Regulator for charging a rechargeable storage device from a photovoltaic cell
TECHNICAL FIELD 
     The present invention relates generally to regulator circuits and, more particularly, to a regulator circuit for regulating the charging of a rechargeable electrical storage device, such as a battery, from a photovoltaic cell. 
     BACKGROUND ART 
     Frequently, particularly in geographical areas where electric power is needed but not readily available, electric energy is generated by a photovoltaic cell array (i.e., a solar cell) and stored in a battery or other rechargeable electrical storage device. Specifically, the photovoltaic cell array generates an electric current that is electrically coupled to the battery. The current is converted into stored electrical energy in the battery, and the battery thus becomes charged. In some instances, the energy stored in the battery is converted by an inverter to a variable-frequency supply voltage which, in turn, drives a load such as an electric motor which may, for example, be connected to a pump. 
     To increase the efficiency of the charging process, a regulator is often coupled between the photovoltaic cell array and the battery to regulate the current delivered to the battery by the photovoltaic cell array. However, prior-art regulators either have failed to achieve a satisfactory level of charging efficiency, or have been too expensive to be practical for use in typical solar-powered battery-charging applications. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a circuit has input terminals for coupling to cell terminals of a solar cell and output terminals for coupling to device terminals of a rechargeable storage device. The circuit regulates charging of the rechargeable storage device by the photovoltaic cell. The circuit selectively establishes and breaks an electrical connection between the input terminals and the output terminals based on a comparison of a signal proportional to the voltage across the output terminals and a first voltage limit. The circuit also selectively establishes and breaks the electrical connection based on a comparison of the signal and a second voltage limit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the several figures and in which: 
     FIG. 1 is a block diagram of a prior-art system in which a regulator may be used for charging a battery from a photovoltaic cell; 
     FIG. 2 is a generalized schematic diagram of a regulator according to the present invention; 
     FIG. 3 is a detailed schematic diagram of the regulator of FIG. 2; 
     FIG. 4 is a waveform illustrating a steady-state charging characteristic of a battery charged through the regulator of FIGS. 2 and 3; and 
     FIG. 5 is a waveform illustrating the first-time charging characteristic of a battery charged through the regulator of FIGS. 2 and 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a system 20 for charging a rechargeable storage device or battery 22 from a photovoltaic cell or solar cell 24. The system 20 includes a regulator 26 coupled between the solar cell 24 and the battery 22. More particularly, the regulator 26 has a pair of input terminals 28, 30 and a pair of output terminals 32, 34. The solar cell 24 has a pair of cell terminals 36, 38, each of which is coupled to a respective one of the input terminals 28, 30 of the regulator 26. The battery 22 (or other suitable rechargeable storage device) has a pair of device terminals 40, 42, each of which is coupled to a respective one of the output terminals 32, 34 of the regulator 26. 
     As noted above, this system 20 for regulating the charging of a battery 22 from a solar cell 24 is known in the prior art. The present invention relates to a regulator that may be employed in the system 20 in place of the regulator 26 for regulating charging of the battery 22. For convenience, the regulator of the present invention is described below in relation to the solar-powered charging system 20 shown in FIG. 1. 
     FIG. 2 generally depicts a regulator 44 according to the present invention. The regulator 44 is a circuit for regulating charging of a rechargeable storage device, such as the battery 22 (shown in FIG. 1), by a photovoltaic cell, such as the solar cell 24 (also shown in FIG. 1). Like the regulator 26, the regulator 44 has a pair of input terminals 46, 48 for coupling to the cell terminals 36, 38 of the solar cell 24. The regulator 44 also has a pair of output terminals 50, 52 for coupling to the device terminals 42, 44 of the battery 22. Further, the regulator 44 includes a switch 54 or other switching means for selectively establishing and breaking an electrical connection between the input terminals 46, 48 and the output terminals 50, 52. Still further, a control 55 is coupled to the input terminals 46, 48 by lines 56 and to the output terminals 50, 52 by lines 57 of the regulator 44 as well as to the switch 54 thereof by a line 58. 
     The structure and operation of the regulator 44, and of the control 55 in particular, is described in detail hereinafter. Generally, the control 55 includes means connected to the lines 57 for providing a signal V T  proportional to a voltage across the output terminals 50, 52 of the regulator 44. The control 55 also includes first means responsive to the signal V T  and to a first voltage limit V 1  for causing the switch 54 to close. The closing of the switch 54 establishes an electrical connection between the input terminals 46, 48 and the output terminals 50, 52 of the regulator 44 when the signal V T  is equal to a first predetermined proportion of the first voltage limit V 1 . In addition, the control 55 includes second means responsive to the signal V T  and to a second voltage limit V 2  for causing the switch 54 to close. Once again, the closing of the switch 54 establishes an electrical connection between the input terminals 46, 48 and the output terminals 50, 52 of the regulator 44 when the signal V T  is equal to a second predetermined proportion of the second voltage limit V 2 . Of course, if V T  is equal to either of the first or second voltage limits, then the respective predetermined proportion will be one. 
     FIG. 3 is a schematic diagram illustrating the regulator 44 in greater detail. The following description provides exemplary values for circuit elements of a regulator 44 adapted for charging a lead-acid automobile battery having a rated voltage of about fourteen volts, which is commonly used in solar charging applications. It will be readily apparent to those skilled in the art, however, that a regulator designed in accordance with the present invention may employ circuit elements having different values for charging other types of batteries, if desired. 
     As shown, the switch 54 comprises a low-voltage field-effect transistor Q1 provided with a heat sink 60 for dissipating heat generated by the transistor Q1. Of course, any other suitable transistor or other switching means may be used instead of the Schottky transistor Q1. When the transistor Q1 is in the conducting or &#34;on&#34; state, it establishes an electrical connection between the input terminals 46, 48 and the output terminals 50, 52, and when the transistor Q1 is in the non-conducting or &#34;off&#34; state, it breaks that connection. 
     A Schottky high-power diode D1 is coupled between the input terminal 46 and the output terminal 50 of the regulator 44, the anode of the diode D1 being coupled to the input terminal 46, and the cathode of the diode D1 being coupled to the output terminal 50. The diode D1 prevents the battery 22 from discharging through the solar cell 24 when the voltage across the battery 22 exceeds the voltage output of the solar cell 24. A heat sink 60 is disposed adjacent the Schottky high-power diode D1 to dissipate heat generated by current flowing through the diode D1. 
     A resistor network 64 is coupled across the output terminals 50, 52 and includes, in the present specific example, a 10-kilohm variable resistor RV coupled in series between a 24-kilohm resistor R1 and a 100-kilohm resistor R2. The variable resistor RV has a wiper terminal 66 which provides a voltage signal V T  proportional to the voltage across the output terminals 50, 52, and, in turn, to the terminal voltage across the battery terminals 40, 42. The constant of proportionality, and thus the voltage signal V T  at the wiper terminal 66, can be adjusted to any desired value by properly setting the resistance of the variable resistor RV, and further adjustments may be made by selecting other suitable values for the resistors R1 and R2, if necessary. 
     A 10-μF smoothing capacitor C1 is coupled between the wiper terminal 66 of the variable resistor RV and the output terminal 50 of the regulator 44 to minimize voltage transients in the voltage V T . The 10-μF capacitor C1 is selected to provide a time constant that will maintain the voltage signal V T  at the desired value thereof (about 7.2 volts). This value of V T  is chosen so that the regulator 44 will regulate the battery voltage of a lead-acid battery 22 to a desired level of about 14.2 volts. It should be noted that this voltage value is specific to lead-acid batteries. 
     A Zener diode ZD having a breakdown voltage of about 7.2 volts and a 2-kilohm resistor R3 are coupled in series between the output terminal 50 and the input terminal 48 to provide a reference voltage signal V R  for comparison with the voltage signal V T . The cathode of the Zener diode ZD is coupled to the output terminal 50 (and thus to the cathode of the diode D1), and the anode of the Zener diode ZD is coupled to the resistor R3. A 10-μF capacitor C2 is coupled in parallel with the Zener diode ZD in order to minimize voltage transients on the reference voltage signal V R . 
     A pair of operational amplifiers 68, 70, connected as voltage comparator circuits, are provided to compare the terminal voltage signal V T  to the reference voltage signal V R  and to turn the transistor Q1 on and off to respectively establish and break an electrical connection between the input terminals 46, 48 and the output terminals 50, 52 of the regulator 44. The inverting input terminal of the operational amplifier 68 is coupled through a 68-kilohm input resistor R4 to the anode of the Zener diode ZD, and the non-inverting input terminal of the operational amplifier 68 is coupled through a 68-kilohm input resistor R5 to the wiper terminal 66 of the variable resistor RV. In addition, a 270-kilohm feedback resistor R6 is coupled from the non-inverting input of the operational amplifier 68 to the cathode of a diode D2, the anode of which, in turn, is coupled to the output of the operational amplifier 68. A 10-megohm feedback resistor R7 is coupled from the non-inverting input of the operational amplifier 68 to the anode of a diode D3, the cathode of which, in turn, is coupled to the output of the operational amplifier 68. 
     In this configuration, the operational amplifier 68 compares the terminal voltage signal V T  to the reference voltage signal V R  and produces a first op-amp output signal from when the voltage across the output terminals 50, 52 is at a first low level of approximately 13 volts until the voltage across the output terminals 50, 52 is at a first high level of approximately 15 volts. This first op-amp output signal is coupled through a diode D4 and a 470-ohm resistor R8 to the gate 72 of the transistor Q1. When the voltage across the output terminals 50, 52 is increasing between approximately 13 volts and approximately 15 volts, the operational amplifier 68 will cause the transistor Q1 to turn on, establishing an electrical connection between the input terminal 48 and the output terminal 52 of the regulator 44 and permitting the battery 22 to be charged by the solar cell 24. The operational amplifier 68 has a relatively wide hysteresis band (about 2 volts) and sets the maximum voltage to which the battery 22 will be exposed. 
     The inverting input terminal of the operational amplifier 70 is coupled through a 10-kilohm input resistor R9 to the anode of the Zener diode ZD, and the non-inverting input terminal of the operational amplifier 70 is coupled through a 10-kilohm input resistor R10 to the wiper terminal 66 of the variable resistor RV. In addition, a 10-megohm feedback resistor R11 is coupled from the non-inverting input terminal of the operational amplifier 70 to the output of the operational amplifier 70. 
     In this configuration, the operational amplifier 70 compares the terminal voltage signal V T  to the reference voltage signal V R  and produces a second op-amp output signal from when the voltage across the output terminals 50, 52 is at a second low level of approximately 14.1 volts until the voltage across the output terminals 50, 52 is at a second high level of approximately 14.3 volts. This second op-amp output signal is coupled through a diode D5 and the 470-ohm resistor R8 to the gate 72 of the transistor Q1. Thus, when the voltage across the output terminals 50, 52 is increasing between approximately 14.1 volts and approximately 14.3 volts, the operational amplifier 70 will cause the transistor Q1 to turn on, establishing an electrical connection between the input terminal 48 and the output terminal 52 of the regulator 44. 
     The operational amplifier 70 has a relatively narrower hysteresis band than the operational amplifier 68 (about 200 millivolts). The center of the hysteresis band of the operational amplifier 70 (i.e., the desired voltage level to which the battery 22 is to be regulated) is selected by setting the resistance of the variable resistor RV to approximately 5 kilohms to attain the desired voltage V T  as described above. The upper and lower voltage limit values of the hysteresis band of the operational amplifier 70 are determined by setting the resistances of the feedback resistor R11 and the resistor R10 to values that will ensure that the battery voltage is regulated to a voltage within an acceptable range of the desired value (in this case, to between 14.1 and 14.3 volts). 
     If desired, a light-emitting diode D6 and a 10-kilohm current-limiting resistor R12 may be coupled in series between the gate 72 of the transistor Q1 and the input terminal 48 of the regulator 44, which is typically grounded as shown in FIG. 3. The optional light-emitting diode D6 is illuminated whenever the transistor Q1 is turned on by either or both of the operational amplifiers 68 and 70 to indicate that the battery 22 is being charged. The resistor R12 limits the current flowing through the light-emitting diode D6. Also, when both operational amplifiers 68 and 70 are off, the resistor R12 reduces the gate voltage of the transistor Q1 to ensure that it turns off. 
     In order to protect the regulator 44 from damage in the event that it is connected to the solar cell 24 with reversed or incorrect polarity, a current-shunting diode D7 may be coupled across the input terminals 46, 48. Also, to protect the regulator 44 from the adverse effects of transient voltage signals, a capacitor C3 and a metal-oxide varistor MOV1 are coupled in parallel across the output terminals 50, 52. 
     A diode D8 is coupled between the anode of the diode D1 (i.e., the input terminal 46 of the regulator 44) to the power terminals of the operational amplifiers 68 and 70. The diode D8 protects the operational amplifiers 68, 70 in the event that the regulator 44 is connected improperly. 
     The regulator 44 may be either incorporated directly into a solar cell 24 so that the solar cell 24 provides a regulated current output, or provided as a separate unit that may be coupled to a solar cell 24 as described above. 
     The operation of the regulator 44 is now described with reference to FIGS. 3-5. 
     FIG. 4 illustrates the steady-state voltage characteristic for charging a lead-acid battery 22 from a solar cell 24 using a regulator 44 according to the present invention. When the battery 22 is charged, the battery voltage is regulated to about 14.2 volts (more specifically, to between 14.1 and 14.3 volts). When a load is connected across the battery terminals 40, 42, the battery 22 begins to discharge, and the voltage across the battery terminals 40, 42 (the &#34;battery voltage&#34;) drops. When the battery voltage drops below about 14.1 volts, indicating that the battery 22 is in need of charging, the transistor Q1 is turned on by the operational amplifier 70, and the battery 22 begins to be charged from the solar cell 24. However, while the load remains connected across the battery terminals 40, 42, the battery 22 continues to discharge and the battery voltage continues to drop because the battery 22 discharges through the load faster than it can be recharged by the solar cell 24. 
     In practice, the load is automatically disconnected when the battery voltage drops to about 11.5 volts and, because the transistor Q1 is on (due to both operational amplifiers 68, 70 being on), the battery 22 is charged from the solar cell 24 through the regulator 44. When the battery voltage rises above 14.3 volts, the operational amplifier 70 turns off, but the operational amplifier 68, and thus the transistor Q1, remains on to allow the battery 22 to be momentarily overcharged or boosted until the battery voltage reaches about 15 volts. This momentary overcharging is an important feature of this invention because it agitates or electrically stirs the electrolytic fluid in the lead-acid battery 22 to ensure that the electrolytic fluid remains homogeneously mixed. The operational amplifier 68 turns off the transistor Q1 when the voltage reaches about 15 volts to prevent evaporation of the electrolytic fluid, thus prolonging the life of the battery 22. 
     When the transistor Q1 turns off at the 15-volt level, the battery voltage decreases to about 14.1 volts. When the battery voltage reaches about 14.1 volts, the operational amplifier 70 turns the transistor Q1 on once again, charges the battery to a voltage of about 14.3 volts, and turns off. This process is continually repeated to maintain the battery voltage between about 14.1 and about 14.3 volts until the battery 22 is substantially discharged once again through a load (e.g., through an inverter and an AC electric motor). 
     As the battery 22 discharges through the load (not shown), the battery voltage begins to drop once again, and the above-described battery-charging process is repeated. 
     FIG. 5 illustrates the voltage characteristic obtained the first time a new battery 22 is charged from a solar cell 24 using the regulator 44. This characteristic is similar to that described above in connection with FIG. 4, except that the first time a battery 22 is charged, the battery voltage remains at approximately 13 volts for a substantial period of time (e.g., nearly twenty-five minutes as shown in FIG. 5). The battery 22 then charges rapidly to approximately 15 volts before the transistor Q1 is turned off. Thereafter, the battery 22 is alternately charged and discharged as described above. 
     The foregoing description is for the purpose of teaching those skilled in the art the best mode of carrying out the invention and is to be construed as illustrative only. Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of this description. The details of the disclosed structure may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications within the scope of the appended claims is reserved.