WO2010015855A2

WO2010015855A2 - Recharging batteries

Info

Publication number: WO2010015855A2
Application number: PCT/GB2009/050979
Authority: WO
Inventors: Martin James Bellamy
Original assignee: G24 Innovations Limited
Priority date: 2008-08-06
Filing date: 2009-08-05
Publication date: 2010-02-11
Also published as: GB0814415D0; WO2010015855A3

Abstract

An arrangement for charging a rechargeable battery (2) from the output current of a photovoltaic transducer (1) is described. When the output voltage of the photovoltaic transducer (1) is at a high level, which indicates that the output current of the transducer (1) is sufficient to charge the battery, the transducer (1) is connected directly to the battery (2). However, if the output voltage falls below this level, then charge from the transducer (1) is stored in a capacitor (7) until such time as the voltage across the capacitor (7) rises above a predetermined level V₂ at which point a switch (11) is closed which causes the capacitor (7) todischarge into the battery (2). When the voltage across the capacitor (7) falls below a lower predetermined level V₃, the switch (11) is then opened so as to terminate the discharge of the capacitor (7). The invention is of particular application to dye-sensitised photovoltaic transducers which generate small output currents in response to low levels of ambient light.

Description

RECHARGING BATTERIES

The present invention relates to the charging of rechargeable batteries, and in particular to charging such batteries using the output electrical current of a photovoltaic transducer.

A photovoltaic transducer responds to incident light by generating a voltage across its terminals which can give rise to an electric current in an external circuit. The term "light" as used throughout this specification is intended to refer not only to visible radiation but also to infrared and ultraviolet radiation. In this way, the power of the incident light is converted into useful electric power. The magnitude of the resulting voltage and/or current depend on a number of factors, including the wavelength and intensity of the incident light and the chemical and physical characteristics of the photovoltaic transducer.

Photovoltaic transducers are normally manufactured as an array of photovoltaic cells which are electrically connected together in series, such that the output voltage is the sum of the individual voltages generated across each of the cells. However, since the magnitude of the generated current depends on the area of the light-sensitive surface of the photovoltaic transducer, the photovoltaic cells could alternatively be electrically connected together in parallel so as to increase the effective surface area exposed to the incident light, the effect of which will be that, whereas the voltage remains the same as with an individual cell, the magnitude of the current generated in an external circuit is increased. It is now possible to manufacture photovoltaic cells which are connected together in series and in which the length of the cells in the array, and hence the area of light-sensitive surface, is selected such that a desired level of current is generated. In practice, such cells can be manufactured in the form of a flexible sheet or web, and the desired length is simply cut from the web at the end of the manufacturing process.

A type of photovoltaic transducer to which the present invention is particularly applicable is the dye-sensitised type. This type of transducer is able to respond to incident light over a wide range of intensity. However, at low levels of incident light, the level of the output current is relatively small, typically measured in microamps.

In order for a rechargeable battery to accept the output current from a photovoltaic transducer as a charging current, the level of the output current must be above a first critical value. If the output current is below this value, the battery will not accept any charge, and the output current will simply be wasted. It is only when the output current rises above this critical value, that the battery will accept any charge.

For example, a commercially available NiMH rechargeable battery has a capacity C of 1 250 mAh, and has a critical value of 15 mA below which it will not accept any charging current, assuming that the battery manufacturer's voltage requirements are adhered to.

Although it would be possible to charge a rechargeable battery from the output current of a photovoltaic transducer simply by connecting the output terminals of the photovoltaic transducer directly to the battery terminals, this is not an efficient way of recharging the battery, since, in conditions of low light intensity, such as at sunrise or sunset or during overcast days, the typical output current of photovoltaic transducers is less than this critical value.

It would therefore be desirable to provide an arrangement which is able to generate a charging current for a rechargeable battery which is always above this critical value.

Thus, in accordance with a first aspect of the present invention there is provided apparatus for receiving electrical charge generated at a variable rate by a photovoltaic transducer and for supplying the charge at a rate above a predetermined threshold to a rechargeable battery, the apparatus comprising: means for receiving the generated charge from the photovoltaic transducer; means for storing the received charge; level sensing means for sensing when the level of stored charge is greater than a first predetermined value and for generating a first output signal in response thereto; and charging means responsive to the first output signal for supplying at least some of the stored charge to the rechargeable battery at a rate above the predetermined threshold.

With such apparatus, it is possible to collect charge generated by a photovoltaic transducer, even in conditions of low ambient light, and to use the charge to generate a charging current of a suitable level for the efficient charging of a rechargeable battery.

Provided that the output current of the photovoltaic transducer and the capacity of the rechargeable battery are appropriately selected for a given application, it is not normally possible to overcharge the battery. For example, if a given application requires a constant current of 1 amp over a period of 5 days, i.e. 120 hours, the required battery capacity C is 120 amp-hours. However, if the local climate provides 5 "sun hours" per day, the average output current required of the photovoltaic transducer will be 120 amp- hours divided by 5 hours, i.e. 4.8 amps. In this example, the average charging current is:

4.8 amps = 120 amp-hours / 25 hours = C / 25.

However, rechargeable batteries can become damaged through overcharging when the charging current exceeds 1 C, so that the problem of overcharging is therefore not likely to arise.

While the stored charge is being supplied as a charging current for the rechargeable battery, it is desirable to continue to monitor the level of charge in the storing means to ensure that it remains sufficient to enable a charging current of a desirable level to be supplied to the rechargeable battery. Thus, the level sensing means is preferably also arranged to sense when the level of charge stored is less than a second predetermined value, which is less than the first predetermined value, and for generating a second output signal in response thereto, and the charging means is then preferably arranged to terminate the supply of the stored charge to the rechargeable battery in response to the second output signal.

With this arrangement, the charging current supplied to the rechargeable battery is always within a predetermined range. By appropriate selection of the first and second predetermined values, it is therefore possible to ensure that the charging current always exceeds the minimum at which the rechargeable battery will accept the charge.

Since the capacity of the storing means is finite, the amount of charge supplied by the storing means in the form of a charging current for the rechargeable battery cannot exceed the actual output current of the photovoltaic transducer for any considerable time. It follows that the long-term average of the charging current will be substantially the same as the long-term average of the output current from the photovoltaic array. In order for the instantaneous value of the charging current to be above the predetermined level, the charging current is therefore generated in pulsed form. Each pulse is initiated when the level of charge in the storing means exceeds the first predetermined value and terminated when the level of charge in the storing means falls below the second predetermined value threshold.

The storing means is conveniently embodied as a capacitor, in which case the level sensing means is preferably arranged to sense the voltage across the terminals of the capacitor. It will be appreciated that, in an ideal capacitor, the voltage across the terminals, is directly proportional to the quantity of charge stored in the capacitor. Thus, the measured voltage will provide a direct indication of the amount of charge in the capacitor.

This voltage can then conveniently be compared with a first reference voltage representing the first predetermined value, e.g. using a standard voltage comparator.

When the level sensing means is arranged to sense when the level of charge stored is less than the second predetermined value, the level sensing means is preferably arranged to compare the sensed voltage with a second reference voltage representing the second predetermined value, e.g. using a second standard voltage comparator.

Alternatively, the storing means may comprise a further rechargeable battery which is capable of accepting varying levels of charging current over a wide range.

The charging means may be embodied as a simple electrical switch which acts to selectively connect the storing means to the rechargeable battery.

The resulting arrangement is particularly simple in structure, since, when the voltage across the capacitor exceeds the first reference voltage, the electrical switch connects the capacitor to the rechargeable battery, whereupon the rate of discharge is at its maximum value and gradually reduces until the voltage across the capacitor falls below the second reference voltage, at which point the electrical switch disconnects the capacitor from the rechargeable battery. By selecting the first and second reference voltages appropriately, it can be ensured that the resulting charging current remains within a desired range for the particular rechargeable battery.

It will be appreciated that, should the level of output electrical current from the photovoltaic transducer be at a level sufficient to act as an efficient charging current for the rechargeable battery, then the function of the storing means would not be required. In this case, it could be desirable to supply the output electrical current directly from the photovoltaic transducer to the rechargeable battery.

Thus, the apparatus preferably further comprises means for determining if a parameter of the electrical output of the photovoltaic transducer is greater than a predetermined value and for generating a third output signal in response thereto, and means responsive to the third output signal for supplying the received electrical charge generated by the photovoltaic transducer directly to the rechargeable battery without passing through the storing means.

The determining means could be embodied as a voltage comparator which detects when the output voltage of the photovoltaic transducer is greater than a set voltage and, in response, operates an electrical switch for diverting the output current from the photovoltaic transducer directly to the rechargeable battery.

Alternatively, the electrical switch could be controlled in dependence on the measured output electric current from the photovoltaic transducer.

It would alternatively be possible simply to allow the electrical output of the photovoltaic transducer to be passed to the storing means at all times, provided that this is not detrimental to the storing means.

A diode is preferably connected between the receiving means and the storing means, such that the output electrical current from the photovoltaic transducer can flow in only a single direction. This effectively prevents any charge from leaking back into the photovoltaic transducer from the storing means.

Although the apparatus defined above could be electrically connected to a separate rechargeable battery, the apparatus preferably includes the rechargeable battery itself, since this enables the charging means to be arranged to supply the charge to the rechargeable battery at a rate which is tailored to the rechargeable battery to be sufficient for the charge to be stored . Equally, although the apparatus defined above could be electrically connected to a separate photovoltaic transducer, the apparatus preferably includes the photovoltaic transducer itself, since this can ensure that the photovoltaic transducer is tailored to the rechargeable battery, for example by selecting the maximum output voltage of the photovoltaic transducer to be approximately equal to the voltage rating of the rechargeable battery. For example, if the photovoltaic transducer comprises an array of photovoltaic cells connected in series, and the output voltage of each cell is 0.5 volts, then a rechargeable battery rated at 1.5 volts can be recharged effectively without the risk of being damaged by overcharging, by selecting as the photovoltaic transducer a series connection of three such photovoltaic cells. Although it is not critical to match these voltages, the resulting arrangement makes efficient use of the output of the photovoltaic transducer.

Alternatively, if the only available combinations of photovoltaic transducer and rechargeable battery are not matched with respect to voltage, it would be possible to include a dc-dc voltage converter between the storing means and the rechargeable battery so as to increase or decrease the voltage level from that of the photovoltaic transducer output so as to match the voltage rating of the battery. This is not always necessary, since it is possible to connect a photovoltaic transducer with a nominal output voltage of 12V directly to a rechargeable battery rated at 1.5V without damaging either the photovoltaic transducer or the rechargeable battery, assuming that the level of the output current of the photovoltaic transducer is within the appropriate range for efficient charging of the rechargeable battery. However, as the difference between the output voltage of the photovoltaic transducer and the rated voltage of the rechargeable battery increases, the charging becomes less efficient.

Conventional solar cells, such as those manufactured from silicon, will generate a significant electric current only when positioned directly in strong sunlight, whereas dye- sensitised photovoltaic transducers are able to generate a significant electric current over a much wider range of values of ambient light intensity, including conditions of relatively low light intensity, such as at sunrise, sunset and in cloudy or overcast conditions, or even in artificially illuminated environments, both indoors and outdoors. The versatility of such photovoltaic transducers means that they are suitable for use in environments where the level of sunlight cannot be easily predicted. In such environments, the amount of current generated by a photovoltaic transducer could change from a low current level to a current level orders of magnitude higher.

It is therefore preferred that the photovoltaic transducer is of the dye-sensitised type and comprises an array of at least one dye-sensitised photovoltaic cell.

It will be appreciated, that the usefulness of the present invention manifests itself particularly well with such photovoltaic transducers, since even low levels of light will cause the photovoltaic transducer to generate at least a small amount of charge, and this can be stored until such time as there is a sufficient level of stored charge to enable the rechargeable battery to be supplied with a suitable level of charging current at which at least some of the charge can be stored.

The photovoltaic transducer can be in the form of a flexible web or sheet. In this way, the transducer can be configured such that it can receive incident light from more than one direction, so as to enhance the amount of light captured and hence increase the output current. Such an arrangement gives rise to a large number of different applications. For example, the photovoltaic transducer could be attached to, or form at least a part of, an item of outer clothing and used to power an electrical device such as a personal stereo player, a mobile telephone or a satellite-based locating system such as a

GPS device.

Furthermore, the apparatus could form part of, or be attached to, a potable container such as a back-pack, a rucksack, a briefcase or suitcase.

The present invention extends to a portable electrical device which is arranged to be supplied with power from a rechargeable battery, in which the electrical device includes apparatus of the above type.

In addition, the portable device may comprise the rechargeable battery.

The present invention further extends to a method of receiving electrical charge generated at a variable rate by a photovoltaic transducer and supplying the charge at a rate above a predetermined threshold to a rechargeable battery, the method comprising: receiving the generated charge from the photovoltaic transducer; storing the received charge; sensing when the level of stored charge is greater than a first predetermined value; and supplying, in response thereto, at least some of the stored charge to the rechargeable battery at a rate above the predetermined threshold.

The method preferably further comprises sensing when the level of charge stored is less than a second predetermined value, which is less than the first predetermine value, and terminating the supply of the stored charge to the rechargeable battery in response thereto.

Preferred embodiments of the present invention will now be described with reference to the accompany drawings, in which:

Figure 1 illustrates a schematic circuit diagram for apparatus of a preferred embodiment of the present invention;

Figure 2 is a flowchart illustrating the first part of the method of a preferred embodiment of the present invention;

Figure 3 is a flowchart illustrating the second part of the method of the preferred embodiment of the present invention;

Figure 4 is a graph illustrating the charging and discharging cycles of a capacitor used as the storing means of the apparatus of Figure 1 ;

Figure 5 illustrates a further embodiment of the present invention in the form of an article of clothing;

Figure 6 illustrates a further embodiment of the present invention in the form of a rucksack; and

Figure 7 illustrates a further embodiment of the present invention incorporated into a mobile telephone.

Referring to Figure 1 , an embodiment of the present invention is described in which the output electrical current from a photovoltaic transducer 1 is used to generate a charging current for a rechargeable battery 2. The photovoltaic transducer 1 is in the form of a flexible dye-sensitised photovoltaic cell array, the output of which is connected via a diode 3 to a two-way switch 4 which is controlled by the output of a first voltage comparator 5. The diode 3 serves to prevent any current from flowing back from the battery 2 into the photovoltaic transducer 1 such as might otherwise occur when the battery voltage is greater than the instantaneous output voltage of the photovoltaic transducer 1. The first voltage comparator 5 is supplied at a first input with the output of the photovoltaic transducer 1 and is supplied at a second input with a reference voltage V₁. The output of the first voltage comparator 5 is supplied as a control signal to the two- way switch 4. The first voltage comparator 5 is supplied with power from the rechargeable battery 2 via a suitable connection. The two-way switch 4 selects one of two possible paths for the output current from the photovoltaic transducer 1 , in dependence on the value of the control signal supplied by the first voltage comparator 5. The first path 6 connects the output current to a charge store in the form of a capacitor 7. The second path 8 bypasses the capacitor 7 and supplies the output current directly to the rechargeable battery 2. This arrangement causes the output current from the photovoltaic transducer 1 to be supplied (a) to the capacitor 7 when the level of the output current from the photovoltaic transducer 1 is below the threshold at which any charge can be received by the rechargeable battery 2 and (b) direct to the rechargeable battery 2 when the level of the output current is greater than this threshold.

The level of charge in the capacitor at any time is monitored by second and third voltage comparators 9, 10, which are also powered by the rechargeable battery 2 using suitable connections. The second voltage comparator 9 is supplied at a first input with a signal representing the voltage across the capacitor 7, and at a second input with a second reference voltage V₂. The third voltage comparator 10 is supplied at a first input with a third reference voltage V₃, which is lower than the second reference voltage V₂, and at a second input with the same signal as is supplied to the first input of the second voltage comparator 9, representing the voltage across the capacitor 7. The output of the second voltage comparator 9 acts to close a switch 11 in the event that the voltage across the capacitor 7 exceeds the second reference voltage V₂. The switch 11 is connected between the output of the capacitor 7 and the rechargeable battery 2. The output of the third voltage comparator 10 acts to open the switch 11 in the event that the voltage across the capacitor 7 falls below the third reference voltage V₃, thereby terminating the supply of charge from the capacitor 7 to the rechargeable battery 2. The value of the third reference voltage V₃ is selected such that the charging current supplied to the rechargeable battery 2 is always greater than the above-mentioned threshold at which the rechargeable battery 2 starts to accept charge.

The operation of the circuit will now be described with reference to the flowcharts of Figures 2 and 3.

Referring to Figure 2, the output voltage of the photovoltaic transducer 1 is first detected (step 12), and a determination made as to whether or not the output voltage is greater than a first reference voltage V₁ (step 13). If the output voltage is not greater than V₁, then the output current from the photovoltaic transducer 1 is stored in a capacitor 7 (step 14), and the output voltage continues to be monitored (step 12). If, however, the voltage is greater than V₁, then the output current from the photovoltaic transducer 1 is supplied directly to the rechargeable battery 2 (step 15), but the output voltage still continues to be monitored (step 12).

Referring to Figure 3, the voltage across the capacitor 7 is sensed (step 16) and a first determination made as to whether the capacitor voltage is greater than the second reference voltage V₂ (step 17). If the capacitor voltage is not greater than V₂, then the method proceeds to continue sensing the capacitor voltage (step 16). If the capacitor voltage is indeed greater than V₂, then the charge is supplied from the capacitor 7 to the rechargeable battery 2 (step 18). The capacitor voltage is sensed again (step 19) and a second determination is made as to whether the capacitor voltage is less than the third reference voltage V₃ (step 20). If the capacitor voltage is not less than V₃, then the method proceeds to continue sensing the capacitor voltage (step 19). If the capacitor voltage is indeed less than V₃, then the supply of charge from the capacitor 7 to the rechargeable battery 2 is terminated (step 21 ), and the method returns to the step of sensing of the capacitor voltage (step 16).

Figure 4 illustrates how the voltage across the capacitor 7 changes with time. At a notional zero time t₀, such as the time at which the photovoltaic transducer 1 is connected to the remainder of the apparatus, or the time at which light starts to be incident on the photovoltaic transducer 1 , there is no charge stored in the capacitor, and so the voltage across the capacitor 7 is zero. The level of charge stored in the capacitor 7, and hence also the capacitor voltage, then starts to rise during a first charging regime 22, until a time ti at which the capacitor voltage reaches the second reference voltage V₂. At this point the capacitor 7 becomes connected to the rechargeable battery 2 so that charge starts to flow out of the capacitor 7 during a discharging regime 23. This is terminated at a time t₂ when the capacitor voltage falls below the third reference voltage V₃, and a further charging regime 24 is initiated which is then terminated again at a time t₃ at which time the capacitor voltage again reaches the second reference voltage V₂. As can be seen from the graph, the duration (t₃ - t₂) of the further charging regime 24 is less than that (t-i - t₀) of the first charging regime, since the further charging regime starts at a point where there is still some charge remaining in the capacitor 7, whereas the first charging regime 22 starts when there is no stored charge.

Since the voltage across the capacitor 7 is proportional to the charge stored in the capacitor 7, it follows that the charging current is proportional to the negative gradient of the graph during the discharge regime 23, and the absolute value of this gradient decreases from an initial, relatively high value at time ti to a final, relatively low value at time t₂. By suitable selection of the second and third reference voltages V₂ and V₃, it is possible to determine the maximum and minimum values of the charging current supplied to the rechargeable battery 2.

However, most rechargeable battery technologies require some form of temperature compensation: a typical level of temperature compensation for a 12V rechargeable battery having six cells is 18 mV per Celsius degree relative to 25 degrees Celsius, so that at 35 degrees Celsius the compensation would be -180 mV.

The values of V₁, V₂ and V₃ are therefore preferably additionally controlled in dependence on the ambient temperature, such that the values of these parameters are higher at low temperatures than at high temperatures, in which case a separate temperature sensor is preferably provided (not shown).

Although in the above arrangements, the two-way switch 4 is operated in dependence on the output voltage of the photovoltaic transducer 1 , it would alternatively be possible to control the switch 4 in dependence on the output current of the photovoltaic transducer 1. Furthermore, although the charge levels in the capacitor 7 are determined by measuring the voltage across the capacitor 7, it would alternatively be possible to determine the level of charge in the capacitor 7 by measuring the current supplied both to and from the capacitor 7 and integrating this over time.

Figure 5 illustrates a jacket 25 to which are attached two identical flexible dye-sensitised photovoltaic transducers 26, which are connected together in parallel to the remaining circuitry as described above with reference to Figure 1. In this case, two diodes are provided, each being connected between a respective one of the two photovoltaic transducers 26 and the remaining circuitry, to prevent the output current from one of the two photovoltaic transducers 26, which may be directed toward the sun, from leaking back into the other photovoltaic transducer 26, which may be directed away from the sun. This circuitry may also be attached to the jacket 25, or may be located elsewhere, e.g. in a pocket of the jacket 25, and be electrically connected to the photovoltaic transducers on the jacket 25. Such an arrangement could be used to power a portable electrical device such as a mobile telephone.

Figure 6 illustrates a rucksack 27 designed to be worn on the back of a user for carrying equipment. The upper and rear surfaces of the rucksack 27 have respective identical flexible dye-sensitised photovoltaic transducers 28, 29 attached thereto, and the remaining circuitry described above with reference to Figure 1 is conveniently located within the rucksack 27 so that it is protected from adverse weather conditions. As with the jacket 25 described above with reference to Figure 5, a respective diode is connected to each of the photovoltaic transducer 28, 29, and the arrangement could be used to power any suitable portable electrical device.

Finally, a mobile telephone 30 is illustrated in Figure 7, to the upper surface of which a photovoltaic transducer 31 is attached. The photovoltaic transducer 31 is connected to the apparatus described above with reference to Figure 1 , which is within the casing of the mobile telephone 30.

Claims

CLAIMS:

1. Apparatus for receiving electrical charge generated at a variable rate by a photovoltaic transducer and for supplying the charge at a rate above a predetermined threshold to a rechargeable battery, the apparatus comprising: means for receiving the generated charge from the photovoltaic transducer; means for storing the received charge; level sensing means for sensing when the level of stored charge is greater than a first predetermined value and for generating a first output signal in response thereto; and charging means responsive to the first output signal for supplying at least some of the stored charge to the rechargeable battery at a rate above the predetermined threshold.

2. Apparatus as claimed in claim 1 , wherein the level sensing means is arranged to sense when the level of charge stored is less than a second predetermined value, which is less than the first predetermine value, and for generating a second output signal in response thereto, and wherein the charging means is arranged to terminate the supply of the stored charge to the rechargeable battery in response to the second output signal.

3. Apparatus as claimed in claim 1 or claim 2, wherein the storing means comprises a capacitor.

4. Apparatus as claimed in claim 3, wherein the level sensing means is arranged to sense the voltage across the capacitor.

5. Apparatus as claimed in claim 4, wherein the level sensing means is arranged to compare the sensed voltage with a first reference voltage representing the first predetermined value.

6. Apparatus as claimed in claim 4 or claim 5, when dependent on claim 2, wherein the level sensing means is arranged to compare the sensed voltage with a second reference voltage representing the second predetermined value.

7. Apparatus as claimed in any one of claims 1 to 6, wherein the charging means comprises an electrical switch for selectively connecting the storing means to the rechargeable battery.

8. Apparatus as claimed in any one of claims 1 to 7, further comprising means for determining if a parameter of the electrical output of the photovoltaic transducer is greater than a predetermined value and for generating a third output signal in response thereto, and means responsive to the third output signal for supplying the received electrical charge generated by the photovoltaic transducer directly to the rechargeable battery without passing through the storing means.

9. Apparatus as claimed in any one of claims 1 to 8, further comprising a diode connected between the receiving means and the storing means.

10. Apparatus as claimed in any one of claims 1 to 9, further comprising the rechargeable battery.

1 1. Apparatus as claimed in any one of claims 1 to 10, further comprising the photovoltaic transducer.

12. Apparatus as claimed in claim 11 , when dependent on claim 10, wherein the maximum output voltage of the photovoltaic transducer is approximately equal to the voltage rating of the rechargeable battery.

13. Apparatus as claimed in claim 11 or claim 12, wherein the photovoltaic transducer comprises at least one dye-sensitised photovoltaic cell.

14. Apparatus as claimed in any one of claims 1 1 to 13, wherein the photovoltaic transducer is in the form of a flexible sheet.

15. Apparatus as claimed in claim 14, wherein the flexible sheet is attached to, or forms, an article of clothing.

16. A portable container comprising apparatus as claimed in claim 14.

17. A portable electrical device arranged to be supplied with power from a rechargeable battery and comprising apparatus as claimed in any one of claims 1 to 9.

18. A portable electrical device as claimed in claim 17, in combination with the rechargeable battery.

19. A method of receiving electrical charge generated at a variable rate by a photovoltaic transducer and supplying the charge at a rate above a predetermined threshold to a rechargeable battery, the method comprising: receiving the generated charge from the photovoltaic transducer; storing the received charge; sensing when the level of stored charge is greater than a first predetermined value; and supplying, in response thereto, at least some of the stored charge to the rechargeable battery at a rate above the predetermined threshold.

20. A method as claimed in claim 19, further comprising: sensing when the level of charge stored is less than a second predetermined value, which is less than the first predetermine value; and terminating the supply of the stored charge to the rechargeable battery in response thereto.

21. A method as claimed in claim 19 or claim 20, wherein the photovoltaic transducer comprises at least one dye-sensitised photovoltaic cell.

22. A method as claimed in any one of claims 19 to 21 , further comprising selecting the charging rate to be optimal for the rechargeable battery.

23. A method as claimed in any one of claims 19 to 22, wherein the maximum output voltage of the photovoltaic transducer is approximately equal to the voltage rating of the rechargeable battery.