In recent years, cells and batteries—especially cells—have become available in many different sizes and shapes besides the old cylindrical cells, transistor batteries and lantern batteries. These are used in watches, cameras, and other microminiature electronic gizmos.
Silver-oxide types
Silver-oxide cells are usually made into button-like shapes, and can fit inside even a small wristwatch. They come in various sizes and thicknesses, all with similar appearances. They supply 1.5 V, and offer excellent energy storage for the weight. They also have a flat discharge curve
Mercury types
Mercury cells, also called mercuric oxide cells, have advantages similar to silver-oxide cells. They are manufactured in the same general form. The main difference, often not of significance, is a somewhat lower voltage per cell: 1.35 V. If six of these cells are stacked.
to make a battery, the resulting voltage will be about 8.1 V rather than 9 V. One additional cell can be added to the stack, yielding about 9.45 V. There has been some decrease in the popularity of mercury cells and batteries in recent years. This is because of the fact that mercury is highly toxic. When mercury cells and batteries are dead, they must be discarded. Eventually the mercury, a chemical element, leaks into the soil and ground water. Mercury pollution has become a significant concern in places that might really surprise you.
Lithium types
Lithium cells have become popular since the early eighties. There are several variations in the chemical makeup of these cells; they all contain lithium, a light, highly reactive metal. Lithium cells can be made to supply 1.5 V to 3.5 V, depending on the particular chemistry used. These cells, like their silver-oxide cousins, can be stacked to make batteries. The first applications of lithium batteries was in memory backup for electronic microcomputers. Lithium cells and batteries have superior shelf life, and they can last for years in very-low-current applications such as memory backup or the powering of a digital liquid-crystal-display (LCD) watch or clock. These cells also provide energy capacity per unit volume that is vastly greater than other types of electrochemical cells. Lithium cells and batteries are used in low-power devices that must operate for a long time without power-source replacement. Heart pacemakers and security systems are two examples of such applications.
Lead-acid cells and batteries
Automotive batteries are made from sets of lead-acid cells having a free-flowing liquid acid. You cannot tip such a battery on its side, or turn it upside-down, without running the risk of having some of the acid electrolyte get out.
Lead-acid batteries are also available in a construction that uses a semisolid electrolyte. These batteries are popular in consumer electronic devices that require a moderate amount of current. Notebook or laptop computers, and portable video-cassette recorders (VCRs), are the best examples.
A large lead-acid battery, such as the kind in your car, can store several tens of ampere- hours. The smaller ones, like those in notebook computers, have less capacity but more versatility. Their overwhelming advantage is their ability to be used many times at reasonable cost.
Nickel-cadmium cells and batteries
You’ve probably seen, or at least heard of, NICAD cells and batteries. They have become quite common in consumer devices such as those little radios and cassette players you can wear while doing aerobics or just sitting around. (These entertainment units are not too safe for walking or jogging in traffic. And never wear them while riding a bicycle.) You can buy two sets of cells and switch them every couple of hours of use, charging one set while using the other. Plug-in charger units cost only a few dollars.
Types of NICAD cells
Nickel-cadmium cells are made in several types. Cylindrical cells are the standard cells; they look like dry cells. Button cells are those little things that are used in cameras, watches, memory backup applications, and other places where miniaturization is important. Flooded cells are used in heavy-duty applications and can have a charge capacity of as much as 1,000 Ah. Spacecraft cells are made in packages that can withstand the vacuum and temperature changes of a spaceborne environment.
Uses of NICADs
There are other uses for NICADs besides in portable entertainment equipment. Most orbiting satellites are in darkness half the time, and in sunlight half the time. Solar panels can be used while the satellite is in sunlight, but during the times that the earth eclipses the sun, batteries are needed to power the electronic equipment on board the satellite. The solar panels can charge a set of NICADs, in addition to powering the satellite, for half of each orbit. The NICADs can provide the power during the dark half of each orbit. Nickel-cadmium batteries are available in packs of cells. These packs can be plugged into the equipment, and might even form part of the case for a device. An example of this is the battery pack for a handheld ham radio tranceiver. Two of these packs can be bought, and they can be used alternately, with one installed in the
“handie-talkie” (HT) while the other is being charged.
NICAD neuroses
There are some things you need to know about NICAD cells and batteries, in order to get the most out of them.
One rule, already mentioned, is that you should never discharge them all the way until they “die.” This can cause the polarity of a cell, or of one or more cells in a battery, to reverse. Once this happens, the cell or battery is ruined.
Another phenomenon, peculiar to this type of cell and battery, is called memory. If a NICAD is used over and over, and is discharged to exactly the same extent every time (say, two-thirds of the way), it might start to “go to sleep” at that point in its discharge cycle. This is uncommon; lab scientists have trouble forcing it to occur so they can study it. But when it does happen, it can give the illusion that the cell or battery has lost
some of its storage capacity. Memory problems can be solved. Use the cell or battery almost all the way up, and then fully charge it. Repeat the process, and the memory will be “erased.” NICADS do best using wall chargers that take several hours to fully replenish the cells or batteries. There are high-rate or quick chargers available, but these can sometimes force too much current through a NICAD. It’s best if the charger is made especially for the cell or battery type being charged. An electronics dealer, such as the manager at a Radio Shack store, should be able to tell you which chargers are best for which cells and batteries.
Photovoltaic cells and batteries
The photovoltaic cell is completely different from any of the electrochemical cells. It’s also known as a solar cell. This device converts visible light, infrared, and/or ultraviolet directly into electric current.
Solar panels
Several, or many, photovoltaic cells can be combined in series-parallel to make a solar panel. An example is shown in Fig. 7-6. Although this shows a 3 × 3 series-parallel array, the matrix does not have to be symmetrical. And it’s often very large. It might consist of, say, 50 parallel sets of 20 series-connected cells. The series scheme boosts the voltage to the desired level, and the parallel scheme increases the current-delivering ability of the panel. It’s not unusual to see hundreds of solar cells combined in this way to make a large panel.
Construction and performance
The construction of a photovoltaic cell is shown in Fig. 7-7. The device is a flat semiconductor P-N junction, and the assembly is made transparent so that light can fall directly on the P-type silicon. The metal ribbing, forming the positive electrode, is interconnected by means of tiny wires. The negative electrode is a metal backing, placed in contact with the N-type silicon. Most solar cells provide about 0.5 V. If there is very low current demand, dim light will result in the full output voltage from a solar cell. As the current demand increases, brighter light is needed to produce the full output voltage. There is a maximum limit to the current that can be provided from a solar cell, no matter how bright the light. This limit is increased by connecting solar cells in parallel.
Practical applications
Solar cells have become cheaper and more efficient in recent years, as researchershave looked to them as a long-term alternative energy source. Solar panels are used in satellites. They can be used in conjunction with rechargeable batteries, such as the lead-acid or nickel-cadmium types, to provide power independent of the commercial utilities.
A completely independent solar/battery power system is called a stand-alone system. It generally uses large solar panels, large-capacity lead-acid or NICAD batteries, power converters to convert the dc into ac, and a rather sophisticated charging circuit. These systems are best suited to environments where there is sunshine a high percentage of the time.
Solar cells, either alone or supplemented with rechargeable batteries, can be connected into a home electric system in an interactive arrangement with the electric utilities. When the solar power system can’t provide for the needs of the household all by itself, the utility company can take up the slack. Conversely, when the solar power system supplies more than enough for the needs of the home, the utility company can buy
the excess. Photovoltaic cells and batteries
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