GB2090084A

GB2090084A - Photovoltaic Battery Charging System

Info

Publication number: GB2090084A
Application number: GB8137075A
Authority: GB
Original assignee: Pennwalt Corp
Current assignee: Pennwalt Corp
Priority date: 1980-12-23
Filing date: 1981-12-09
Publication date: 1982-06-30

Abstract

A photovoltaic battery charging system comprises a photovoltaic cell 12 connectable to the battery 16 to be charged and comprising means for preventing an overcharge. The overcharge preventing means comprise a comparator 40 which compares the battery voltage with a reference voltage and provides a differential voltage signal to control the pulse width of a signal from a pulse generator 50 which in turn controls a transistor or SCR switching means 10, controlling the intervals for which the cell 12 is operable to charge the battery 16. Switching means 10 continuously short circuits the cell 12 when the battery is fully charged. <IMAGE>

Description

SPECIFICATION Photovoltaic Battery Charging System This invention relates to photovoltaic battery charging systems and more particularly to such systems incorporating a switching arrangement for preventing overcharging of the battery being charged.

Energy systems are in demand which do not depend on non-renewable or expensive energy sources. Moreover energy systems are in demand which are usable in remote locations such as many remote villages, for example, not having convenient access to conventional electric power.

Transmission of electric power to such locations would, in many instances, be both financially prohibitive as well as a less then efficient use of energy. The remote locations may be scattered throughout a territory wherein each location's individual electrical requirements may be quite small. Both demands may theoretically be satisfied cheaply and efficiently by photovoltaic cell solar electric power in combination with a suitable battery storage system.

Referring to the accompanying drawings: Figures 1 and 2 are schematic illustrations of prior art series type and linear shunt type regulators respectively for preventing overcharging of a battery from a photovoltaic cell array. Such systems are known and usually comprise a voltage regulator to prevent damage to the batteries by overcharging.

One type of regulator used in these systems is a series regulator, as depicted in Figure 1 of the accompanying drawings. In this arrangement a variable impedance device such as a transistor 10 is inserted in series with the photovoltaic cell array 12. A blocking diode 14, suitably a Schottky diode, prevents discharge of battery 1 6 through the photovoltaic cell array. Battery 1 6 is of a type which is rechargeable, typically a lead-acid battery, or a series of such batteries.

Load 1 8 may be a lamp, TV set, a village water pump, or the like. Sensing and control circuits 20 may, for example, conveniently comprise a voltage divider for sampling battery voltage and a differential amplifier for controlling current to the base of transistor 10. Thus, current fed into the base of the transistor determines whether battery 1 6 is charging. A major disadvantage of the series regulator is loss of efficiency due to the series resistance of the variable impedance device.

A more widely used type of regulator for applications of this nature is shown in Figure 2 of the accompanying drawings and is a linear shunt regulator utilizing a fixed resistance 24 and a variable impedance device such as a transistor 10 in parallel with photovoltaic cell array 12. Since the transistor operates in the linear mode, the full output of array 12 must be dissipated by transistor 10 and fixed resistance 24 when battery 1 6 is in a fully charged state, thus necessitating a massive heatsink as well as.

transistors and resistors having high power handling capabilities.

The present invention overcomes the disadvantages of the prior art devices, i.e.

efficiency of the regulator is not lowered by the introduction thereinto of series impedance, nor is the presence of a large heatsink required.

It is stated that the short circuit current of a photovoltaic solar cell is substantially identical to its maximum current at the optimum power level.

Due to this inherent current limiting action of the photovoltaic cell, it can be shown that controlled shorting of the cell output may be a practical method of regulating the charging current to a storage battery.

In accordance with this invention, therefore, there is provided a photovoltaic battery charging system comprising a photovoltaic solar cell, means for connecting the cell to the battery to be charged, means for preventing overcharging of said battery and means for preventing discharge of said battery through said cell when connected thereto, wherein said overcharge preventing means comprise:: a comparator circuit for sampling the voltage of the battery being charged, comparing said voltage with a reference voltage and providing a voltage corresponding to the difference therebetween; a means in series with said comparator circuit for generating a pulse signal the width of which is controlled by the differential voltage signal provided by the comparator circuit; and switching means connected serially across the terminals of said cell for regulating current from said array to said battery in accordance with said generated pulses.

The invention is further described with reference to Figures 3 and 4 of the accompanying drawings, in which: Figure 3 is a schematic illustration of a theoretical switching short regulator for preventing overcharging of a battery from a photovoltaic cell array.

Figure 4 is a schematic illustration of an embodiment of a switching short regulator of the present invention for preventing overcharging of a battery from a photovoltaic cell.

Thus, reference should now be made to Figure 3 wherein a switching device SW, typically a transistor, SCR, and the like, is connected across the output terminals of photovoltaic cell array 1 2. When battery 1 6 reaches a fully charged condition, SW is closed to thereby short the output of array 12 and effectively preventing further charging of the battery 1 6. Blocking diode 14 prevents switching device SW from discharging the battery back through the array.

After battery 1 6 has discharged, switch SW is opened to thereby permit the full output of the photovoltaic array to be applied again to the battery. It is apparent from the above description that very little power is dissipated in the switching device since it is operated in the saturated mode.

An embodiment of the theoretical circuitry of Figure 3 is illustrated in Figure 4. A comparator circuit 40 samples battery voltage 42 between suitable series resistors R connected across the output terminals, and compares it to a reference voltage 44. The difference voltage 46 from comparator controls the width of the pulse generated by pulse width modulation circuit 50.

Signal 52 from pulse width modulation circuit 50 is applied to base 54 of transistor 10 switching it on or off and thereby regulating the charging current 56 to battery 16 by varying the interval that photovoltaic cell array 12 is effectively shorted. With a discharged battery, transistor 10 is switched off to thereby allow the full output of the array to be applied to the battery. Conversely, when battery 16 is fully charged, transistor 10 will be switched on, thereby shorting the output of the photovoltaic cell array.

Claims

1. A photovoltaic battery charging system comprising a photovoltaic solar cell, means for connecting the cell to the battery to be charged, means for preventing overcharging of said battery and means for preventing discharge of said battery through said cell when connected thereto, wherein said overcharge preventing means comprise:: a comparator circuit for sampling the voltage of the battery being charged, comparing said voltage with a reference voltage and providing a voltage corresponding to the difference therebetween; a means in eries with said comparator circuit for gener awing a pulse signal the width of whic is controlled by the differential voltage signal provided by the comparator circuit; and switching means connected serially across the terminals of said cell for regulating current from said array to said battery in accordance with said generated pulses.

2. A system according to claim 1, wherein said switching means is a transistor.

3. A system according to claim 2, wherein said transistor is operated in a saturated mode.

4. A system according to claim 1, 2 or 3, wherein said switching means operate to short circuit the cell output when the battery is in a fully charged condition.

5. A system according to claim 1, substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.