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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Flashlight with batteries recharged by solar cells. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Alternative energy sources It is not known why, but every time when it becomes necessary to use a flashlight, the batteries in it turn out to be dead. Common situation? Apparently, many of us use the flashlight so infrequently that the batteries gradually self-discharge, and as a result, when they become needed, it turns out that they have already used up their energy. In this case, the unusable manganese-zinc batteries are replaced with nickel-cadmium cells. An ingenious way out, until a flashlight is needed and it is discovered that there are no elements in it. It’s also good if they have been connected to the charger since the last use, or in extreme cases, if you can find them in the dark. In short, you need a flashlight that is always ready for use, i.e., the batteries in it must be freshly charged. This requirement is met by a flashlight recharged from the sun. There is no need to remove the batteries from it, they are always in a charged state. Flashlight device The clever part of the device is the flashlight itself, which includes a magnetic holder that is attracted to many metal surfaces. The holder consists of two magnetic rods pressed into a plastic housing. An insulated wire was attached to each magnet and passed inside the tube to the elements. The other part of the design is a solar-powered charger. On the surface of the charger, two steel strips are fixed, the distance between which corresponds to the distance between the magnetic rods of the flashlight. Each strip is connected to the corresponding output of the charger. When not in use, the flashlight is simply magnetized to the steel strips of the charger. This will ensure electrical contact between the charger and the batteries of the flashlight, which are recharged by solar cells. When it is necessary to use a flashlight, it, along with freshly charged batteries, is "torn off" from the charger. Nickel-cadmium batteries Nickel-cadmium batteries, commonly referred to as nickel-cadmium cells, are somewhat different from most dry cells, such as the manganese-zinc battery commonly used in flashlights. As the battery discharges, it loses some of its voltage. This effect is manifested in the brightness of the flashlight bulb. As the battery drains, the glow becomes more and more dimmer until it stops altogether. In contrast, nickel-cadmium cells hold the voltage quite stable during the discharge. This can be seen from the constancy of the glow up to a deep charge. After the element is discharged, the voltage on it quickly drops and the glow stops. On fig. 1 for comparison shows the dependence of the voltage on the degree of discharge of the elements of the two mentioned types. As you can see, to determine the remaining life of a manganese-zinc cell, you simply need to measure the voltage across it. For a nickel-cadmium element, this is not so easy to do. An 80% discharged cell produces the same voltage as a freshly charged cell. Thus, when recharging a nickel-cadmium cell, some difficulty arises. Until the element is completely discharged, we cannot judge its condition. In addition, nickel-cadmium cells are very sensitive to overcharging, which can damage them. So a partially discharged cell poses a really tricky question: how much charge can it take on?
Recharging nickel-cadmium cells To better understand the principle of operation of the charger, you must first familiarize yourself with the operation of the nickel-cadmium cell itself. You can start consideration with a completely discharged element. To charge it, you need to pass current through it. Due to its design, the nickel-cadmium cell has a fairly high internal resistance, which is inversely proportional to the amount of charge accumulated in the cell: the lower the charge, the higher the resistance. Due to the presence of internal resistance, part of the energy of the charging current is converted into heat. Therefore, it is necessary to start the charge with a small current, otherwise the energy dissipated in the internal resistance in the form of heat will lead to the failure of the element. As the charge increases, the internal resistance of the cell decreases. The lower the resistance, the less heat is dissipated and the more efficiently the charge of the cell flows. In addition, more charging current can now be passed through the cell, which will further speed up the charging process. In practice, it is possible to complete the charge cycle at a current significantly higher than the initial current. However, it is very difficult to regulate and maintain such a charge mode. For simplicity, manufacturers recommend the maximum safe current regardless of battery condition. For disk nickel-cadmium cells, this current does not exceed 330 mA. Even a completely discharged cell with a high internal resistance can be charged with such a current without fear. However, the answer to the question has not yet been received: what amount of charge will not harm the element? The charging current mentioned above can only be maintained until the battery is fully charged. This usually takes 4 hours. If you continue recharging, there is a danger of overcharging the cell, which can lead to a decrease in battery life or, worse, destruction of the cell. Thus, if the battery is only half discharged, it can be easily recharged without even knowing it. This is why the manufacturer recommends slow recharging. For a disk element, the charging current should not exceed 100 mA. With slow charging, you can charge the cell without fear of overcharging for the recommended 14 hours required to charge a fully discharged cell. In fact, it is possible to constantly lightly charge the element without fear of its destruction: the charge rate is quite low and excess energy is easily dissipated by the element. Battery Charger In this case, it was decided to choose a low battery charge rate. A complete diagram of the charger and flashlight is shown in fig. 2. To limit the charging current flowing through the nickel-cadmium cells, an incandescent lamp was included in the circuit.
Incandescent lamps with a tungsten filament have a specific characteristic. Cold filament has very low resistance. As the filament heats up, its resistance increases more than 10 times. By turning on such a lamp in series with nickel-cadmium cells, it is possible to partially compensate for the internal resistance of the battery. When a fully discharged battery is connected to a solar battery, the charging process occurs as follows. The solar battery creates a current in the circuit that flows through the nickel-cadmium cells and the incandescent lamp. The current is limited by the total resistance of the battery cells and the lamp filament. At first, most of the energy is absorbed by the battery due to its high internal resistance. A smaller part of the energy is released on the lamp, since at this moment its filament has a relatively low resistance of the order of 7 ohms. Regardless of internal resistance, nickel-cadmium batteries have their own voltage limit of 1,5 V per cell. In other words, the total battery voltage during charging under any condition is limited to about 3 V. With a small limiting resistor (lamp filament resistance of 7 ohms), batteries quickly reduce the output voltage of the solar array to about 3 V. As the battery charges, its internal resistance decreases, which in turn causes an increase in the current flowing through the cells and through the lamp, as well as the resistance of the lamp. In fact, the lamp makes up for the loss of battery resistance, and the charging current remains more or less constant. Flashlight As the resistance of the lamp increases, the voltage across it increases. But since the voltage on the battery is fixed, this leads to a gradual increase in the output voltage of the solar array. This trend continues until the battery is fully charged. At this point, the operating point on the solar array's current-voltage characteristic will have shifted so that a voltage of 2 V will be applied to the current-limiting lamp. At this voltage, the filament resistance is 25 ohms, limiting the charging current to 80 mA. No further increase in current or voltage will occur, since the operating point is at the bend of the volt-ampere curve of the photovoltaic converter (Fig. 3). We can say more: this current is so small that nickel-cadmium cells can be charged for an arbitrarily long time.
In addition to limiting the charging current, the lamp is an indicator of the presence of the charging process. A bright glow corresponds to a large current flowing through the elements. A weak glow or its absence indicates almost no charging current. Solar battery A 5 volt battery is great for two reasons: 5 volts is enough to charge the nickel-cadmium cells, and it also leaves power for the indicator lights. The simplest solar battery, consisting of 11 elements, more or less meets the above requirements. For such devices, small sickle-shaped elements can be used, as they are very cheap and develop sufficient power. Such elements usually generate a current of 80-100 mA. The requirements for the solar battery are quite mild, however, it must, together with the lamp, provide regulation. Although the solar cell could generate 5V at 80mA, the choice was quite arbitrary. If you have a solar panel that generates 6V at 100mA or more, it will work just fine. The extra voltage will be dissipated across the lamp, keeping the current at the desired level. Charger design The base of the charger is made from a rectangular piece of wood measuring 5x10 cm2 (any short block will do). If warm tones are preferred, then you can choose a block of mahogany or use a painted pine or spruce block. The final product looks like shown in Fig. 4.
Two steel strips are fixed on the front surface of the base. Any magnetic material will work, such as steel tape used to frame wooden containers. Such steel is thin, elastic and is a good conductor of electricity. First, solder the conductors to the undersides of the strips, and then drill holes for them in the bar. The strips are placed at the same distance as the magnets on the flashlight and are glued to the base with glue or epoxy. One of the conductors is connected to the solar battery, the other is soldered to the lamp base. The remaining output of the solar battery is attached to the outer (threaded) part of the indicator lamp. Finally, a hole with a diameter of 0,9 cm is drilled in the lower part of the base, a signal lamp is inserted and glued into it. To test the device, you simply need to short-circuit the contact strips with a wire, and the lamp should light up. If the photovoltaic converter is illuminated by the sun, the lamp will glow brightly. Finalization of the design of the flashlight Finally, it is necessary to modify the design of the flashlight. The principle is clear from Fig. 5. First you need to attach a flexible conductor to each magnetic rod. This can be done in different ways, depending on the design of a particular flashlight. You can solder the conductors using enough flux and being careful not to melt the plastic case. You can drill holes in the magnetic rods (if, of course, you have access to them) and fix the conductors in them with small screws or rivets.
After that, it is necessary to drill a hole in the flashlight body so that the conductors can be pulled inward. If the body of the flashlight is metal, the conductors are protected by an insulating sleeve (or other suitable element) to prevent abrasion of the insulation and short circuit. With a plastic flashlight, of course, less work. One conductor is soldered to the center terminal of the lamp socket of the flashlight so that after reassembly, the same reliable contact between the positive terminal of the battery and the lamp base is ensured (the conductor is laid at some distance from the rotating parts). The second conductor from the magnetic rod is passed into the base of the flashlight housing, where the spring is located. It is necessary to cut it to length and remove the spring. A diode is connected to the circuit. The diode terminal marked with a strip is soldered to the conductor, and the anode (unmarked) terminal is soldered to the spring. The diode is placed near the wider end of the spring so that it cannot be damaged by compression. A piece of flexible plastic tube is put on the diode to avoid a short circuit to the flashlight body. The diode has two functions. First, it prevents the battery from discharging through the solar panel at night. Secondly, when the flashlight is connected to the charger in reverse polarity, the diode will not pass the current and protect the batteries from countercharging. Now you need to finally assemble the flashlight, it is ready to go. It is best to place the charger on the wall so that the lens of the flashlight is facing down and not dirty.
Some recommendations Make sure the polarity is correct when connecting the flashlight to the charger. With one polarity, there will be a charge, with the other, there will be no charge due to the blocking diode. If the flashlight is not charging, it is necessary to swap the conductors coming from the solar battery. One more tip: nickel-cadmium cells, unfortunately, have "memory", for example, they can remember the discharge cycle. Let's say the flashlight is used for 15 minutes a day and then recharged again. The battery will remember this and will be "lazy". She will “feel” that her working day is 15 minutes. What happens if the flashlight is needed for 30 minutes or more? It will stop working after 15 minutes! It is worth the batteries to work out completely for 15 minutes, and they will refuse to last longer. To avoid this, it is necessary to periodically turn on the flashlight and fully discharge the batteries, and then reconnect them to the charger. A full charge of the batteries should last for 2 hours. Author: Byers T.
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