WO1994029940A1

WO1994029940A1 - Dry cell recharger

Info

Publication number: WO1994029940A1
Application number: PCT/GB1994/001211
Authority: WO
Inventors: Andrew David White
Original assignee: Innovations Group Limited
Priority date: 1993-06-03
Filing date: 1994-06-03
Publication date: 1994-12-22
Also published as: AU6852694A; GB9311462D0

Abstract

A recharger applies a high frequency (about 100 kHz) preferably square wave pulsed d.c. current to a primary or secondary cell (BT1) to be recharged. An oscillator (26) supplies square waves to a current buffer (20), which draws current at terminal (16) when the input is on to cause the battery (BT1) to be recharged via resistor R2. A load (R20) is connected across the battery (BT1) by transistor (Q1) during the charging pulses, and the voltage on the battery terminals (21, 22) is allowed to float between charging pulses. Comparators (28, 29) prevent charging when the battery voltage is below 1 V or above 1.6 V, respectively corresponding to unchargeable and fully charged conditions of a primary cell battery (BT1). This can be changed to 0 V and 3.6 V for secondary e.g. NiCad, cells. Switch S3 allows for the charging current to be varied depending on battery size. The circuit is arranged such that the falling edges of the charging current pulses produce a short spiked reverse pulse. The above features may be used with a pulsed charging current in general, in place of the high frequency current.

Description

Dry Cell Recharger

The present invention relates to a method and apparatus for recharging dry cells and dry cell batteries, for example of the alkaline, zinc chloride or zinc carbon types. The invention relates especially, though not exclusively, to the recharging of primary dry cells, i.e. cells not designed to be recharged. Primary dry cell batteries are a convenient and popular source of electrical energy. They are designed to produce electricity for an extended period before becoming completely discharged, and normally no regard is given to the possibility of their recharge. They are merely disposed of when exhausted. The electricity which they produce is, however, expensive. Therefore, to reduce expense, and maximise the useful life of primary dry cell batteries, a number of methods have been proposed to allow their recharge. Many of these methods apply a pulsed d.c. current from a half-wave rectifier to the battery in the reverse direction to the battery discharge direction. To provide useful results, it has been found necessary to provide a shunt resistor across the rectifier to allow a small reverse current (i.e. in the normal battery discharge direction) to flow through the cell during the intervals between the half-wave-rectified charging current pulses. It is thought that this small reverse current produces a more even replating of, for example, zinc on the battery electrodes.

Such methods, however, are not ideal, and it is an object of the present invention to provide an alternative and advantageous recharging method and apparatus. According to the present invention, there is provided a method of recharging a dry cell by applying a pulsed high frequency d.c. current to the cell in the reverse direction to that in which the cell discharges. The invention also provides a dry cell rec arger comprising means for applying a pulsed high frequency d.c. current to a cell. The invention extends to recharging dry cell batteries, and reference to dry cells should be taken to include reference to dry cell batteries also.

By high frequency is meant a frequency at least an order of magnitude greater than that of a mains a.c. power supply. The frequency is preferably between 10kHz and 1000kHz, and the invention has been found to work especially well with frequencies between about 50 and about 600 kHz. A frequency of about 100 kHz is especially advantageous. Such high frequencies contrast with the prior art, in which the half-wave-rectified current from a standard a.c. supply will only provide a pulse frequency of about 50 Hz.

The present invention provides an efficient and practical recharging method. It is thought that the use of a large number of small high frequency current pulses, as opposed to fewer much longer pulses, promotes a more even coating of material on the cell electrodes, and reduces the number of dendritic growths within the cell.

In a preferred form, the cell is allowed to float during the periods between the charging pulses. The prior art practice of using a small reverse current between the charging current pulses is not necessary. The high frequency current pulses may be produced in any suitable manner, and make take any suitable waveform. Preferably, the current is of a substantially square waveform, and is preferably produced by a high frequency oscillator, the output of which is coupled to the cell via a current buffer which may have an open collector configuration. The buffer could comprise a circuit for switching a terminal of the cell between high and low voltages in response to the high or low going phase of the waveform.

Preferably, the recharger is designed to allow the cell, acting as a leaky capacitor, to produce short spiked reverse current pulses in response to the falling edges of the charging current pulses. In this regard, a square charging current waveform is particularly advantageous.

These reverse current spikes can further improve the recharging, and are thought to promote an even replating of the cell electrodes in a similar manner to that of the small reverse current in the prior art.

In a particularly preferred form of the invention, the cell is on load during charging. It is possible to leave the cell on load between charging pulses, but it is preferred that the load be removed between pulses. Accordingly, means may be provided to switch a load, for example a load resistor, across the dry cell during each charging pulse, and to remove the load during the intervals between the charging pulses. A load in the region of about 50 to about 60 Ω has been found to produce particularly advantageous results, as has the use of a square charging current waveform with the load, and the floating of the cell between pulses. The load is thought to promote a more even replating of the cell electrodes.

Although at first sight a. load will not affect the charging, it has been found that the interaction of the load with the inherent reactance of the cell results in a complex current waveform with a reverse spike, which has been found to significantly improve the effectiveness of the recharging.

Preferably, the voltage developed across the cell is monitored throughout the recharging process, and the recharging may be halted when the cell voltage is determined to be below a lower limit and/or above an upper limit. If a load is provided, this should be removed from the cell when charging is stopped.

The lower limit may be set, for example at 1 volt, such that a cell voltage lower than the limit indicates that the cell is old and has been completely or substantially discharged, in which case it is generally unsuitable for recharge. The upper limit may be set, for example at 1.6 volts, such that a cell voltage higher than the limit indicates that the cell has been recharged as far as possible. In this case, further charging would be of little benefit and could even cause cell damage.

To provide optimum recharging conditions, the size of the recharging current may be altered in accordance with the specific type and/or size of cell to be recharged. This may be done by varying the size of a series resistor through which the recharging current flows, by varying the voltage of the charging current, or by varying the width of the charging pulses, for example by varying the mark/space ratio of the waveform of the above-mentioned oscillator system. Preferably an open-collector current buffer is controlled to sink or source a variable current.

The charging current will inherently tend to diminish during recharge with increase in cell voltage, because the voltage difference between the cell and charging voltages is reduced. The size of the current will, of course, affect the time taken for recharge to occur.

All of the above-mentioned features, such as the loading of the cell during recharge, the floating of the cell terminals between charging pulses, and the production of a spiked reverse pulse at the falling edge of a charging pulse, have been found to be advantageously applicable to rechargers which use pulsed charging currents in general, and not just to rechargers using high frequency pulses. The invention therefore extends to rechargers in general which include means for applying a pulsed charging current to a cell, and which incorporate any of the above features, such as means for loading the cell during charging pulses, and/or means for floating the voltage on the cell terminals between charging pulses, and/or where the charger is adapted to produce a reverse spiked pulse at the falling edge of a charging pulse.

The invention has also been found to work well when used to charge secondary cells, such as NiCad batteries. Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a recharger in accordance with a first embodiment of the invention; and Figure 2 is a more detailed diagram of a second embodiment;

Figure 3 is a circuit diagram according to a third embodiment; and

Figure 4 shows a charging current waveform of Figure 3.

Referring to Fig.l oscillator 1 produces a high frequency, approximately 100kHz, train of square pulses of set mark/space ratio. This is supplied, via a switch circuit 2, to a current buffer 3, of open collector configuration, the output of which is fed via a series resistor Rl to a battery 4 which is to be recharged. Resistor Rl, along with the charge voltage defined by the current buffer 3, and the charge current pulse widths defined by the mark/space ratio of the oscillator 1, together define the size of the charging current.

This may be varied depending on battery type.

A load 5 is provided across the battery 4, and a switch 6 puts the battery on load only during the application of the charging pulses. Switch 2 cuts off the output of oscillator 1 from the buffer 3 during the non-charging interval between the pulses, and the switch 2 and buffer 3 allow the battery terminal voltage to float.

Voltage detector 7 constantly monitors the battery voltage. If the voltage is below about 1.0V, the battery is considered to be too old and to have been discharged too much for recharging to be practical. If the voltage is above about 1.6V, charging is considered to be complete (at this point further charging would provide little benefit and could even damage the battery) . In either case, the detector 7 opens switch 2 to cut off the oscillator signal and thereby halt recharge. It further opens switch 6 to prevent discharge of the battery 1 across the load 5.

Referring to Fig. 2, a circuit similar to that of Fig. 1, but without a load, is shown in more detail. A square wave signal 8 of set mark/space ratio is applied, via a resistor RO and transistors T3 and T4, to transistors Tl and T2. Assuming T3 and T4 to be enabled to switch, Tl conducts when signal 8 is high, and is non-conductive when signal 8 is low. The opposite is true for T2. Thus, when signal 8 is high, a current at about 1.7 volts is applied to the battery 10 via resistor Rl, and when the signal 8 is low, the battery discharges through resistor R2. Thus, the circuit allows a high frequency pulsed current to be applied to the battery to effect a recharge.

The battery voltage VBAT is monitored via filter RC1, and comparators OP1 and OP2 determine whether this voltage is within preset limits VRl and VR2, here being 1.6V and 1.IV respectively. If VBAT (filtered) is below the VR2 threshold, then the OP2 output is low, and T4 and T3 cannot turn on Tl or T2. If VBAT is above VR2, then the OP2 output is high, and the signal 8 may flow through transistors T3 and T4 to Tl and T2 to produce the desired current waveform on the battery 10 and to begin recharge. If

VBAT exceeds VRl, then OP1 goes high. This changes the reference VR2 of OP2, and forces OP2 low again, thus ending the recharge process.

The high voltage rail of the circuit is provided with a voltage regulator 11, and circuit RC2 removes heavy spikes present on the V+ and V- rails, caused by signal 8. Light emitting diodes 12 and 13 respectively provide an indication of an old or fully charged battery.

Figure 3 is a schematic diagram of a preferred form of battery charger in accordance with the invention. The charger is designed to charge up to 4 cells simultaneously and may be used for charging primary cells, such as zinc-carbon or manganese-alkaline cells or secondary cells such as nickel-cadmium cells.

Each cell is connected at terminal 21 to a positive DC supply VDD, e.g. of 5V, and at terminal 22 to a current sinking circuit 20. The circuit 20 is preferably an open-collector current buffer which controls the flow of current through the cell to ground via resistor R2. The circuit 20 may be constructed as a custom integrated circuit consisting of four current buffers for the respective cells (referred to herein as channels 1 to 4) and in the preferred embodiment also incorporates three inverters which are used to form an oscillator as described below.

The circuit contains resistors R21 to R25 and capacitor C6 which are common to all four channels. Three inverters are connected to the lines labelled 5, 6, 7, 11, 10, and 12 to form a feedback loop in which resistors R24 and R25 and capacitor C6 define the oscillator time constant. The oscillator outputs on lines 10 and 11 are in anti-phase; two of the batteries on channels 1 and 2 are supplied with current simultaneously whilst the other two on channels 3 and 4 are supplied simultaneously but in the periods in which channels 1 and 2 are turned off. This reduces the peak current which has to be supplied by the charger. All the components to the right of resistor R28 are repeated for the other three channels, but are not shown in the figure.

The battery charger is powered by a mains power supply, e.g. producing 9V DC, which is supplied to the input terminal 23. A voltage regulator 24 produces a regulated 5V output which is supplied to the charging circuit and also via a filter R8, C5 to a reference voltage generating circuit 25 which will be described later.

The oscillator 26 supplies square waves at about 100kHz to the current buffer 20 via resistor R28. The current buffer draws current at terminal 16, when the input is on, and this causes the battery to be charged via the resistor R2 as mentioned above. Terminal 17 of circuit 20 produces an output essentially in anti-phase to that of terminal 16, in other words terminal 17 goes high when the circuit 20 is sinking current. The output from terminal 17 turns on transistor Ql which has the effect of connecting load resistor R20 across the battery during the periods when it is being supplied with current. During these charging periods light emitting diode D4 lights to provide an indication to the user that the cell is being charged. The amount of current drawn by circuit 20 is controlled by a reference input at terminal 8. In the embodiment shown, the voltage at this terminal may be switched by switch S3 to three different values, to provide three different fundamental currents. The user selects the appropriate current level depending on the type and size of cell being charged. The circuit 20 may include timing components arranged to compensate for the switching time of transistor Ql to ensure that the cell is put on and off load in synchronism with the turning on and off of the charging current.

The terminal voltage on the battery is monitored by circuit 27. This consists of two comparators 28 and 29 with their outputs wired together. When either output goes low the input of circuit 20 is effectively grounded by diode D5, but when both outputs are high diode D5 is reverse-biased and pulses may pass from resistor R28 to circuit 20.

The battery terminal voltage is supplied to first terminals of the comparators 28 and 29 via a filter comprising resistor R3 and capacitor C2. The second input terminals are supplied with respective reference voltages.

The reference voltage supplied to the inverting input of comparator 28 is set by variable resistor 30 and the reference voltage to the non-inverting input of comparator 29 is provided by variable resistor 31. The former is set to 3.4V for primary cells and the latter to 4V. It should be appreciated that as the battery charges the voltage at terminal 22 decreases with respect to ground. For example, with a fully discharged battery of less than IV output, the voltage at the lower terminal 22 will be above 4V with respect to ground and therefore the output of comparator 29 will go low to stop the charging. Between IV and 1.6V the terminal voltage is between 4V and 3.4V so the outputs of both comparators are high and the diode D5 is cut off. When the battery voltage is above 1.6V the terminal is below 3.4V and the output of comparator 28 goes low, again blocking the oscillator pulses from the current buffer circuit 20. Comparator 28 is provided with hysteresis by resistors R27 and R29 and it is found in practice that the charging process cycles on and off relatively slowly (e.g. every 20 seconds) once the cell is fully charged. An interesting advantage of the hysteresis circuit is that an unserviceable cell discharges relatively rapidly and the relatively rapid flashing of LED D4 indicates to the user that the cell is unserviceable.

It will be noted that the reference voltage generating circuit 25 comprises a switch S2 which may be used to alter the reference voltages when nickel-cadmium cells are being charged. These have a lower terminal voltage of about 1.3V when fully charged. Closing switch S2 causes the first reference level to be set to 3.6V and the second to 5V. This means that there is no lower limit on the battery voltage below which charging will not take place, and charging is stopped when the battery voltage reaches 1.4V. Referring to Figure 4, the charge current waveform as seen on a storage oscilloscope is shown. During a charging cycle (a) , the current varies due to the rate of change of the impedance which is dependent on battery age, type, condition, etc. The negative spike produced at (b) is caused by internal inductive effects within the battery which result in a type of back EMF. The spikes are thought to help to reduce dendritic growth within the battery. During phase (c) , the cell is allowed to float back to its open circuit state, with the load removed and this feature has also been found to improve the charging of the cell.

The above embodiments all relate to the application of a high frequency pulsed d.c. current to a cell. However, the various features described in the above embodiments, such as the loading of the cell during charging, the floating of the cell terminal voltages between charging pulses, and the production of a short spiked reverse pulse, may all be applied to rechargers in general, and not just to those using high frequency currents. The above embodiments may easily be adapted to produce lower frequency pulses of charging current.

Claims

1. A method of recharging a dry cell, in which a pulsed high frequency d.c. charging current is applied to a cell in the reverse direction to that in which the cell discharges.

2. A method of recharging a dry cell according to claim 1, wherein the cell is put on loading during the charging pulses.

3. A method of recharging a dry cell according to claim 1 or 2, wherein the voltage on the cell terminals is allowed to float between charging pulses.

4. A method of recharging a dry cell according to claim 1, 2 or 3, wherein a short spiked reverse current is produced in response to the falling edge of a charging current pulse.

5. A dry cell recharger comprising means for applying a pulsed high frequency d.c. current to a cell to be recharged.

6. A dry cell recharger according to claim 5, wherein means are provided for enabling the voltage on the cell terminals to float during periods between charging pulses.

7. A dry cell recharger according to claim 5 or 6, wherein means are provided for putting the cell on load during the charging pulses.

8. A dry cell recharger according to claim 7, wherein the load is removed from the cell between charging pulses.

9. A dry cell recharger according to any of claims 5 to 8, wherein the recharger is adapted such that a short spiked reverse current pulse is produced in response to the falling edge of a charging current pulse.

10. A dry cell recharger comprising means for applying a pulsed charging current to a cell to be recharged and means for putting the cell on load during the charging pulses.

11. A dry cell recharger according to claim 10, wherein the load is removed from the cell between charging pulses.

12. A dry cell recharger comprising means for applying a pulsed charging current to a cell to be recharged and means for floating the voltage on the terminals of the cell between charging pulses.

13. A dry cell recharger comprising means for applying a pulsed charging current to a cell to be recharged and wherein the recharger is adapted such that a short spiked reverse current pulse is produced in response to the falling edge of a charging current pulse.

14. A dry cell recharger according to any of claims 5 to 13, wherein the charging current has a substantially square waveform.

15. A dry cell recharger according to any of claims 5 to 14, wherein the curent is provided by an oscillator whose output is coupled to a current buffer.

16. A dry cell recharger according to claim 15, wherein the current buffer has an open collector configuration.