GB2260868A - Recharger for dry cells and dry cell batteries - Google Patents

Abstract

The recharger comprises an A.C. source 10 connected to the dry cell or cells via a current control circuit 11 having parallel paths 14, 15 with oppositely directed diodes D1, D2 in series with respective transistorised trigger-operated back-to-back voltage follower circuits 12, 13. The resistor values in circuits 12, 13 are chosen so that for fully discharged cells the ratio of the reverse current to the forward, charging. current is 1:5, and this ratio approaches 1:1 as the cells become substantially fully charged so that the net d.c. component is then substantially zero. The reverse current path 15 is conductive for longer than the forward path 14 during each A.C. cycle, the circuit 13 effectively establishing an approximately constant reference current value corresponding to the value to which the forward, charging, current asyntopically declines. <IMAGE>

Description

"DRY CELL AND DRY CELL BATTERY RECHARGER" This invention relates to rechargers for normally-disposable dry cells and dry cell batteries.

The principles involved in recharging batteries are well known.

Moreover, circuits claiming to offer recharging capabilities for normally-disposable dry cells have been disclosed in the technical press from time to time, going back as far as 1960.

An occasional sudden total failure of such cells during recharging has been properly associated with the appearance of dendritic growths within the respective cells and measures have been proposed for reducing such occurrences, for example by providing current reversal during the recharging process. Typical of circuits having the required performance is a halfwave rectifier circuit having a series current limiting resistor and a shunt resistor across the rectifier to provide the current reversal in each negative half-cycle of the a.c. supply.

It should be noted that rechargers for secondary cells and rechargers for Ni-Cad rechargeable cells both provide uni-directional current, albeit of a pulsating characteristic.

The aforesaid dry cell recharging circuits, though believed to be successful in their recharging capabilities, have severe drawbacks in use, even for a qualified user. Because such circuits lack means for controlling the rectified current pulses, the only form of control is to limit the charging period. However, during recharging progressively less energy is stored in the cell being recharged. The excess energy is dissipated in the cell as heat.

This situation worsens as the charging progresses. On approaching maximum charge the cell exhibits a negative resistance characteristic, resulting in a surge of energy, virtually all of which is dissipated in heat. Under these circumstances a dry cell outgases profusely and it can rupture. Therefore, careful monitoring is required during the recharging process which is made more difficult by the fact that the observer will seldom be aware of the state of charge in the battery at the commencement of the charging process.

In the more successful of the aforesaid circuits the ratio of reverse current to forward (charging) current is arranged by virtue of the circuit parameters to be in the ratio of 1:5 However, these circuits are passive circuits lacking any means to control the current pulses and hence the aforesaid ratio remains substantially constant throughout the charging process. Work carried out by the present applicants has shown that the optimum ratio of 1:5 is only true in respect of conditions which exist at the commencement of a recharging cycle with a fully discharged battery and preferably the ratio should increase during the charging process to an optimum value of unity when approaching full charge.

Thus it is an object of the present invention to provide a circuit for recharging normally-disposable dry cells and dry cell batteries with which no supervision is required during the recharging process. Such circuits can therefore be used b) the lay-person with complete safety.

It is a further object of the invention to provide a circuit as aforesaid in which the energy delivered to the cell undergoing charge is progressively restricted as the battery approaches a fully recharged state.

It is a further object of the invention to control dendritic growth within the cell under charge by substantially increasing the ratio of reverse current to the charging current towards unity as the charging cycle progresses.

In accordance therewith the invention provides a rectifier charging circuit connected between an a.c. source and the cell or cells or battery under charge, said circuit having two parallelly connected loops containing respectively oppositely biased diodes and in which each circuit contains in series with its respective diode a trigger operated back-to-back voltage follower circuit.

Hereinafter the invention is described by way of examples in relation to the accompanying drawings, in which: Figure 1 shows a circuit configuration suitable for batteries of the Types AA, C, D; Figure 2 shows a circuit configuration suitable for recharging PP3-type batteries; Figure 3 reproduces a chart showing the charging characteristics of the circuits of Figures 1 and 2 over a charging cycle; and Figure 4 is a representative waveform diagram for the circuit of Figures 1 and 2 showing the currents flowing through the cell over each cycle of the a.c. source.

Referring to the drawings, there are shown in Figures 1 and 2 functionally identical circuits differing visually only in the form of the connections to the respective secondary windings of the a.c. mains transformer 10 used to provide a source of alternating current. In Figure 1 the voltage between points a, b is nominally 4.5 volts, in Figure 2 this voltage is nominally 9 volts. Each circuit has connected across points c, d the dry cell or battery for recharging. Figure 1 is configured for charging batteries of the type AA, C and D, the resistance values of the circuit differing in accordance with the type of battery being charged so as to obtain optimum charging characteristics for each battery type.

Figure 2 is configured for charging batteries of the type PP3.

Table 1 shows a table of values of the components to adapt the circuit of Figure 1 for each of the battery types AA, C and D.

TABLE 1 Battery Type AA C D F1 (Thermal) 102"C 102"C 102"C D1 or D2 1N4007 IN4007 IN4007 TR1/2/3/4 BC337 BC337 BC337 R1 3K3 820R 330R R2 12R 3R9 2R2 R3 62R 20R 11R R4 4K7 3K9 1K2 Table 2 shows a corresponding list of values for the circuit components of Figure 2 when used to charge PP3-type batteries.

TABLE 2 F1 (Thermal) 1020C D1 and D2 IN4007 TR1/2/3/4 BC337 R1 4K7 R2 56R R3 270R R4 18K The circuits of Figures 1 and 2 each comprise a rectifier 11 having respective diodes D1, D2 and respective currentmodulator circuits 12, 13 in each of two parallel loops 14, 15.

Modulator circuit 12 is formed by the components R1, TR1, TR2 and R2. Modulator circuit 13 is formed by the components R3, TR4, TR3 and R4.

In positive half-cycles of the source voltage between points a and b diode D1 is forward-biased and, subject to the control exerted by the current-modulator circuit 12, permits a charging current to flow through the dry cells connected between points c, d.

During this period diode D2 is biased to block any flow of current through the current-modulator circuit 13.

In the negative half-cycles of the source voltage diode D1 is reverse-biased and prevents current from flowing through the current-modulator circuit 12. Diode D2 is forward-biased and permits a current flow through the dry cells in the discharge direction via the current-modulator circuit 13.

The operation of the current-modulator circuit 12 is as follows.

The instant source voltage across a, b is compared by the modulator circuit 12 with the voltage across c, d (the cell terminal voltage).

When the source voltage rises during a positive going half cycle of the supply above the cell terminal voltage by a sufficient amount current flows through the cells to be charged, in the charging direction.

At the commencement of the positive half-cycle TR1 and TR2 are turned off. As the supply voltage climbs and passes through the trigger point (i.e. nominally at 1.2 volts above the cell terminal voltage) the transistor TR1 becomes conductive. This allows current to commence flowing with a magnitude which is limited by the value of the resistor R2. A volt drop is established across R2 causing transistor TR2 to turn on which then bleeds a current from the base of TR1 via the collector of TR2.

A s t h e voltage reduces during the second part of the positive half-cycle TR1 is turned off at an instant which is determined by the magnitude of the current bleed.

The result of the above operation is to produce a total forward-conduction "charge" area, for current passing through the dry cells in the charging direction, which is inversely related to the combined cell voltage. As the cells recharge and their terminal voltages approach the fully-charged nominal value so the current through the circuit is reduced asymtopically to a minimum level which is very much less than the initial charging current when commencing charging of the cells with the latter in a fully discharged condition.

During the negative half-cycles of the a.c. source voltage D1 is reverse-biased and becomes non-conducting, the voltage across R2 falls to zero and TR2 is turned off.

Diode D2 now becomes forward-biased and, when the transistor TR3 switches on at its trigger point, current is allowed to flow out of the cell being recharged in the discharge direction via the resistor R3.

Modulator circuit 13 is essentially similar to modulator circuit 12 but the circuit value of R3 is selected relative to the value of R2 so as to limit the current flowing through the circuit to provide an initial current ratio, with reference to the charging current, of 1:5 for a fully discharged cell.

Moreover, in respect of the operation of this circuit, the circuit triggers ON when the source voltage reaches a value corresponding to the cell terminal voltage less about 1.2 volts. The current drain through TR4 is less, hence the switch off point is later in the half-cycle. The result of these differences is that whilst the current flow is controlled to a level corresponding to the aforesaid ratio, the duration of conduction through modulator circuit 13 relative to the period of conduction in modulator circuit 12 is longer. Overall the effect is that modulator circuit 13 establishes a reference value of current, the value of which remains approximately constant during the charging process. By means of the selection of the circuit components this value is made to correspond to the value of current to which t h e charging current asymtopically declines.

It is seen therefore that as the cells become substantially fully charged the net d.c. component of current flowing therethrough becomes substantially zero. There remains a small ripple current which the dry cells are capable of withstanding without injurious effects. This should be contrasted with the prior circuits in which towards the end of the charging process there remains a substantial d.c. component of current flowing through the cells in the charging direction which, if allowed to continue, would eventually result in the appearance of the negative resistance characteristics referred to earlier. This flow of direct current, even if halted before the cells become completely charged, will cause unwanted heating of the cells.

It will be inferred from the above-described sequence of operation that because with the circuits of Figures 1 and 2 the forward current through the dry cells reduces to a value equal to the magnitude of the discharging current, the aforesaid ratio increases during the charging process progressively towards unity. This increase in the ratio is found to have a beneficial influence on the occurrence of dendritic growths which in turn leads to a longer life for the cells being recharged.

Battery fault conditions in relation to the circuits of Figures 1 and 2 are manifest either as high cell impedance with abnormally high cell voltage or short-circuit with almost no cell voltage. In the former situation, during the positive halfcycles when the a.c. voltages reaches a value of voltage corresponding to the cell voltage plus 1.2 volts, no further circuit triggering will be possible and charging will cease. In the latter circumstances the circuit will attempt to feed current to the cell but the current is kept to safe levels by the limiting resistors R2 and R3. To cope with any extreme conditions which might occur, thermal fuse F1 cuts off the mains supply.

Figure 3 shows the terminal voltage characteristics. It is to be observed that these charging characteristics are parabolic in form, which results from the charging current dropping off fairly rapidly in the first part of the charging process and diminishing substantially asymtopically as charging progresses beyond the aforesaid first portion, to a near zero value. The characteristic demonstrates that whilst the cells being recharged become charged up to around 95 per cent of their capacity within a normal charging period (a s s urn i n.g t-h a=-t they were substantially discharged at the commencement of the charging period) they never become completely charged, no matter how long the charging is allowed to proceed.

Figure 4 shows a typical waveform characteristic for the circuits of Figures 1 and 2. The upper waveform trace represents the a.c. source voltage as applied across the fullwave rectifier.

The lower waveform trace shows the current flowing through the cells undergoing charge during a cycle of the mains frequency.

As the cells become fully charged the peak amplitude of the cell current diminishes and the areas under the positive and negative half-cycles approach equality.

Claims (6)

1. A dry-cell or dry-cell battery recharger comprising a rectifier charging circuit, connected between alternating current source means and the cell or battery connection means, having two parallelly connected loops containing oppositely biased diodes in which each loop contains, in series with the respective said diode, a trigger operated back-to-back voltage follower circuit.

2. A recharger as claimed in Claim 1 wherein each voltage follower circuit comprises a pair of transistor devices in which the emitter of one device of each pair is connected to the base of the other device of the pair whilst the base of the first said device in each pair is connected to the collector of the second said device in each pair.

3; A recharger as claimed in Claim 2 wherein in one of said loops the collector and base electrodes of the first said transistor device of the respective voltage follower circuit are ohmically connected to the aforesaid respective diode whilst the emitter and base electrodes of the corresponding second said transistor device are ohmically connected to the cell or battery connection means and in the other loop the collector and base electrodes of the first said transistor device are ohmically connected to cell or battery connection means and the emitter and base electrodes of the second transistor device are ohmically connected to the respective diode.

4. A recharger as claimed in any preceding claim wherein the circuit components are chosen such that with respect to a charging current of the cell or battery in the forward conducting loop current flows for a relatively short variable period of each alternate half-period of the alternating source whereas in the reverse conducting loop current flows for substantially the whole of the intermediate half-periods of said source.

5. A recharger as claimed in any preceding claim having circuit values adjusted for safe charging of Ni-Cad cells and batteries.

6. A recharger substantially as described herein with reference to the accompanying drawings.