DEVICE AND A BATTERY

TECHNICAL FIELD

The present invention relates to a circuit and process for supplying a load through a battery charged by a photovoltaic device.

In particular the present invention relates to an interconnection circuit among a photovoltaic device, a battery capable of being charged by the photovoltaic device and a load which is in turn supplied by the battery.

BACKGROUND

It is known that photovoltaic devices, such as for example solar cells, can be used together with batteries to provide sources of renewable and portable electrical power for use in supplying a load, such as for example a temperature or humidity sensor or an electronic display.

The solar cells, the batteries and the load cannot however be merely connected directly to each other. It is known therefore that an interconnection circuit is used to carry out the following functions in particular:

converting the output voltage generated by the photovoltaic device into a charging voltage for the battery (every battery has a predetermined charging voltage that depends on the technology used to manufacture the battery);

fixing the working point of the photovoltaic device at the maximum power point, which is a function of the quantity of light incident upon the photovoltaic device; maximising the battery's charging efficiency, provided by the ratio between the charging energy and the energy stored in the battery. A specimen charging efficiency curve C is shown in Figure 1 ; this depends on the charging current (or voltage) of the battery itself;

making it possible to connect up the devices using different technologies such as for example PCB standards, circuits on plastic or in any event flexible substrates made through the use of printing techniques, etc.

This interconnection circuit is also intended to perform amplification of the voltage from the solar cell.

A single photovoltaic device produces a low voltage, typically of the order to 0.5 - 1 V, which is incompatible with most electronic applications. In order to resolve this problem a plurality of photovoltaic devices connected in series is typically used to supply electronic circuits, but these in-series connections have many disadvantages such as for example a low geometric fill factor, non-optimal adjustment of the current if the photovoltaic device should be shaded, etc. For this reason it is not recommended that a large number of photovoltaic devices should be used in series in preference to amplification of the output voltage from a limited number of photovoltaic devices in series.

Of the circuits which make it possible to achieve this voltage amplification, circuits based on condensers, also known as charge pumps, an illustration of which is shown in Figure 2, are known. Charge pumps comprise a plurality of diodes 1 connected in series, between which there are condensers 2 in parallel.

These circuits use simple cheap components such as condensers and diodes, unlike other known circuits such as inductive switching converters, and their functioning can be easily calibrated by simply changing the switching intervals of the components through modifications to the software controlling the circuit itself.

Because of these characteristics they can also be used in low cost applications such as sensors, displays, network nodes in wireless sensor networks and the like. These circuits nevertheless have the disadvantage that when the photovoltaic device is in shaded conditions, such that the voltage delivered is reduced to a level below the battery charging voltage, the interconnection circuit automatically disconnects the photovoltaic device from the battery, which must itself then supply the load while waiting to be charged by the photovoltaic device when the latter is again capable of providing a sufficient voltage.

Typically the minimum illumination level which can be used in these circuits is therefore that through which the photovoltaic device generates a power equal to that consumed by the electrical load. At lower intensity levels the photovoltaic device is isolated from the circuit.

Thus, when shaded, the energy produced by the photovoltaic device which is insufficient to charge the battery is dispersed. When the power generated by the photovoltaic device is less than that consumed by the battery it is no longer used.

These circuits are not therefore suitable for use in the field of low-cost small-area devices, regardless of energy considerations, and/or whether they are operating in closed environments under low light conditions, which are essential for the development of wireless sensor or industrial or domestic automation networks.

The main limitation associated with the supply of these devices through photovoltaic devices lies in the extreme variability of the illumination conditions in such contexts.

Despite the fact that commercial, domestic or industrial environments may be

characterised by strong illumination peaks, the light intensity may then fall sharply following unforeseen events such as shading by a user/operator, a shadow over a window in the premises, or the fact that artificial lights are switched off.

Document WO 2010/015856 describes an arrangement for charging a re-chargeable battery from the combined output current of three photovoltaic transducers. However the structural characteristics of the system disclosed in this document would not allow reducing the sizes and weight of the circuit beyond a certain limit.

OBJECT OF THE INVENTION

The object of the invention is therefore to provide a circuit and a process for supplying a load capable of using the energy produced by the photovoltaic device even when the latter is below the minimum level necessary for charging the battery, making use of all the light incident upon the photovoltaic device even under low intensity conditions, thus increasing the overall light energy converted into electrical energy. At the same time a reduction in the sizes, volumes and weight of the circuit together with an increased easiness of manufacturing is desirable.

SUMMARY OF THE INVENTION

These and other objects are accomplished through a circuit whose characteristics are defined in claim 1 , using a process as defined in claim 10.

According to an embodiment of the present invention we provide a interconnection circuit to supply a load through a charging battery charged by a photovoltaic device in which the battery has a predetermined current of maximum charging efficiency, the circuit comprising: a first printable converter connected to the output of the photovoltaic device; a printable buffer capacity connected to the output of the first converter; a second printable converter connected in series to the first converter; a battery connected in parallel to the output of the second converter; said load being connected in parallel to said battery; a control device configured to compare the voltage collected at the buffer capacity with a predetermined threshold value and if the voltage is equal to or greater than said threshold value, to control the second converter so that it transfers current from the buffer capacity to the battery; if the voltage is lower than said threshold value, to deactivate the second converter until the voltage at the buffer capacity reaches said threshold value. In a preferred embodiment the printable components of the circuit are made of one or more of the following components: capacitor, diodes, transistors and are obtained by printing over a substrate, wherein the printing is done with any known techniques which include: flexography, screen printing, ink jet printing. In a possible embodiment of the present invention the substrate is a photovoltaic cell, so that the circuit is printed directly on a photovoltaic cell and the printed components of the circuit are transparent.

The first and/or the second converter can be variable gain step-up converters.

In an embodiment of the present invention the first converter is arranged to perform a voltage amplification based on the value of the voltage generated by the photovoltaic converter.

In a preferred embodiment the second converter is arranged to transfer an average current greater than or equal to the value of the current which provides the maximum charging efficiency to the battery.

In a second aspect of the present invention it is provided a method for supplying a load through a battery charged by a photovoltaic device through an interconnection circuit as described above, comprising the steps of: switching on the photovoltaic device; acquiring the output voltage of the photovoltaic device; measuring the maximum power point of said photovoltaic device; modifying the gain of the first converter so that it performs an output voltage amplification; acquiring the voltage collected at the buffer capacity;

comparing the voltage collected at the buffer capacity with a predetermined threshold value and if the voltage is equal to or greater than said threshold value, controlling the second converter so that it transfers current from the buffer capacity to the battery; if the voltage is lower than said threshold value, deactivating the second converter until the voltage at the buffer capacity reaches said threshold value.

FIGURES

Further characteristics and advantages of the invention will become apparent from the following detailed description, provided purely by way of a non-limiting example, with reference to the appended drawings, in which:

Figure 1 , which has already been described, shows a graph of the charging efficiency of a battery;

Figure 2 shows a diagram of an interconnection circuit according to the known art; Figure 3 shows a circuit diagram of an interconnection circuit according to the present invention; and

Figure 4 is a block diagram of a process for supplying a load through a battery charged by a photovoltaic device according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

To sum up, the invention is a new interconnection printable circuit which makes it possible to use the energy produced by a photovoltaic device even when this is below the minimum level required to charge a battery supplying a load. The service life of the battery is significantly increased in this way.

The circuit according to the present invention uses simple cheap components such as condensers and diodes, thus reducing costs and the volume of the circuit and making it compatible with processes for the production of printed circuit electronics.

Figure 3 shows a circuit diagram of an interconnection circuit 10 according to the present invention. This circuit comprises a photovoltaic device 12, for example a solar cell, to the output of which is connected a first converter 14, preferably a converter of the variable gain step- up type, of the type illustrated in Figure 2. A buffer capacity 16 is connected to the output of first converter 14. Finally a second converter 18, preferably a converter of the variable gain step-up type, is connected in series to first converter 14 (and therefore downstream from buffer capacity 16). A battery 20 and a load 22 are connected in parallel to the output of second converter 18.

First converter 14 is intended to perform a voltage amplification which depends on the value of the input voltage, corresponding to the voltage generated by photovoltaic device 12. In a known way first converter 14 makes it necessary for photovoltaic device 12 to work at its maximum power point regardless of the voltage which has accumulated in buffer capacity 16.

Variable gain is achieved by using a circuit having switched capacity whose switching voltage is controlled through pulse width modulation (PWM). This makes it possible to adjust the duration of the pulse controlling first converter 14, varying the voltage gain as a consequence. The PWM modulation system is derived in a way which is in itself known from the value of a first input voltage, corresponding to the output voltage of the photovoltaic device, which is provided to a logic component, for example a

microcontroller 24, which has the task of reading the voltage, determining the maximum power point of photovoltaic device 12 and controlling first converter 14.

Second converter 18 is intended to transfer an average current which is greater than or equal to the value of the current providing the maximum charging efficiency for battery 20. Second converter 18 operates intermittently.

When photovoltaic device 12 can provide sufficient power to charge battery 20 with a current equal to or greater than the value of the maximum charging efficiency current for battery 20, second converter 18 merely transfers all the power to battery 20.

When photovoltaic device 12 is not capable of providing sufficient power, second converter 18 operates intermittently; when it is active it transfers a current equal to the maximum charging efficiency current for battery 20, discharging buffer capacity 16. When it is exhausted no current flows to battery 20 and all the current generated by photovoltaic device 12 is accumulated in buffer capacity 16.

On average power equal to that produced by photovoltaic device 12 is transferred to battery 20, but battery 20 is always charged at its maximum efficiency.

Interconnection circuit 10 according to the present invention does not completely disconnect photovoltaic device 12 when it is not providing sufficient current to supply load 22, leaving this task to battery 20 alone. Because of circuit 10 it is therefore possible to make the best use of photovoltaic device 12 even in these circumstances, because it nevertheless provides its own partial contribution to power generation. This makes it possible to reduce the power drawn from battery 20 while at the same time increasing the range of illumination in which photovoltaic device 12 can be used.

Second converter 18 also fixes the maximum current value that can be transferred to battery 20, in a way which is in itself known. This parameter must be less than the maximum overcharging current, to avoid damage to battery 20 through current overloading.

Second converter 18 is also constructed with a switched capacity conversion circuit of the type in Figure 2. By enabling or disabling the charging phase of such second converter 18 it is possible to cause it to operate intermittently, acting on the value of its input voltage (second input voltage), which is that present in buffer capacity 16. When the second input voltage to second converter 18 exceeds a predetermined first threshold, for example equal to twice the charging voltage of the battery, a logic circuit, for example the microcontroller mentioned above, switches on second converter 18 until the second input voltage falls below a second threshold, equal for example to the battery charging voltage. Energy flows continuously between battery 20 and load 22; intermittently, while battery 20 is in operation, second converter 18 connects it to buffer capacity 16 in the manner described above in order to charge it.

A further advantage of the invention lies in the fact that connection circuit 10 makes it possible to charge the battery even under conditions of low lighting (and therefore low energy). Even if the power source (photovoltaic device 12) is not capable of providing sufficient energy as required by load 22, it is not disconnected but causes buffer capacity 16 and battery 20 to operate in parallel to supply load 22, saving the battery's energy. In addition to this buffer capacity 16 also acts as an uncoupler between first converter 14 and second converter 18, making it possible to charge battery 20 at the maximum point of the charging efficiency curve C and at the same time transferring the maximum quantity of energy originating from photovoltaic device 12.

As mentioned above the printable circuit according to the present invention includes only components which can be easily printed (i.e. capacitor, diodes and transistors) In addition to that, according to a possible embodiment of the present invention the components can be transparent, therefore printable directly on a photovoltaic cell without modifying the performances and without occupying more space than that already occupied by the photovoltaic cell. In this way it is possible to greatly improve the efficiency of the photovoltaic cell without the need to add any external components: all components, including the photovoltaic cell, can be printed directly on the same common substrate with a single process. This integration provides big advantages in term of costs of manufacturing and in terms of size and volume of the final circuit.

A process for supplying a load via a battery charged by a photovoltaic device according to the present invention will now be described with reference to Figure 4.

In a first step 100 of the process photovoltaic device 12 is active and provides an output voltage (or current). In a step 102 microcontroller 24 acquires the output voltage from photovoltaic device 12, determines the maximum power point of photovoltaic device 12 in step 104 and adjusts the gain of first converter 14 in step 106 so that it performs an amplification of the output voltage.

Subsequently, in step 108, microcontroller 24 acquires the voltage which accumulates at the terminals of buffer capacity 16 and in step 110 compares it with a predetermined threshold value which has been pre-set according to the characteristics of battery 20. If the voltage received is equal to or greater than said threshold voltage, microcontroller 24 activates second converter 18 in step 112 so that the latter transfers current from buffer capacity 16 to battery 20.

If the voltage is below the threshold voltage, microcontroller 24 deactivates second converter 18 in step 114 until the voltage at the terminals of buffer capacity 16 reaches the threshold value pre-set according to the characteristics of battery 20.

Once the steps listed above have been completed the process resumes from step 102 and continues iteratively.

When the output current of photovoltaic device 12 is sufficient to maintain the voltage at the terminals of buffer capacity 16 above the pre-set threshold value, second converter 18 is no longer deactivated during the iterative cycles and the output current is transferred to battery 20.

When the output current of photovoltaic device 12 is not zero but nevertheless insufficient to maintain the voltage at the terminals of buffer capacity 16 above the set threshold value, second converter 18 is repeatedly activated and deactivated. During the deactivation period the output current to photovoltaic device 12 is accumulated in buffer capacity 16, causing the voltage at its terminals to increase until it exceeds the threshold voltage. During the activation period the output current to photovoltaic device 12 is insufficient to counterbalance the current transferred to battery 20 by buffer capacity 16; this causes the voltage at the terminals of buffer capacity 16 to drop until the threshold voltage is exceeded.

The iterative nature of the process enables the system to react to any changes in lighting conditions, continually adjusting its parameters so that it always guarantees maximum efficiency in energy transfer.

Of course, without altering the principle of the invention, the embodiments and details of embodiments may be varied extensively with respect to what has been described and illustrated purely by way of a non-limiting example without thereby going beyond the scope of protection of the present invention as defined by the appended claims.