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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Battery charge regulator for solar cells. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Alternative energy sources Power supply of various devices is possible directly from solar cells. However, such a simple connection of solar cells is possible only if the absence of sunlight and, therefore, power supply practically does not lead to undesirable consequences. In many cases, it is necessary that electrical appliances and equipment work even in the absence of sunlight. To do this, you need to store solar energy generated during the day in batteries for later use. The most suitable for these purposes are lead-acid batteries. Lead acid batteries Lead-acid batteries are actually made up of several individual cells connected in series. Each element, which develops a voltage of up to 2 V, contains two lead plates placed in a weak solution of sulfuric acid. When an electric current flows through the cell, a reversible electrochemical reaction takes place, and electrical energy is stored in the cell, which, if necessary, can be used later. Despite the apparent simplicity, in reality, the process of charging a battery is quite complicated. The lead-acid battery is a sensitive electrical device that must be handled with care, especially when charging. In support of this, let's follow the various stages of a typical charging cycle. The charge of the battery begins when voltage is applied to the cell plates, as a result of which an electric current begins to flow through it. It leads to the occurrence of an electrochemical reaction that changes the chemical composition of the plates and the electrolyte of the battery cell. The rate of this reaction depends on the magnitude of the charging current. The greater the current, the faster the reaction proceeds. Ultimately, it is the charge associated with this current that is stored in the cell for later use. The battery accumulates more and more charge, and eventually saturation occurs. Essentially, the chemical reaction stabilizes or balances out, and further charge accumulation ceases. Equilibrium occurs when most of the sulfate ions that were absorbed from the sulfuric acid solution by the lead plates during the battery discharge cycle return from the plates to the solution. In this case, the plates again acquire metallic properties and begin to behave like electrodes placed in an aqueous solution (an excellent medium for electrolysis). The charging current begins to decompose the water in the electrolyte into elemental components (hydrogen and oxygen). This process can be noticed without even knowing about its existence, by observing the so-called "boiling" of the battery. This term is erroneously used because of the external similarity of the bubbling of gas bubbles during electrolysis with boiling. It is more correct to call this effect gas evolution. Gassing starts when the battery has about 70-80% of its full charge. If the battery had been charged at the same rate, the gassing would have damaged the battery cells. However, the rate of electrolysis causing outgassing is proportional to the current flowing through the cell. The lower the current, the slower the water decomposes and the weaker the gas evolution. You can significantly reduce the devastating effects of outgassing by reducing the charging current when signs of outgassing appear. Although it stops completely only in the absence of current, the amount of charging current can be reduced to such a level that the quality of the battery is not degraded when the charge is accumulated. At the last stage of the charge, the battery is charged with a current, the value of which is usually a small part of the initial charge current. This current slowly charges the battery and thereby prevents intense gas evolution. After the battery is fully charged, it can be disconnected from the power source. Due to the presence of impurities in the electrolyte and changes in the chemical composition of the plates, internal currents arise in the battery cells, which reduce the accumulated charge over time. Eventually the battery will self-discharge. Battery charge regulators Obviously, the current required to charge the battery depends on the state of charge of the battery cells. This implies the need to create a charge regulator that evaluates the state of discharge of the battery and, depending on it, controls the charging current. There are three ways to charge lead-acid batteries. When charging from solar cells, the most suitable method is with a two-stage charging cycle (Fig. 1).
First of all, let's assume that the battery is completely discharged. Let's start passing current through the elements. Since the charging cycle of the battery must correspond to the period of generation of useful electrical power by the solar cells, it is desirable that the battery be charged in the shortest possible time. The optimal charging mode will be one in which gas evolution will begin approximately 4 hours after the start of charging the battery. This time corresponds to the highest intensity of solar radiation during daylight hours, usually in the range of 10-14 hours. Regardless of seasonal changes and weather conditions, it is at this time of day that the maximum return from solar cells can be achieved. This charging time corresponds numerically to a charging current of 20 A for every 100 Ah of battery capacity, if, of course, solar cells allow such a current to be received. For example, a 75Ah battery should be charged with 15A. After a 4-hour charge at a fixed rate, the battery will have 80% of its full charge before gassing begins. The next step is to reduce the charging current to a lower level. The value of this current is usually 2-5% of the battery capacity. For a battery with a capacity of 75 Ah taken as an example, the charging current at the final stage of charging can be 1,5-3,75 A. Depending on the selected current, it will take another 4-10 hours for the final charge of the battery. At this speed, it takes more than a day to fully charge the battery. However, in advanced power devices, batteries are usually in a fully charged state most of the time of operation, and their complete discharge is extremely rare. Backup (compensation) recharging of batteries After the final charge of the battery, it is recommended to additionally apply a reserve (compensation) recharge current to it. The value of this current is usually 1-2% of the total battery capacity. This additional third stage of battery charge complicates the design of the charge regulator. You can get out of the situation by combining the second and third charging stages, using the same current as the final current or backup charging current, the value of which is 2% of the battery capacity. As a result, the design of the regulator is simplified and its reliability is increased. Regulator design For normal operation of the charge regulator, which satisfies the charging current requirements listed above, it is necessary to know the state of charge of the battery at any time. Fortunately, the battery itself provides the key to solving this problem: there is a well-established relationship between the amount of charge stored in the battery and the voltage across it. As can be seen from fig. 2, this relationship is almost always linear.
The charge area of interest to us lies within 70-80% of the battery's full charge. It is when this degree of charge is reached that gas evolution begins and it is necessary to change the charging current. For a 12-volt battery, the voltage at this point is 12,6 V. A fully charged battery develops a voltage of 13,2 V. By determining the voltage on the battery, you can adjust the charging current. If the voltage is below 12,6 V, then the battery cells contain less than 80% of the charge and the regulator provides full charging current. When the voltage on the battery rises above 12,6 V, it is necessary to reduce the charging current to the level of the charging current. The voltage on the battery is monitored by a special device (comparator), which is nothing more than a conventional amplifier with a very high gain. Indeed, the comparator included in the circuit shown in Fig. 3 can be used as an operational amplifier.
The comparator compares two voltages - measured and reference, supplied to its inputs. The inverting input of the comparator (-) is supplied with a reference voltage from the zener diode D2. This voltage sets the trigger level of the device. The battery voltage is divided by resistors R1 and R2 so that it approximately equals the stabilization voltage of diode D2. The voltage divided by the resistors is applied to the non-inverting input (+) of the comparator from the potentiometer slider for fine adjustment of the switching threshold. If the battery voltage decreases so much that the signal at the non-inverting input drops below the limit determined by diode D2, a negative voltage will be established at the output of the comparator. If the battery voltage rises above the reference voltage, the comparator output will be positive. Switching the sign of the voltage at the output of the comparator will provide the necessary regulation of the charging current. The principle of operation of the charge regulator The charging current is regulated by an electromagnetic relay. The relay is controlled through transistor QI by the output voltage of the comparator. A negative voltage at the output of the comparator means that the battery is discharged and full charging current is required (transistor Q1 is closed). Therefore, the collector current is zero and the relay is turned off. Normally closed relay contacts shunt the current limiting resistor Rs. When the relay is turned off, the resistor is removed from the circuit and the full current from the solar cells goes to the battery. As the state of charge increases, the voltage on the battery increases. Gas evolution begins when the voltage reaches 12,6 V. The comparator, set to this level, switches (positive at the output of the comparator). The transistor opens and the collector current turns on the relay. The relay contacts that shunted the resistor Rs open.
Now the charging current from the solar cells must overcome the resistance of the limiting resistor. The value of this resistor is chosen so that the value of the charging current is 2% of the battery capacity. In the table in fig. 4 shows the values of Rs depending on the capacity of the battery. There is some uncertainty around the comparator switching voltage. Let, for example, the voltage on the battery rise to 12,6 V, exceeding the threshold. Under normal conditions, this will change the output voltage of the comparator, operate the relay and decrease the charging current. However, the battery output voltage depends not only on the state of charge, but also on other factors, and therefore it is not uncommon to observe a slight decrease in voltage after turning off a large charging current. It is quite likely, for example, that the voltage drops by several hundredths of a volt (up to 12,55 V). How will the scheme work in this case? Obviously, the comparator will switch back and the high charging current mode will be restored. Since the battery voltage is very close to 12,6V, a sudden increase in current will undoubtedly cause the voltage to surge to a level higher than 12,6V. As a result, the relay will turn off again. Under these conditions, the comparator will switch back and forth near the trip voltage. To eliminate this undesirable effect, called "yaw", a small positive feedback is introduced into the amplifier using a resistor, creating a hysteresis dead zone. With hysteresis, the comparator requires a larger change in voltage than before to operate. As before, the comparator will switch at 12,6 volts, but for it to reset, the battery voltage must drop to 12,5 volts. This eliminates the oscillating effect. The serial connection of the diode D1 in the charging circuit protects the battery or discharge through the solar cells in the dark (at night). This diode also prevents the charge regulator from drawing power from the battery. The regulator is fully powered by solar cells. indicator device An indicator device is introduced into the charge controller, designed to display the mode of operation of the controller at any time. Although the indicator is not a necessary part of the device (the regulator will work without it), nevertheless, its presence increases the convenience of working with the regulator. The indicator device (Fig. 3) consists of two comparators and two light emitting diodes (LEDs). The inverting input of one comparator and the non-inverting input of the other are connected to a zener diode that generates a reference voltage. The remaining inputs of the comparators are connected to the output of the comparator that controls the charging current. The upper comparator is triggered and turns on the LED LED1 when the regulator operates in high charging current mode. If the regulator switches to the feeding current mode, the upper comparator turns off, and the lower comparator is activated and turns on the LED LED2. charge regulator design The charge regulator is mounted on a printed circuit board (Fig. 5), the placement of circuit components on which is shown in Fig. 6. Particular attention should be paid to the placement of semiconductor elements (to avoid erroneous connection of leads). The finished circuit is placed in any (preferably waterproof) case. For these purposes, a small plastic box is quite suitable. If the case is opaque, to indicate the operating modes, drill a hole for the LEDs in its cover. It is also necessary to make a hole on the side of the housing for the output of connecting conductors.
Powerful Regulators The described regulator can control a charging current of about 5 A. Its value is limited by the properties of the contactor of the electromagnetic relay used. The relay contacts are rated for current up to 3 A, and it is quite natural to ask why it is recommended to use them up to 5 A. This can be given the following explanation. When the contacts open a circuit, a small electrical arc usually occurs between them. The arc leads to phenomena similar to electric welding, and notches appear on the surface of the contacts. The greater the flowing current, the stronger the effect of the electric arc. To prevent such a process in the circuit of the described regulator, the relay contacts are shunted with a small resistance. Therefore, a significant part of the energy is absorbed by the resistor, and not dissipated in the electric arc. Thus, the contacts, without being destroyed, can regulate currents in excess of the rated current. If it is required to increase the regulated current, it is necessary to use a more powerful relay in the circuit, switched on by the contacts of a low-current relay, as shown in fig. 7.
To install a second relay, the PCB drawing needs to be modified accordingly. Start by removing the jumpers going to the relay contacts. This disconnects the contacts from the current limiting resistor. Now use these pins to drive a more powerful relay. It is also necessary to replace the diode D1 and the current-limiting resistor Rs with a diode and resistor capable of withstanding high currents. It makes more sense to place both of these elements off the board near the relay, since they dissipate more heat than the previous circuit elements. Connect the battery and solar cells directly to the power relay using thick wires, and use thin wires to power the regulator circuit from the positive output of the solar cells. Low Power Regulator There may be such a case when the electric energy of a small solar battery is not enough even to power the relay. Then the relay can simply be replaced by a transistor. To this end, you can remove the relay RL1 and the transistor Q1 that controls it and connect a pnp transistor to the resistor Rs, and its base to the resistor R5. On fig. 8 shows the electrical circuit after complete modification.
When the voltage at the output of the comparator is positive, the transistor is turned on and full charging current flows to the battery. When the regulator switches to boost charge mode, the comparator output becomes negative, the transistor turns off and the charging current now flows only through the Ra resistor, bypassing the transistor. The advantage of this circuit over the relay circuit is that its operation is not limited to 12 V. The device can regulate the charging of batteries rated for voltages of 3-30 V. Of course, it is necessary to change the values of the resistors and R2 and the type of diode D2 in order to bring together the values of the voltage falling on the potentiometer VR1 and reference on the zener diode. The current is limited to about 250 mA. The printed circuit board itself serves as a heatsink that allows you to remove excess heat from the used transistor. The heat sink pad is formed on the reverse side of the board and does not require any insulation. Calibration Only four connections need to be made to connect the regulator. Two - to the positive and negative terminals of the solar array and two, respectively, to the positive and negative terminals of the battery. After installing the regulator in the charger, it is necessary to calibrate the circuit and, in particular, adjust its sensitivity to voltage changes so that the current switches at the right moment. To do this, first let the battery discharge slightly. Then the VR1 potentiometer slider is turned clockwise until it stops (according to the diagram, to the upper position). The relay contacts will then close. The voltage on the battery as it is being recharged is monitored with a voltmeter. When it reaches 12,6 V, the potentiometer VR1 slider rotates in the opposite direction until the relay turns off. This will correspond to the "recharge" charge. Unfortunately, the charging voltage of a battery also depends on its temperature. The colder the battery, the more voltage is required to charge. This changes the threshold voltage at which the regulator should operate. The graph in fig. 9 shows the response voltage as a function of temperature.
An error in setting the trip voltage can in principle be neglected. If the temperature of the battery during charging is relatively stable and positive, which can be ensured in one way or another, for example by covering it well, then small temperature changes will practically not affect the operation of the regulator.
Author: Byers T.
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