeable battery or cell is reached. A further embodiment of die present invention provides for an improved method for measuring the current-off resistance-free voltage V^,. It has been found that by utilizing a relatively long current-off period, e.g. 50-1000 msec, and measuring the V^. near the end of die period, die influence of individual cell design on charger performance can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS The above features, and odier provisions and variations diereof, are described in detail hereafter. Certain general features with respect to battery charging, and the theoretical and governing aspects thereof, and typical charging characteristics and circuits which achieve those characteristics, are also discussed. Thus, the discussion hereafter is made in association with the accompanying drawings, in which: Figure 1 is a typical charge acceptance curve for a battery, showing the co-relation of charge current, and undercharge and overcharge zones, with d e state of charge of a battery or cell being charged, as die state of charge progresses in time; Figure 2 shows the thermal effects of discharge, charge, and overcharge, of a typical small nickel cadmium cell, at a relatively low charging rate of 1C; Figure 3 shows typical current, cell temperature, and accumulated charge characteristics of a nickel cadmium cell charged under very rapid conditions in keeping with the present invention; Figure 4 shows another set of curves for a somewhat larger cell charged over a slighdy longer period of time, showing also the effects on internal cell pressure; - - Figure 5 shows typical current and temperature conditions of the cell being charged and discharged under rather heavy duty conditions using a charger of the present invention; Figure 6 is a set of curves similar to those of Figure 4, but where the cell is cold and is charged at cold ambient temperatures; Figure 7 is yet another set of curves showing typical charge, temperature and current characteristics of a high capacity battery which was charged over a relatively short period of time; Figure 8 is a circuit of a typical charger in keeping with d e present invention; Figures 9 (a), (b), and (c), are typical curves showing the effects of various reference voltages as they change in time, and d e various effects they have on a charging current where the charging current is, in any event, being reduced below a maximum charging current in keeping with the state of the charge acceptance capability of the cell or battery being charged; Figure 10 is a family of typical charging curves for battery packs having various capacities, all charged with an identical maximum current; Figure 11 is a curve showing charging current against time, where certain predetermined time periods from the beginning of a charge cycle are indicated; Figure 12 is a state of change and condition diagram co-relating die various states of charge that might occur or fault conditions that might occur during charging of a battery, with reference to the time periods shown in Figure 11; Figure 13 shows a family of charge current versus time curves, where the condition of diermal runaway may occur; Figure 14 shows the imposition of forced charge current control on the charge current, having reference to the typical time periods referred to in Figure 11; Figure 15 shows yet another charge current versus time condition with respect to predetermined time periods with different control criteria than suggested in Figure 14; Figure 16 is yet another charge current versus time curve showing another criterion by means of which thermal runaway may be precluded; and Figure 17 is a figure similar to Figure 11 but showing finishing charge condition; and carrying with it a related set of time curves showing the correlation of the terminal voltage and die resistance free voltage of the rechargeable battery or cell being charged. Figure 18A shows a three-dimensional family of voltage decay curves taken every 10 seconds (and plotted every 2 minutes) during 500 msec interruptions of 5A (8C) charging current passing through commercial X brand AA NiCd cells. Figure 18B is a two-dimensional sample of Fig. 18A showing voltage as a function of elapsed charging time for current-off time of 15 and 495 msec. Figure 19A is similar to Fig. 18A except that cell brand Y was used. Figure 19B is a two-dimensional sample of Figure 19A showing voltage as a function of elapsed charging time for current-off time of 15 and 495 msec. DESCRIPTION OF THE PREFERRED EMBODIMENTS Having regard to Figure 1, the basic principles of the present invention, and die considerations of charging characteristics and charge acceptance characteristics of a rechargeable battery or cell are now reviewed. What Figure 1 shows is a characteristic relationship of charge current against state of charge of the battery or cell, it being understood that the state of charge varies as charge is delivered to d e battery or removed from it. A typical curve 10 is shown, which signifies the maximum charge current that the battery is capable of accepting - i.e., converting charge current into stored chemical energy -- as a function of its state of charge. This curve essentially divides the figure into two zones; the zone 12 that is under the curve 10 being the undercharge zone for die battery or cell, and die zone 14 diat is above the curve 10 being the overcharge zone for the battery or cell being charged. Also shown is a line 16 which intersects with a line 18, and die curve which comprises lines 16 and 18 may be considered to be die curve of an ordinary prior art charger which charges a battery or cell at a constant current until such time as a particular terminal voltage is reached -- which voltage is intended to represent 100% of the state of charge of the battery or cell -- at which time the constant current is terminated and reduces either to zero or tc a trickle charge. The trickle charge is shown at 20, and is, in any event, an extension of curve 10 past line 18 (i.e., after the occurrence of the condition represented by line 18). By 100% state of charge, it is understood that the battery or cell being charged has reached 100% of its capacity of stored energy, measured in ampere- hours. However, state of charge per se is not a characteristic that can be specifically measured except by fully discharging die cell or battery to determine what the state of charge was at the time that the discharge operation was begun. On the od er hand, die capability of the battery or cell to accept charge is a function of its state of charge; and if higher current is fed to die battery or cell than it is capable of accepting to increase its state of charge, then heat and gases are produced within the battery or cell. The area 22 which is above the curve 10 but below the curve 16,18 represents a domain or set of conditions where overheating will occur under charge conditions such as proposed by curve 16,18. Chargers in keeping with the present invention will follow curve 16, which may be at IOC ~ the charging current level as shown in Figure 1, but may be at any other value such as 3C or 4C, or even up to 20C - until such time as the state of charge condition is reached where curve 16 intersects curve 10. At that time, and as discussed in greater detail hereafter, adjustment of die charge current is made. Thus, curve 10 represents the charge acceptance curve. If a charging current is fed into the battery or cell, and as die state of charge increases, the electrochemical carriers within the battery or cell are one by one converted from d e discharged into the charged state. Thus, as charge progresses, there are fewer and fewer carriers still available for conversion, and die ability of the battery or cell to accept charge decreases. If, at diat time, when the reduced density of carriers means that the battety or cell is incapable of accepting a current beyond a certain limit, forcing the charge current beyond diat limit does not result in faster charging, but results in elevating the electrode potentials to the electrolysis level - which results in the evolution of gases and excess heat. Overcharging of the battery or cell occurs, and damage may begin to be experienced within the battery or cell. It is that circumstance which charging circuits of the present invention seek to overcome, by recognizing when the charge carriers are no longer capable of handling an incoming charge current and diereby reducing die incoming, current to match the decreasing ability of the carriers to handle the incoming charge current. Those features are discussed below. The following discussion is directed particularly at nickel cadmium cells, or batteries made up of a number of nickel cadmium cells for such use as in hand-held power tools, camcorders, and die like. At their equilibrium cell voltage of approximately 1.2 volts per cell, nickel cadmium cells have a negative temperature coefficient in the order of -4mV/"C. As noted above, die cells are charged at a higher charger terminal voltage, so that there is a driving voltage which is the differential between the charger terminal voltage and die rest cell voltage. Moreover, it is noted diat even the industry which provides nickel cadmium cells to the market -- especially those having sintered electrodes - recognizes the capability of those cells to tolerate very high discharge rate circumstances of up to IOC; whereas the recommended charging rate is usually only about 0.1C. The charge reactions within a nickel cadmium cell are as follows: 2Ni(OH) 2 + 20H- -> 2NiOOH*H 2 0 + 2e (1) Cd(OH) 2 + 2e -> Cd + 20H- (2) It should be noted diat the limitation of the low recommended charging rate of about 0.1C is not related to the charge reactions noted above, but radier to die overcharge reaction (3) -- shown below - which may occur on the positive electrode of a nickel cadmium cell, and to die overcharge reactions (4) and (5) -- shown below -- which occur on the negative electrode of a sealed nickel cadmium cell, or the overcharge reaction (6) - shown below - which occurs at the negative electrode of a vented nickel cadmium cell. Those overcharge reactions are: 40H- -> 2H 2 0 + 0 2 + 4e (3) 0 2 + 2Cd + 2H 2 0 -> 2Cd(0H) 2 (4) 0 2 + 2H 2 0 + 4e -> 4OH- (5) 2H 2 0 + 2e -> H 2 + 20H- (6) When sealed cells are being charged, die oxygen evolution which comes as a result of overcharge reaction (3) generates pressure within the cell, and diat in turn accelerates the consuming reactions of the evolved oxygen as shown in overcharge reactions (4) and (5). However, it should be noted d at at a low charging rate of about O.IC, for example, the pressure within the sealed cell remains at about 1 atmosphere; whereas, at a charging rate of only 1.0C, the pressure within the sealed cell could exceed 10 atmospheres for ordinary cells, and even 5 atmospheres for special high rate cells which have special designs to enhance oxygen recombination. Thus, faster charging is only possible if the charging current is controlled, or stopped, before significant overcharge conditions of die cell occur. Moreover, the recombination reactions are slower at low temperatures, so that if the cell temperature is reduced by about 40 "C, the operating pressure within the cell will usually double. Therefore, for cold batteries there is a significant risk of the cell safety valve opening, which would result in loss of the electrolyte from the cell, especially if the cold cell is being rapidly charged od er d an by die circuits and mediods of die present invention. Still further, the overcharge reactions (3), (4), and (5), for sealed cells, may also result in significant heating within the cell. On the odier hand, the charge reactions (1) and (2) will generally result in a slight cell cooling due to die negative heat of reaction of about 0.06 kcal/Ah. That negative heat of reaction may or may not be masked by heat evolution within the cell due to the internal resistance of the cell. Also, the discharge reaction of the cell will naturally show the opposite heat effect to the negative heat of reaction of reactions (1) and (2). It has been determined that if the cell were to be thermally insulated, die charging reaction at very low charge rates could cool die cell by about 10 * C but the subsequent overcharge period could increase the cell temperature by at least 20 * C for every 10% of capacity over the full charge of the cell. This is illustrated in Figure 2 where charge and discharge, and overcharge, are all contemplated at a 1C rate. Curve 24 shows a rise in temperature from less than 10 'C to above 30' C over slighdy more than one hour of discharging, with a reduction in temperature for the next hour of charging to about 25 "C, but then a significant increase in temperature over the next half hour or so of overcharging. Thus, the present invention is fully aware of the fact that die overcharge phase as noted in Figures 1 and 2 may occur even before the charging of the cell has been completed. However, at high charge rates, the ability of the cell to accept charge may fall below the rate of charge even when the state of charge is only at a fraction of its full capacity, so that the overcharge reactions may set in, with die commensurate heat and pressure consequences, well before the cell is fully charged. Most of the prior art demonstrates charging methods where the charging current is delivered in short, high energy pulses that are separated by zero current intervals during which information about the state of charge is obtained. As described above, that information may strictly be the resistance free terminal voltage, or as in Macharg or Mullersman it may relate to a determination of voltage decay rate during the interval of zero charge current. Some of the heat which is due to die overcharge reactions can be eliminated; but by delivering short pulses of high current and therefore subjecting the cell to high voltage drops, irreversible heating due to die internal resistance of the cells will occur. It follows that controlling the charging current to just below the level where overcharge conditions begin, as demonstrated in Figures 1 and 2, is ideal, since both over-pressure and over-temperature conditions should be avoided. Circuits of the present invention, where current interruptions of only a few milliseconds are employed, are able to detect die onset of overcharge reactions within the cell and d ereby reduce d e charging current to a safe level so as to charge the cell at the highest current possible without overcharging. In other words, circuits of the present invention will follow the charge acceptance curve of the cell or battery after the time that the constant current curve 16 intersects the charge acceptance curve 10, as shown in Figure 1. For example, reference is made to Figure 3, where the charge, temperature and current characteristics of half-height sub-C cells having capacities of about 650 mAh are shown. It should be noted diat die initial current as shown in curve 26 is delivered at almost 18C for about 3 minutes, whereby nearly 90% of the charge of the cell is delivered in the first 5 minutes as shown at curve 28. At the same time, curve 30 shows that the internal temperature of the cell increased by only about 10 * C, where the initial temperature of the cell was at about ordinary room temperature. Figure 4 shows still further curves for sub-C cells having capacities of about 1200mAh. However, the charge curve 32, cell temperature curve 34, and charging current curve 36, are joined by curve 38 which shows internal cell gauge pressure. The charging is a nominal 15 minute charge at a current rate of about 4C, so that at about 12 or 13 minutes the current began to reduce and d e charge reached about 90% of its rated capacity. It will be noted that a negligible increase of pressure occurred within the cell, and diat die increase in pressure leveled off at about 18 minutes when the charge current was reduced to zero. The temperature within the cell actually decreased very slighdy over the entire charging operation. The purpose of Figure 5 is to show typical current and temperature conditions by curves 40 and 42, respectively, for ordinary cells of the sort whose charging characteristics are shown in Figure 4. Here, however, the charging current is slighdy higher, at about 5C. During the discharge interval shown at 44, it will be noted diat die internal temperature of the cell increased, whereas in the subsequent charging cycle 46 the internal cell temperature decreased. The same increase in temperature during discharge and decrease of temperature during charge are repeated at intervals 48 and 50. The electrochemical cooling effect combined widi d e heat losses are therefore shown to remove the heat produced during each discharge period as die cell is subsequently charged. Referring now to Figure 6, another set of curves is shown which are similar to those of Figure 4. However, in this case, the cell temperature is very cold, being in the range of -15 to -10 * C. It will be noted diat in this case the charging current curve 52 has not reached the maximum of about 5 amperes which is limited by die switching power source within the charging circuits, as the ability of the cell to accept charge was depressed due to die low temperature. There was a certain warming tendency of d e temperature, curve 54, as the charge, curve 56, increased; but it is clear that as the cell warmed up and was capable of accepting a higher current, it would not overcharge and, indeed, die temperature began to decrease. The charger was operated for a 20 minute period, at which point 60 it turned off. It was then immediately restarted, and it will be noted diat die charging current continued to follow the curve 52 downwardly, as expected. The pressure within the cell increased over d e entire period d at the charging operation continued as shown in Figure 6, but uiere was clearly no excessive or dangerous buildup of cell pressure at the low cell temperatures being experienced. Finally, as to typical charging curves for various kinds of cells and batteries, Figure 7 shows a measured charge current curve 62, a charge curve 64, and a temperature curve 66, which exhibit the charge characteristics of a nickel cadmium aircraft battery charged at an initial charging rate in the order of 200 amperes. The capacity of the battery, a large vented nickel cadmium battery having sintered plate construction, is 40 Ah. What is particularly of note is that, as shown at curve 64, approximately 85% of the charge was delivered to die battery in 10 minutes. Moreover, the internal temperature of the battery - which was charged at ambient room temperature - increased less than 10" C. The battery was fully recharged in less than 30 minutes. A Typical Charging Circuit of the Present Invention: Figure 8 shows a typical charging circuit, with a battery pack set up as discussed hereafter. The circuits are much simplified to show essential components, but not necessarily all components, of an actual charger. Moreover, several circuit additions which affect the variable reference voltage in keeping with the present invention, and which are dependent on such factors as temperature and charge current, are shown but may not appear in any specific charger. Briefly, the circuit of Figure 8 is as follows: A source of electrical energy 70 is provided; that source may be such as 115 volts AC or 12 volts DC, or otherwise. 115 volts AC is a standard household voltage in North America, 12 volts DC is standard automobile voltage, suggesting therefore that the circuits of the present invention can be arranged to operate under ordinary household voltage circumstances or to be powered such as from the cigarette lighter of an automobile. An output 72 is provided, to which electrical charging energy is delivered; and a rechargeable battery or cell which is shown generally at 74, is connected across the output 72. A switching inverter circuit 76 of the buck type is provided in series with the source 70, and a typical switching device is MOSFET Ql. Obviously, the rate of the amount of energy being delivered to die output 72 and rechargeable battery or cell 74, and dierefore the charging current, may be regulated by switching the switching inverter 76 between its conducting and non-conducting states. That switching control is, itself, under the control of a control circuit 78 which may be a pulse widdi modulated controller. The control circuit 78 is proportionally controlled by a control input 84 whose input is fed from sensing circuits as discussed hereafter. As a matter of convenience, the output of amplifier 86 is connected to die controller 78. Further, timing pulses coming from the timer 82 to the enable input 80 of the control circuit 78 control short interruptions of the charging current. In an alternative approach to the portion of the circuit of Figure 1 by which the power ~ that is, the charging current « is delivered to d e input, it is evident that other power delivery systems that are controllable may be utilized. For example, the switching inverter device 76 is shown as utilizing a MOSFET, but it could as easily be substituted by odier power control devices, as is well known in the art. Therefore, in its broadest sense, a power converter device is inserted in the series between the source 70 and die output 72. Typical examples of such a power converter device may include bipolar transistors, silicon controlled rectifiers, gate turn-off thyristors, linear regulators, or switching magnetic amplifiers, in circuits of different types and topologies. Sensing means are provided to detect die terminal voltage of the rechargeable battery 74. They include a sensing line 88 connected to a resistor RS, so that the signal at the negative input of comparator 90, connected also to resistor R25 to form a voltage divider to ground, is a function of and direcdy related to die detected terminal voltage of the battery or cell 74. The other input to the comparator 90 is the reference voltage which is generated within the charging circuit, and is independent of die terminal voltage of the battery or cell 74. The reference voltage is found at line 92, and it is initially set during calibration of the circuits off the divider network R31 and PI. As noted hereafter, the value of the reference voltage on line 92 is, however, algebraically affected by compensation which is arranged for temperature of the battery or cell 74 or the ambient, depending on d e mounting of temperature sensitive device 94. The value of the reference voltage on line 92 may also be algebraically affected by die value of the charging current. The temperature compensation circuit shown generally at 96 and die charge current compensation circuit shown generally at 98 are connected dirough jumpers Jl and J2, respectively, so d at dieir output values are algebraically added at jimction 100. Whenever a difference occurs between the voltage reference on line 92, and d e detected terminal voltage at junction 102 - the inputs to comparator 90 - an output signal from the comparator 90 is delivered on line 104 to a control input of latching means 106 which, for convenience, is shown as a D-type flip-flop. Control output from latching means 106 is fed on line 108 through network R34, C14 which has values so as to give it a slow time constant, to one of the input terminals of amplifier 86. The network C15, R40 which is a feedback network from the output of amplifier 86 to the other input thereof, is one having a fast time constant. In general, the speed of die controller 78 is such as to operate the switching inverter circuit 76 at frequencies of 20 Khz to 30 Khz (and in some instances, up to 100 Khz). The switching inverter circuit 76 of the buck configuration consists of the MOSFET switch Ql, diode D2, and die inductor LI. Typically, the switching inverter circuit 76 is turned completely off for a period from 0.5 msec up to about 20 msec, and usually in the range of about 1 to 3 or 5 msec, under die control of the enable input 80. The filter capacitor C6 filters out the switching frequency of the switching inverter circuit 76, so as to preclude any unwanted effect of high frequency at the output 72. Obviously, the duty cycle of the switching inverter device 76 is controlled by die controller 78 under its logic enable signal 80 and its proportional control signal 84. As noted, die control signal input 84 is influenced by the output of the amplifier 86. Its output is influenced by the value of the charging current if the charge current sensing circuit 98 is operable, as described hereafter. In any event, the controller 78 can therefore be controlled at any time diat charge current is flowing, but in keeping with the conditions of d e operation of the charging circuit and the conditions of die charging current and d e sensed terminal voltage of the rechargeable battery or cell 74, all as controlled by die output 108 of the latching device 106; and d at output is, in turn, a function of die output of the comparator 90 on line 104 as determined by the comparison of the sensed terminal voltage value at junction 102 and die reference voltage on line 92. The action of d e described feedback circuit is such that, in its linear operating region when charging current is greater than zero but less than the maximum current, it will adjust die value of die charging current so that die battery voltage sensed at jimction 102 at the time the output of the comparator 90 on line 104 is latched into the latching means 106, shall equal the value of the reference voltage on line 92. As noted, d e reference voltage on line 92 is or may be further influenced by die value of the charging current and/or the output of the temperature sensitive device 94, if circuits 96 and/or 98 are active. It is worth noting, therefore, that the provision of the pulsed timing signal on line 110 to the enable input 80 of controller 78, and to the clock input of the latching means 106, provides for ON-OFF control of the switching inverter 76 under the control of the timer 82. Needless to say, a clock pulse can also be provided on line 112 instead of from line 110 as a consequence of control under die alternating current source being delivered also to input 70, and being triggered thereby. In some cases, a pressure sensitive device such as a specifically mounted strain gauge or the like, having a variable resistance depending on die pressure it is exposed to, may be mounted within the rechargeable battery or cell being charged. That is shown at 95, and it may be connected to jumper J3. The action of die pressure sensitive device 95 affects the voltage at junction 100 in much the same manner as the action of diermistor 94 affects the voltage at junction 100. In an alternative arrangement, the output of circuits 96 and 98, which are the temperature compensation circuit and die charge current compensation circuit, respectively, (or of the pressure sensitive device RP), may instead of being connected so as to algebraically affect the reference voltage on line 92, be connected at junction 102. Such a connection is shown by the dashed line 114. In that case, the value of the reference voltage on 92 remains substantially constant, but the odier input to the comparator 90 from the junction 102 is algebraically affected by die operation of the circuits 96 and/or 98. In general, a further inverter amplifier would also be inserted in the line 114, so that die effect of the output of circuits 96 and/or 98 is algebraically added to jimction 102 in the opposite sense dian they would be at junction 100 so as to affect the value of the reference voltage on line 92. The effect of the temperature compensation as a consequence of die operation of the temperature sensitive device 94 and die temperature compensation circuit 96, is now discussed. Obviously, one principle purpose of temperature compensation is to avoid diermal runaway. Moreover, the charging circuit of the present invention must be capable of operating over wide temperature ranges, where the chargers or the batteries being charged, or both, may typically be found in ambient temperatures ranging from -20 * C to +50 * C. A temperature sensitive device 94 is provided. That device may be a tiiermistor, a temperature sensitive resistor odier d an a thermistor, a two terminal temperature sensitive active device, or a multi-terminal temperature sensitive active device. In any event, the output of the temperature sensitive device is a function of die temperature of the device, and diat output changes in accordance widi die temperature of the device. The temperature of the device is dependent on die manner in which it is mounted: die temperature sensitive device may be mounted in such a manner that its temperature is affected by the ambient in which the recharging circuit or the rechargeable battery or cell is placed; or it may be mounted in such a manner that its temperature is affected by the internal temperature of the rechargeable battery or cell. In the former condition, die temperature sensitive device may simply be mounted in a position diat it is near the outside of die case of the charger, or it is near the mounting arrangement where the rechargeable battery or cell is placed. In die latter case, the temperature sensitive device may be mounted in such a manner that it is urged into close and intimate physical contact with the case or shell of the rechargeable cell or battery being charged, so that changes of internal temperature within the cell or battery affect the temperature of its case or shell and d ereby are noted by die temperature sensitive device. The temperature sensitive device 94 is mounted in such a manner that its physical mounting is noted at jacks 116 and 118. Moreover, in the circuit of Figure 8, jack 118 is arranged that a switch 120 is open if the temperature sensitive device is in place, and is closed if it is not. In any event, the value of R20 may be such that diere is an approximately linear voltage output at the junction of R19 and R20 over the range of, say, -10 * C to -i-60'C. Changing the value of R19 will allow adjustment of die magnitude of temperature compensation, so diat if it is known for example that the battery charger will operate only with a specific battery type such as nickel cadmium or nickel-hydride, or lead-acid, different values of R19 may be chosen. The fixed resistor R21 is connected in the circuit if the temperature sensitive device 94 is removed and the switch 120 at jack 118 closes. It will be noted d at a further component identified as N19 is included in the circuit, in series with resistor R19 and jumper Jl. That component N19 may be a network or device which has a complex, non-linear output, whereby the operation of the circuit as it is affected by die operation of the temperature sensitive device 94 may be more sensitive in certain ranges of temperature being sensed than in other ranges. Moreover, an amplifier 122 is provided, and it has an input from the same junction of resistors R19 and R20, and R21. In the event that the circuit including d e temperature sensitive device 94 should accidentally open, amplifier 122 will generate an RTO (temperature sensor open) signal on line 124. That line goes to an input of OR-gate 109, which is arranged so d at any signal at either of its inputs will cause LED 126 - which is a red LED - to illuminate. At the same time, the signal from the output of OR-gate 109 is fed to die latching device 106 at input R, and it causes die latching device to shut down and thereby to inhibit further charging current at the output 72. It will also be noted diat a second comparator 91 is provided, as well as a second latching device or flip-flop 107. The inputs to the comparator 91 are from the voltage sensing line 88, and d rough a voltage divider R30/R29 from the reference voltage line 92. The purpose of the comparator 91 is faulty cell sensing in the battery or cell 74. If the sensed resistance free terminal voltage of the rechargeable battery or cell 74 reduces because die cell (or a cell of the battery) is faulty, then an intolerable difference between the sensed value of the resistance free voltage and die reference voltage is noted by die comparator 91. That being the case, the output of the comparator is fed to the latching device or flip-flop 107, where a low voltage output on line 111 occurs. That low voltage output on line 111 is fed to an input of the OR-gate 109; the odier input of which is the RTO signal on line 124. As noted above, a signal at either input of OR-gate 109 causes the LED 136 to illuminate, and at d e same time applies the shut-down signal to input R of latching device 106. Although associated άrcuitiy is not specifically shown, it is obvious that the operation of the pressure sensitive device 95 may be quite similar to that of the operation of the temperature sensitive device 94. Thus, the pressure sensitive device 95 is shown connected to jumper J3, but it may also be connected dirough a similar circuit as is the temperature sensitive device 94; and in any event, its operation and the output from the pressure Sensitive device 95 as it may be affected by die internal pressure of the rechargeable battery or cell 74, affects the voltage at junction 100 (or at junction 102, as discussed above). It has also been noted that die reference voltage on line 92 (or the signal representing the sensed terminal voltage at junction 102) may be algebraically affected by die value of die charging current being fed to die rechargeable battery or cell 74. That is the function of the circuit 98. Here, a current sense line 128 leads from the negative side (in this case) of the output 72, which is connected also to the positive side of die current sensing shunt R5, through smoothing filter R15, C18, to an input of amplifier 130. The gain of the amplifier 130 is determined by die value of R16/R17. The output of the amplifier 130 is connected dirough R18 and jumper J2 to the junction 100 -- or, as suggested above, in some circumstances to junction 102. In any event, the arrangement is such that, in its most simple embodiment, die output of the amplifier 130 will swing from its highest value to its lowest value when die charging current being delivered to die rechargeable battery or cell 74 and as sensed by line 128, is within a predetermined range. For example, the output of amplifier 130 for a 10 ampere charger may be at its highest value when the charging current is above 3 amperes, and at its lowest value when the charging current is below zero. Moreover, that output may swing essentially linearly over the predetermined range of current values. It should also be noted that die presence of diode D14, when it is included, has die effect of limiting the range of operation of die circuit 98 to only those output voltages of amplifier 130 which are greater than the voltage at junction 100, (or at junction 102, as discussed above). Thus, in the case shown in the circuit of Figure 8, amplifier 130 will exercise linear control of the reference voltage on line 92 only when the charging current is below 3 amperes, and above 1 ampere. This allows for appropriate charge current compensation over a wide variety of rechargeable cells and batteries where the type of rechargeable battery or cell to be charged is specified for a particular charger circuit. Thus, specific values of components, as to their resistance, threshold voltages, etc., can be chosen. Still further, it is noted d at counter 132 can function as a timer. Its function is to provide a signal to junction 100 at die end of a predetermined charge period, whereby the value of the voltage reference on line 92 may be lowered. This is particularly useful when charging lead-acid batteries, whereby a slighdy higher initial voltage during the fast charging portion at die beginning of the charge circuit may be followed by a lower float charge voltage. Moreover, it is noted diat latching circuit 106 has an output on line 134 which goes to LED 136. The LED 136 is a green LED, and its function is to provide a visual signal confirming that the electrical charging circuits are working and diat charge current is being fed dirough the output circuit 72. The signal on line 134 is complementary to the signal which is on line 108. If, for example, there is a constant signal on line 108 because the state and condition of die rechargeable battery or cell 74 is such that there is a continuous charging current flowing to it, then line 108 is continuously high and die output to line 134 is continuously low, thereby permitting LED 136 to be continuously illuminated. On die other hand, as the output begins to regulate, so that the charge current begins to reduce due to the modulation of die power controller or switching inverter, then the illumination of LED 136 becomes discontinuous. Widi appropriate values of other circuit components to adjust time constants, and die like, the LED 136 will begin flashing at a visible rate. Several other features demonstrated in the typical charger circuit of Figure 8 require to be discussed. For example, it is noted diat current sense line 128 goes not only to amplifier 130, but also to amplifier 86. However, it has also been noted that the time constant of the circuit C15, R40 is a fast time constant, whereas the time constant of R34, C14 is a slow time constant. Therefore, the operation of the controller 78 - usually a pulse widdi modulated controller, as noted -- can be controlled by d e output of the latching circuit 106 at any time that charge current is flowing to the output 72 and is sensed on die line 128; but that control is in keeping with the conditions being sensed and controlled as a consequence of the input to the comparator 90, and its output, either as a consequence of the value of the sensed resistance free terminal voltage as it is algebraically affeαed on line 92 or at junction 102 by die output of die temperature compensation circuits 96, the charge current compensation circuit 98, or the pressure sensitive device 95. Of more significance, may be the arrangement of an additional resistor RSI, which is shown as being an integral part of the internal structure of the rechargeable battery 74 (it is not relevant when a single cell is connected across the output 72). This provides a means whereby the specific value of the reference voltage on line 92 may be matched at junction 102, as a consequence of die terminal voltage of the battery 74 which is, itself, a consequence of the number of cells in the battery that are connected in series or in a series/parallel arrangement. In particular, it is appropriate for the value of die reference voltage on line 92 to be specific, that is, for there to be a particular reference voltage value per cell. Therefore, it is appropriate for the charging circuit to have some means of determining how many cells there are in series, so as to set the output voltage accordingly. This problem may come as a consequence of the requirement for a charging circuit of the present invention to charge battery packs for different kinds of hand-held power tools, each of which may have a different voltage. Battery packs may also have different voltages for certain kinds of toys, camcorders, radio operated models of vehicles and vessels, etc. Very often, adjustment for various terminal voltages of battery packs can be made by changing a switch setting on the charger, but if the switch is improperly set a dangerous overcharge condition may arise. By the present invention, the provision of a resistor RSI within a battery pack is sufficient to provide appropriate information to d e charging circuit as to die voltage necessary to be delivered to the output 72. However, it must be noted diat such battery chargers as are adapted to provide variable output voltage for battery packs that have the resistor RSI installed within them, do not have die voltage sense line 88 in the manner as shown in Figure 8 in solid lines; rather, the voltage sense line is broken as at 140, and is picked up on line 142. The resistor RSI replaces the resistor RS (shown in Figure 8), which must be deleted when resistor RSI is to be used. The value of the resistor RSI which is within the battery pack is, therefore, a function of d e number of cells that are in the rechargeable battery 74. What that means is that a voltage drop occurs through the string RSI and R25, so that die voltage at terminal 102 is essentially the equivalent of the single cell resistance free terminal voltage as discussed and contemplated above. For a battery having n. cells, the resistance of resistor RSl would be equal to (n - 1) times the resistance of resistor R25. Ordinary commercially available resistors are sufficient to provide the voltages and resistance string necessary, and it is usual to utilize semi- precision resistors that have a resistance accuracy of about 1% of rated value. There is an inherent fail safe characteristic, because if the line 142 should fail for some reason, the charger will sense a continuously low or zero voltage, and in keeping with other features discussed hereafter, it will alarm and turn off. Obviously, the operation of the circuit of Figure 8 can be dependent upon a number of factors, and die fine control of the circuits may be as a consequence of certain properties of die rechargeable battery or cell being sensed -- notably, its internal temperature, the charge current being delivered to it, or its internal pressure -- or changes in those properties being sensed. Specifically, changes in either the internal temperature or the internal pressure of the rechargeable battery or cell being charged may affect the operation of die charger, such that die charge current being delivered to die rechargeable battery or cell 74 may be altered, or d e charging operation terminated. Such changes may be as a consequence of there having been a predetermined increase of the internal temperature of the rechargeable battery or cell being sensed, or a predetermined increase of the internal pressure being sensed; or even upon a predetermined absolute temperature or a predetermined absolute internal pressure of the rechargeable battery or cell being reached. Figures 9 (a), (b) and (c), are provided to show die effects of various reference voltages as they change in time, and to show die various effects that the changing reference voltage has on the charging current where the charging current is in the mode diat it is being reduced from below a maximum charging current in keeping with the state of charge acceptance capability of the rechargeable battery or cell being charged. Each of Figures 9(a), 9(b) and 9(c), is a family of three curves: the principle curve is a curve showing the variation of charge current with time; beside die principle curve is a curve showing the commensurate change of reference voltage and its relationship to the charge current; and die diird curve is below the principle curve and shows the change of the reference voltage with time. Obviously, the scale of current is the same in the principle curve and die curve to its side, and die scale of time is the same in the principle curve and die curve below it. Figure 9(a) shows a charge current curv'e 150, and two curves 152 and 154 showing the relation of the reference voltage against current in curve 152, and of die reference voltage against time in curve 154. The charge current in curve 150 begins as a constant current, and dien reduces to a particular value, shown at 156, at which time the reference voltage in curve 152 and 154 begins to change until a further value at 158 is reached, at which time the reference voltage in curve 152 and 154 reached its lower value. If the reference voltage had not altered at a charge current value shown at 156, the charge current would have continued to follow the curve as shown at 160; and if the reference voltage had been at die level diat it adopted at die charge current value shown at 158, the charge current would have followed die curve 162. Figure 9(b), on the odier hand, shows die situation where the gain of the amplifier 130 has been reduced, so d at die effect on the reference voltage on line 92 changes gradually from its higher to its lower value as die charge current varies from its full value to zero. Thus, the charge current in curve 164 remains constant at a value shown at 166 until such time as charge current regulation begins. At that time, the reference voltage as shown in curves 168 and 170 begins to decrease; it being noted diat die charge current reduces from the maximum value shown at 166 at the same time that the reference voltage in curve 170 begins to decrease. Having regard to Figure 9(c), a stepwise change in the reference voltage is shown. Here, the charge current in curve 174 follows the same general pattern as suggested in curve 150 of Figure 9(a), except that at a specific instant in time indicated at 174 diere is a stepwise change in the value of the reference voltage as shown in curves 176 and 178. The stepwise change which may occur may be as die result of the charge current having reached a particular current threshold as shown at value 180, or as a consequence of the timer having timed out a predetermined charging period. The stepwise change in the reference voltage will generally cause a discontinuity of d e charge current, as shown at the blip 182. Otherwise, the discontinuity in charge current may be followed by recovery as indicated in Figure 9(c); and in any event it will be noted d at die charge current after the blip 182 follows the lower curve similar to curve 162 in Figure 9(a). Obviously, the practice of varying the reference voltage as the charging process is continuing is one of considerable importance, particularly so as to preclude die possibility of thermal runaway in nickel cadmium cells, as well as for optimization of die charging cycle. Optimization results in lower energy requirements and therefore higher energy efficiency, and will itself preclude unnecessary or perspectively harmful damage to die rechargeable battery or cell as a consequence of overcharging, and the like. The Methods of die Present Invention: The present invention provides several methods for terminating the finishing charge while recharging rechargeable batteries and cells. The first few steps of the differing mediods are substantially constant in all events, and comprise die following steps: (a) providing an electrical charging current from a source thereof, such as source 70, to an output 72 across which a rechargeable battery or cell 74 may be connected; (b) periodically interrupting the flow of electrical charging current under die operation of the controller 78, and determining die resistance free terminal voltage of the rechargeable battery or cell 74 being recharged. The determination of the resistance free terminal voltage is made during the interval when the flow of electrical charging current has been interrupted. The sensed resistance free voltage is compared at comparator 90 with a reference voltage which is independent of d e rechargeable battery or cell 74; (c) for a fixed and predetermined period of time, the electrical charging current is delivered to the output 72 at the lesser of a predetermined maximum current value, or the ability of the rechargeable battery or cell 74 to accept a charging current. Thus, in the circumstance where the rechargeable battery or cell 74 has a greater ability to accept electrical charging current than is the maximum predetermined current value, the electrical charging current is delivered to die output 72 at a constant value which is equal to the maximum current value allowed by the charger; following the first fixed period of time noted above, die electrical charging current continues to be delivered to the output at die maximum value for a second variable time period. However, that second variable time period lasts only for so long as the sensed resistance free voltage of the rechargeable battery or cell 74 is less than the independent reference voltage. Thus, the second variable time period ends at die first instance when the sensed resistance free voltage reaches the same value as the independent reference voltage. At that time, the electrical charging current is permitted to reduce, continually or stepwise, in such a manner that die sensed resistance free voltage and die independent reference voltage remain at the same value. Several optional procedures exist at this stage: In die first instance, a timer may be operated from the beginning of the charge cycle so that, following a third predetermined period of time measured from the beginning of the charge cycle, the electrical charging current is reduced to a trickle current in the event that the charge current is still at its maximum value. Moreover, a timer can also be operated from the beginning of the charge cycle so that at the end of a fourth predetermined period of time which follows the instant when the electrical charging current has begun to be reduced, die electrical charging current is forcibly altered to a predetermined value of from zero to a predetermined lower charging current which is below the maximum current value. That value may be a trickle charge, or a finishing charge as discussed hereafter. In the alternative, during die period of time which follows the second variable time period and while the electrical charging current is permitted to reduce, die value of the charging current can be constandy sampled on a periodic basis and compared with die value of die electrical charging current at at least a prior sampling thereof. In that case, when an increase of the electrical charging current has been sensed, die control circuit means are operated so as to force the electrical charging current to continue to reduce. To illustrate the above, reference is now made to Figures 10, 11, and 12. Figure 10 shows a family of typical charging curves for battery packs having various capacities, all charged with an identical maximum current. Figure 11 is a curve which shows charging current against time, with certain predetermined periods from d e beginning of the charge cycle being indicated. Figure 12 is a state diagram which co-relates the various time periods and states of charge that might occur, or the fault conditions diat might occur, during charging of a battery, with reference to the time periods diat are shown in Figure 11. First, with reference to Figurfe 10, a family of typical curves is shown. Here, various capacities of battery packs ranging from 600 mAh to 2500 mAh, are shown. Each is assumed to be charged with circuits and mediods according to the present invention, and with a peak current at about 7.5 amperes, as shown at 190. Curve 192 is for a battery pack having a capacity of 600 mAh, curve 194 is for a battery pack having a capacity of 1000 mAh, curve 196 is for a battery pack having a capacity of 2000 mAh, and curve 198 is for a battery pack having a capacity of 2500 mAh. There is, very clearly, a similarity of appearance among the curves. However, the curves also show that a fixed timing period of 20 minutes is just sufficient, under ideal conditions, for a battery pack having a capacity of about 1800 mAh to 2500 mAh; and far too long for a battery pack having a capacity of 500 mAh or 600 mAh up to about 1000 mAh or 1200 mAh. Obviously, it is the best practice to terminate the charge cycle as soon as practicable so as to preclude diermal runaway, and so as to provide a signal to the user that die battery pack has been charged. Thus, a cycle of variable total length is proposed, consisting of a period of constant current, the length of which is determined by die ability of the battery pack to accept charging current under the conditions when die sensed resistance free voltage is less than the independent reference voltage, followed by a fixed time period of declining current during die time when the sensed resistance free voltage is equal to the independent reference free voltage. That cycle may last for 8 or 10 minutes. Referring to Figure 11, however, the detail is now provided. First, the time period Tl is for a fixed period of time, and terminates at time t 2 . The charging current shown in curve 200 is permitted to rise to its maximum during period Tl (which, for purposes of the present discussion is assumed to include die time period TA) and reaches a predetermined maximum current value if the battery will accept current at that value, as noted above. Assuming diat it does, a second variable time period T2 continues with delivery of charging current at the maximum value for so long as the sensed resistance free voltage is less than the independent reference voltage. T2 is terminated at time t 3 . Thereafter, the electrical charging current is permitted to reduce as shown at 202 in such a manner that the sensed resistance free voltage and die independent reference voltage remain at the same value, as discussed above. In die first option described above, a timer is operated from the beginning of the charge cycle so that at die end of a predetermined period of time T3, at time t^ if the charge current is still at its maximum value as indicated at 204, die electrical charging current is reduced to a trickle current as indicated at 206. At the same time, however, if the charge current follows curve 202, at the end of a fourth predetermined period of time which occurs at time t^, which follows from time t 3 by a predetermined period of time, die charging current may than be forcibly altered to a predetermined value. That value may be 0, or it may be a trickle current, or a finishing charge current as is discussed hereafter. The other option following step (c) as discussed above, is discussed in greater detail hereafter with reference to Figure 13. Still further, the methods of die present invention contemplate that die internal pressure of the rechargeable battery or cell being charged may be monitored. Thus, the operation of the charger may be altered, or the charging terminated, upon a determination of diere having been a predetermined increase of the internal temperature or the internal pressure of the rechargeable battery or cell being charged; or upon a condition arising where a predetermined absolute internal temperature or a predetermined absolute pressure of the rechargeable battery or cell being charged, is reached. During the period of time diat die charging cycle continues, the rechargeable battery or cell is constandy monitored to determine if the cell or at least one cell in the battery being recharged is faulty. That monitoring is carried out by a determination of die sensed resistance free voltage, and a determination if the sensed resistance free voltage suddenly alters from its previous value. That determination is made by d e comparator 91 as described above; or such other means as may be provided to send an explicit faulty cell signal. In any event, when a faulty cell is detected by whatever method, obviously a fault has occurred, and die electrical charging current is instandy discontinued and an alarm signal is given to that effect. However, it may be that it is appropriate for the faulty cell testing to be inhibited for a first predetermined period of time TA, which is shorter than the first fixed period of time Tl. That permits certain electric chemical settling to occur in the cell or battery when it is first connected to die charger, especially if the cell or battery is cold, or deeply discharged. Reference is now made to Figure 12, where values are shown that are parπcularly intended for a charger having a maximum current of 7.5 amperes, and intended to charge battery packs having capacities ranging from 600 mAh up to 2500 mAh, as discussed in association with Figure 10. When the power is turned on from its off status as shown at 210, the pretest period TA at 212 exists for 37 seconds. The charger then goes to the status of period Tl; and at time t 2 which occurs at 150 seconds, period Tl as shown at 214 terminates. If a fault is detected, as shown at line 216, an alarm status as at 218 is entered, and die charging cycle is terminated. However, the charger itself is not turned off, requiring a manual operation as shown at 220. In the normal course of events, period T2 is now entered, and for nickel cadmium batteries having capacities in the range of 600 mAh to 1000 mAh, a maximum charging current in the order of about 7.5 amperes, and a resistance free charging voltage being in the range of about 1.3 to 1.5 volts per cell, the time period T2 terminates at time t 3 no more than about 8 or 10 minutes from the beginning of the charging cycle. For nickel cadmium batteries or cells having capacities in the range of 2000 mAh to 2500 mAh, and similar charging current and charging voltage values as discussed above, die time period T2 ends at time t 3 no more than about 15 or 20 minutes from the beginning of the charging cycle. Time period T2 is shown at 220. Here, at time t 3 , several options might occur. If a fault is sensed, tiien die alarm status 218 is entered. The variable time period T2 as shown at 222 occurs for so long as the full charge current is delivered -- in other words, for so long as the sensed resistance free voltage is less than the reference voltage. That period ends with die equality of the sensed resistance free voltage and die reference voltage. Then, the charging current follows curve 202 from Figure 11, status 224 in Figure 12, terminating at time t 4 . However, at time t υ at the end of time period T3 as at 226, if the charge current is still at its maximum value as at 204, it is immediately reduced to a trickle value as at 206. Then, following time t^ a beeper stage 228 is reached where an audible alarm is sounded to alert the user that the charge is complete. Likewise, as shown on line 230, at the end of 1200 seconds from die beginning of the cycle, if the current remained at its maximum value, die beeper stage 228 is reached. Referring now to Figure 13, one further feature is discussed. Here, a family of charge current curves is shown, where one of them shows the condition when thermal runaway may occur. Curves 240 and 242 show die conditions where a normal reduction in charging current with time occurs. However, if the temperature of the rechargeable battery or cell increases, the charging current may follow the curve 244 in the manner discussed above. In diat case, die charging current begins to increase once again, even though die condition of die sensed terminal free voltage being equal to the reference voltage may still apply. Figure 14 shows an option whereby thermal runaway may be precluded. During the period of time d at die electrical charging current is permitted to reduce, die value of the electrical charging current may be constandy sampled on a periodic basis and compared with die value of die electrical charging current at at least a prior sampling thereof. This may be accomplished e.g., by a digitally controlled sampling circuit, or by a simple analog circuit monitoring the value of the first derivative of the current value, specifically designated as a "valley detector". When, as in the case of curve 246, an increase of the electrical charging current is sensed, control circuit means take over and force the electrical charging current to continue to reduce as shown at 248. If the charging current were permitted to continue at the value shown in 250, thermal runaway would be precluded, but a high residual current is fed to d e rechargeable battery or cell, and d at is not necessary. Likewise, the electrical charging current may be reduced in the controlled manner as shown by curves 252 and 254, where the slopes of d ose curves are chosen arbitrarily, but more or less approximate the anticipated charge acceptance curve of a particular or specific battery or cell being charged. Moreover, referring to Figure 15, which is a variation of Figure 11, forced current reduction in curve 260 rather than as at curve 262 may be chosen, if it is determined diat for the particular battery or cell being charged, an acceptable state of charge will still be reached in an acceptably short period of time. Referring now to Figure 16, a specific criterion to preclude thermal runaway is illustrated. Here, the charge current as shown at curve 264 is permitted to reach a so-called current valley as indicated at 266; but if a current increase of a predetermined magnitude shown at δl, and indicated at 268, is detected, dien at diat point the current is forced to reduce to a trickle charge as shown at 270. Alternatively, die charging current may be forced to reduce to zero. •4b- Finally, the concept of a finishing current for rechargeable batteries or cells is discussed with reference to Figure 17. It must be noted diat certain circumstances arise where a rechargeable battery or cell will not be absolutely 100% fully charged if the final charging current reduces to a trickle current. Theoretically, a single cell -- especially such a cell as a high rate nickel cadmium cell which has rolled electrodes -- may be considered to be an infinite number of cells that are arranged in parallel one with another. Essentially, diat means that die voltage between the electrodes at a distance from the portion of the electrodes where the current connections are made to diem, may differ very slighdy than the voltage between the electrodes at the point where the current connections are made to them ~ which is the same point where the resistance free terminal voltage is sensed. It has been found, however, that providing a finishing charge at a current value of between about 0.5C and 3C, will charge the battery or cell to a full 100% of its charge capacity in a short time. This is also especially true in the case of vented lead-acid batteries, where there is a substantial volume of free electrolyte which floods and covers the plates of die cells in the battery. The present invention thereby provides for a predetermined finishing charge current value to be fed to d e rechargeable battery or cell being charged, at a point in time near the end of die charge cycle. That point in time is determined as being die instant when die charging current reaches the value of the predetermined finishing charge. Then, a further charge period of constant current charging at the predetermined finishing charge current value is initiated. There are various methods contemplated herein for determining die termination point of the finishing charge period. One such method is by determining either a further predetermined period of time measured from the beginning of the charge cycle has expired, or when the sensed resistance free voltage of the rechargeable battery or cell being charged has inσeased above the value of the independent reference voltage by a predetermined amount. This is explained by reference to Figure 17. The major portion of Figure 17 replicates Figure 11, and die same designations and reference numerals are applied. However, it will be noted diat diere is also a co-relation of die terminal voltage and d e resistance free voltage, in the same time scale. Curve 280 shows the resistance free voltage rising until the time t 3 , at which point it becomes substantially constant as shown at 282. It will be noted diat die terminal voltage 284 of the cell rises with die resistance free voltage until the time t 3 , and die difference between those voltages as shown at 286 remains constant until time t 3 , because the current in the cell is constant as at 200. Then, as shown at curve 202, the current reduces, and dierefore die terminal voltage of the cell reduces as at 288. As the current 202 reduces to die finishing charge value shown at 290, and intersects that value, it tiien assumes the new constant current charging at the finishing charge current value. It will be noted diat d e resistance free voltage continues to remain constant until the lapse of a certain period of time which is dependent upon die battery or cell being charged, but which is near the end of die charge cycle, and dien the resistance free voltage and the terminal voltage begin to rise again as shown at 292 and 294. Once again the difference between those voltages is constant, but is lower in absolute value, because the finishing charge current is lower than the initial maximum charge current. When the resistance free voltage increases by a value shown at 296, the finishing charge current is terminated. Odierwise, the finishing charge current may continue if the resistance free voltage does not follow the curve 292 until a further predetermined period of time ton-, as shown at 298, is reached. The V p curve of Fig. 17 shows a typical second rise at 292 (after the first rise 280), followed by a voltage plateau 297a or voltage peak 295a. The rise in the voltage curve at 292 is characterized by an inflection point 292a, the point at which the rate of voltage increase begins to decrease. In other words, die inflection point 292a is the point at which the derivative of V^, with respect to time is at a maximum. Similarly, the voltage plateau 297a and voltage peak 295a may be located. To perform these operations, the claimed device may contain a means for compiling data such as values for V w at time t, and means for processing die data to locate the inflection point, voltage peak or voltage plateau. A preferred embodiment would comprise a microprocessor for compiling and processing the data, and locating the characteristic change points by calculating the derivative curves and locating the maximum described above. The battery charger would dien terminate the finishing charge when at least one of the characteristic -points has been located. Alternatively, two other preconditions with respect to the sensed resistance free voltage might also be determined as being criteria to terminate the finishing charge current. They are a determination diat die resistance free voltage begins to decrease, in the manner as shown at 295 in Figure 17, or if the sensed resistance free voltage no longer continues to increase but remains constant in the manner as shown at 297. In either of those further instances, as well as the condition noted above when die resistance free voltage increases by a predetermined amount, the finishing charge current may be terminated and diereby avoid any significant overcharge condition of d e rechargeable battery or cell being charged. As a further embodiment, die charger may be equipped with an ampere-hour (Ah) counter. The counter will record die total charge delivered to die battery or cell during a selected period of time. The counter may be coordinated via a microprocessor so as to terminate the finishing charge when a certain preselected value of total current delivered during a set time has been reached. For example, certain battery manufactures prescribe a certain total Ah charge for the saturation or finishing charge. The microprocessor would dien be set with diis Ah value measured from eid er to N or t^ as shown in Fig. 11. The microprocessor would dien communicate a signal so as to terminate the finishing charge. In addition, die Ah counter may be coordinated with die microprocessor so as to measure total charge delivered from die point of voltage peak or plateau as described above. This method is particularly advantageous when used widi charging vented lead-acid batteries. A still further embodiment of die invention relies on a relationship between the time t 4 -t 3 and topp- . With reference to Fig. 11, the inventors have found d at die duration of die decreasing current charge at T3, i.e. t 4 -t 3 , depends on die construction and condition of d e battery, the preselected value of reference voltage, and battery temperature. Further, it has been found d at a battery with a steeper slope for die T3 curve segment, i.e. a shorter T3 time segment, requires a relatively short finishing charge. For batteries with a less sloped curve, a relatively longer finishing charge is needed. In a specific example, it was shown that where a charger was set with a finishing current equal to about 20% of the initial high rate charging current, good results were obtained when finishing time t^-t,, was set approximately equal to t 4 -t 3 over a wide range of cell capacities. Naturally, a different ratio of the two time segments may be used under differing conditions, e.g. different ratios of initial and finishing current, and different classes of batteries. Accordingly, a microprocessor may be employed in coordination with a timing means to record d e time t 4 -t 3 and set die finishing time as a function thereof. Obviously, the mediods and imposition of die finishing charge current as shown in Figure 17, may be imposed as well in the event that die charge current shows a tendency to increase, as indicated at 300. An embodiment incorporating an improved mediod for measuring the resistance free voltage during die current interruption will now be described. The charging method described in the Europ. Patent Appln. 311,460 uses a constant, predetermined value of reference voltage V,^ to permit charging as close to the maximum rate as possible without causing significant overcharging. In most cases it is easy to select a suitable value for a given battery and to use temperature compensation to insure diat a proper value is used at temperatures substantially different than ambient. For example, battery packs may contain the necessary information to set die V,^, correcdy for packs with different numbers of cells. However, there are also small differences between cells of the same size produced by different manufacturers, which result in slighdy different requirements with respect to V^. This is particularly important in sealed NiCd cells, which often differ in the design of d e negative electrode and dierefore have a different ability to recombine oxygen with the cadmium. Recombination of oxygen causes depolarization of die positive electrode and dierefore contributes to the cell voltage decay after the opening of the charging circuit. Figures 18A and 19A show families of voltage decay curves (taken every 10 seconds, but plotted only every 2 min) during 500 msec interruptions of 5A (8C) charging current passing through commercial AA NiCd cells produced by two different manufacturers and designated as X and Y. Inspecting the initial decay curves, taken before any overcharge reactions are taking place, it can be seen that for both cells there is an initial sharp drop, corresponding to die IR portion of the voltage and to electrical decay phenomena with very short time constant. The remaining portion of each 500 msec long decay curve is fairly flat, corresponding to processes with fairly long time constant, like charge equilibration within the active mass. Looking now at the decay curves taken after the initial 4 minutes, which reflect increasing degree of overcharge reactions taking place in parallel with the charging reaction, a third rate process with an intermediate time constant is seen, which causes most of the curvature during die first half of the voltage decay intervals. This is caused by the onset of an increasing degree of oxygen production by the overcharge reaction, causing the potential to increase significandy during the current-on periods. During the current-off period die oxygen is being consumed (recombined) and die potential falls off as rapidly as die recombination reaction permits. The rate of oxygen consumption/recombination is strongly catalysed by the presence of nickel or other catalysts in the electrodes and will therefore differ from cell to cell based on die cell design. The cell X in figure 18A shows relatively poor catalysis for oxygen consumption and consequendy shows significant potential increase during the initial part of the decay curve, compared to much more catalysed cell Y in Figure 19A. Consequendy, cell X will require slighdy higher V,^ to be charged with d e same small degree of overcharge (e.g. 3% of total current) as cell Y. Conversely, if the same V,^ is used for both cells, cell X will be charged at lower than optimal rate or cell Y will be charged too fast, allowing too much overcharge. Previously, short current-off intervals were used and die resistance-free voltages were sampled only several msec after interrupting the current. It has now been discovered d at d e problem of differing cell design can be gready reduced by extending die current-off periods substantially, so diat die resistance free voltage can be sampled after the initial oxygen decay has been completed, i.e. 100-500 msec after each current interruption. This is clearly visible by comparing Figures 18B and 19B, where the resistance free voltages sampled at short and long intervals after current interruptions are plotted. While there is significant difference between the corresponding curves taken at 15 msec intervals, the difference between the curves taken at 495 msec intervals is much less. Therefore, the performance of existing charging systems is much improved by using resistance free voltage sampled at 50-1000 msec and preferably 100-500 msec, whereby the influence of cell design on charger performance is much reduced. . In a preferred embodiment, die above is incorporated as an element in one of the several embodiments of a charger described in this application. It should be clear that die mediods of the present invention, and indeed die circuits of the present invention, may be accomplished by adopting die use of appropriate solid state devices. For example, a programmable logic array, a microcontroller, a single chip microcomputer, or an application specific integrated circuit might control the operation of the circuits so as to permit alternative controls and to permit variable reference voltage control « where the value of d e variable reference voltage is a function of a property such as the internal temperature or the internal pressure of the rechargeable battery or cell being charged. Thus, the various charge characteristics, particularly as discussed with reference to Figures 9 through 17 described above, may be accomplished by adopting such solid state devices as noted immediately above. There has been described representative circuits and various alternative embodiments thereof, whereby very fast charging operation of rechargeable batteries and cells may be effected. A number of different but related mediods, all having regard to the manner of operation of the circuits of the present invention, have also been described.

Claims: What is claimed is: 1. A method of recharging rechargeable batteries and cells, comprising the steps of: (a) providing an electrical charging current from a source thereof to an output across which a rechargeable battery or cell may be connected; (b) periodically interrupting the flow of electrical charging current to said output and determining the resistance free terminal voltage of the rechargeable battery or cell being recharged during the interval when said flow of electrical charging current has been interrupted, and comparing the sensed resistance free voltage with a reference voltage independent of die rechargeable battery or cell being recharged; (c) wherein for a first fixed and predetermined period of time, said electrical charging current is delivered to said output at the lesser of either a predetermined maximum current value or a current which said rechargeable battery or cell can accept without any substantial rise in its internal temperature; and wherein following said first fixed period of time said electrical charging current continues to be delivered to said output at said maximum value for a second variable time period which exists for so long as said sensed resistance free voltage of the rechargeable battery or cell being recharged is less than said independent reference voltage, whereby said second variable time period is terminated at die first instance when said sensed resistance free voltage reaches the same value as said independent reference voltage, and said electrical charging current is permitted to reduce in such a manner that the sensed resistance free voltage does not exceed said independent reference voltage; (d) operating a timer from the beginning of the charge cycle so that (1) following a diird predetermined period of time from the beginning of die charge cycle, the electrical charging current is reduced to a predetermined value of from zero to a predetermined low charging current in the event diat d e charge current is still at said maximum value; and (2) at die end of a fourth predetermined period of time which follows the instant when the electrical charging current begins to be reduced, the electrical charging current is forcibly altered to a predetermined value of finishing charge current of from zero to a predetermined low charging current which is below said predetermined maximum current value; and (e) operating a means for terminating the finishing charge. 2. The method of claim 1, wherein the means of step (e) comprises a timing means, and wherein the finishing charge is terminated at the end of a fifth predetermined period of time measured from the beginning of the fourth period of time, the fifth period of time being a function of die difference between the end of die second period of time and die beginning of the fourth period of time. 3. The method of claim 2, wherein the finishing charge current is about 20% of the value of the electrical charging current in step (a), and the fifth period of time is approximately equal to the difference between the end of the second period of time and die beginning of the fourth period of time. 4. The method of claim 1, wherein the means of step (e) comprises a means for measuring total charge delivered to the battery or cell, wherein the finishing charge is terminated when a predetermined value of charge delivered has been reached. 5. The method of claim 4, wherein the value of charge delivered is measured from the beginning of the first period of time. 6. The method of claim 4, wherein the charge value is measured from the beginning of the fourth period of time. 7. The method of claim 1, wherein the means of step (e) comprises a means for detecting changes in the resistance free voltage with respect to time during step (d) (1) wherein the finishing charge is terminated when said means detects a change in the resistance free voltage which is characteristic of the onset of overcharge. 8. The method of claim 7, wherein the finishing charge is terminated when either of the voltage peak or voltage plateau is detected. 9. The method of claim 7, wherein the finishing charge is terminated when d e second inflection point is detected. 10. The method of claim 9, further comprising operating a processing means, wherein said processing means compiles data for values of resistance free voltage with respect to time, and locates the inflection point by -18- computing the maximum of the first derivative of resistance free voltage with respeα to time. 11. The method of claim 9, further comprising operating a means for measuring total charge delivered to die battery or cell, wherein the finishing charge is terminated when a predetermined value of charge delivered has been reached, said value of charge being measured from when the second inflection point has been detected. 12. A charger for recharging rechargeable batteries and cells, comprising: (a) means for providing an electrical charging current from a source thereof to an output across which a rechargeable battery or cell may be connected; (b) means for periodically interrupting the flow of electrical charging current to said output and determining the resistance free terminal voltage of the rechargeable battery or cell being recharged during die interval when said flow of electrical charging current has been interrupted, and comparing the sensed resistance free voltage with a reference voltage independent of die rechargeable battery or cell being recharged; (c) wherein for a firs fixed and predetermined period of time, said electrical charging current is delivered to said output at the lesser of either a predetermined maximum current value or a current which said rechargeable battery or cell can accept without any substantial rise in its internal temperature; and wherein following said first fixed period of time said electrical charging current continues to be delivered to said output at said maximum value for a second variable time period which exists for so long as said sensed resistance free voltage of the rechargeable battery or cell being recharged is less than said independent reference voltage, whereby said second variable time period is terminated at the first instance when said sensed resistance free voltage reaches the same value as said independent reference voltage, and said electrical charging current is permitted to reduce in such a manner that die sensed resistance free voltage does not exceed said independent reference voltage; (d) a timing means which operates from the beginning of the charge cycle so that (1) following a third predetermined period of time from -*9- the beginning of the charge cycle, the electrical charging current is reduced to a predetermined value of from zero to a predetermined low charging current in the event that die charge current is still at said maximum value; and (2) at the end of fourth predetermined period of time which follows the instant when the electrical charging current begins to be reduced, die electrical charging current is forcibly altered to a predetermined value of finishing charge current of from zero to a predetermined low charging current which is below said predetermined maximum current value; and (e) a means for terminating the finishing charge. 13. The charger of claim 12, wherein the means of step (e) comprises a timing means, and wherein the finishing charge is terminated at the en of a fifth predetermined period of time measured from the beginning of the fourth period of time, d e fifth period of time being a function of die difference between the end of die second period of time and die beginning of the fourth period of time. 14. The charger of claim 13, wherein the finishing charge current is about 20% of the value of the electrical charging current in step (a), and the fifth period of time is approximately equal to the difference between the end of die second period of time and die beginning of the fourth period of time. 15. The charger of claim 12, wherein the means of step (e) comprises a means for measuring total charge delivered to the battery or cell, wherein the finishing charge is terminated when a predetermined value of charge delivered has been reached. 16. The charger of claim 15, wherein the value of charge delivered is measured from the beginning of the first period of time. 17. The charger of claim 15, wherein the charge value is measured from the beginning of the fourth period of time. 18. The charger of claim 12, wherein the means of step (e) comprises a means for detecting changes in the resistance free voltage with respect to time during step (d) (1) wherein the finishing charge is terminated when said means detects a change in the resistance free voltage which is characteristic of the onset of overcharge. 19. The method of claim 18, wherein the finishing charge is terminated when eidier of the voltage peak or voltage plateau is detected. 20. The charger of claim 18, wherein the finishing charge is terminated when the second inflection point is detected. 21. The charger of claim 20, further comprising a processing means, wherein said processing means compiles data for values of resistance free voltage with respect to time, and locates the inflection point by computing the maximum of the first derivative of resistance free voltage with respect to time. 22. The charger of claim 20, further comprising a means for measuring total charge delivered to die battery or cell, wherein the finishing charge is terminated when a predetermined value of charge delivered has been reached, said value of charge being measured from when the second inflection point has been detected. 23. A charger for charging rechargeable batteries including: (a) means for supplying an electrical charging power to a rechargeable battery so as to rapidly charge said rechargeable battery; (b) means for periodically interrupting said electrical charging power at said rechargeable battery for a preselected time interval so as to permit the detection of die internal resistance free voltage of said battery during the interruption of said electrical charging power, wherein said preselected time interval is of a length sufficient to allow for the substantial recombination of any oxygen produce during charging and the internal resistance free voltage is detected after the substantial recombination of the oxygen, (c) means for comparing said internal resistance free voltage of said battery with a preselected reference voltage; and (d) means for reducing said electrical charging power when said internal resistance free voltage of said battery at least equals said preselected reference voltage so as to reduce die rate of charging of said rechargeable battery so as to maintain said internal resistance free voltage at a value which does not exceed said preselected reference voltage. 24. The charger of claim 23, wherein the resistance free voltage is detected 50-1000 msec after commencement of each interruption. 25. The charger of claim 24, wherein the resistance free voltage is detected 100-500 msec after commencement of each interruption. 26. The method of claim 1, wherein die interruption is of a length sufficient to allow for the substantial recombination of oxygen produced during charging and wherein the resistance free voltage is determined after such substantial recombination.

Inventors: ETTEL VICTOR ALEXANDER, NOR JIRI K, SOLTYS JOSEF V, ETTEL, VICTOR, ALEXANDER, NOR, JIRI, K., SOLTYS, JOSEF, V.
Applicants: INCO LTD, NORVIK TECHNOLOGIES INC, INCO LIMITED, NORVIK TECHNOLOGIES, INC.
Application year: 1993
Main classification: H02J7
IPC codes: H01M10, H02J7