What is a dry cell? How do cells and batteries work? How Do Cells And Batteries Supply Current? How does the chemical reaction occur in cells?
If we had no better sources of electrical energy than simple cells, we could not make much use of electric current. To supply more current, other types of cells have been developed. The type that you have probably seen most often is known as a dry cell. It is used in flashlights and portable radio sets. Compared with a wet cell, a dry cell is fairly convenient to carry. For no liquid spills out if the cell is tipped or shaken. Yet a useable dry cell is not really dry except on the outside. The inside must be kept moist. Otherwise the cell will not supply current. When the inside becomes dry, the cell is useless.
You can see how the main parts of a dry cell are arranged by studying the picture on page 208. It shows what a large dry cell looks like cut in half from top to bottom. The outer zinc can is the negative electrode, while the long carbon rod is the positive electrode. Though not a metal, carbon is a conductor of electric current. Between the zinc can and the carbon rod is a moist mixture of chemicals. They include ammonium chloride, zinc chloride, and manganese dioxide. The solution of ammonium chloride in water is the electrolyte.
Except for using some different materials, a dry cell produces current in the same way that a simple cell does. Ammonium chloride acts chemically with the zinc. Zinc chloride, ammonia, and hydrogen are formed. The zinc chloride remains in the solution, while the ammonia and hydrogen are set free as gases at the carbon electrode. As a result of the chemical action, the zinc can becomes negatively charged and the carbon rod positively charged. When the two electrodes are connected, an electric current flows from one to the other.
Bubbles of gas on the carbon electrode will reduce the current just as those on the copper electrode do in a simple cell. A dry cell has some chemicals that help prevent this, however. The zinc chloride acts with the ammonia to form ammonium chloride, while the manganese dioxide acts with the hydrogen to form water. Then the ammonium chloride dissolves in the water and provides more electrolyte. These changes take place rather slowly. So when a dry cell supplies a large current for too long, the ammonia and hydrogen cannot be removed fast enough. As a result, the current becomes smaller. If the cell is disconnected for a while, the chemicals have time to remove the gas bubbles. The cell can then supply a larger current again.
A new dry cell has a pressure of about 1.5 volts. The size of the cell makes no difference. If a higher voltage is needed, we can connect two or more cells in series to form a battery. The voltage of each cell is added to that of the others. As in any series connection all the current flows through each cell, one after another. The positive electrode of one cell is connected to the negative electrode of the next cell and so on. A flashlight battery usually has two a three small dry cells. To connect them in series, a piece of brass on top of the carbon rod is pushed against the bottom of the next zinc can.
As in a simple cell, the zinc in a dry cell is used up. When holes are eaten through the zinc can the water inside evaporates. Since the cell no longer supplies current, it is worn out. The old cell must then be replaced with a new one. For sob purposes, we can use a better type of cell. This is known as a storage cell. Of course, it does not store current. Like other electric cells, it stores chemical energy that can be changed into electrical energy. Unlike the others, however, the storage cell can have its stored chemical energy replaced. Then it will supply current again.
A storage cell produces current by chemical action, as other cells do. Lead is the negative electrode, while lead peroxide is the positive electrode. A solution of sulfuric acid in water is the electrolyte. When the two electrodes are connected, an electric current flows from one to the other. The sulfuric acid acts chemically with each electrode to form lead sulfate. As this material covers both of them, the cell becomes discharged. Then it can no longer supply current. To charge the cell again, a current must be sent through it in the right direction. This changes the electrodes back to lead and lead peroxide. Electrical energy is changed into chemical energy, which is stored in the cell. So it will now supply current again.
The pressure of a storage cell is about 2 volts. This is not enough for most uses. Ordinarily, a number of these cells are connected in series to form a storage battery. For example, the storage battery in an automobile has either three or six cells. Lead bars across the top connect them in series. Each cell adds its voltage to that of the others. So the total voltage of the battery is either 6 or 12 volts. Even higher voltages are used in storage batteries that operate telephones or provide an emergency source of electrical energy for hospitals, factories, and the like.
Storage batteries must be well built to supply large currents. The electrodes are made up of sets of plates connected together. Each plate has a metal frame filled with either lead or lead peroxide, depending on whether it is part of a negative or a positive electrode. The different sets of plates are fitted close together in the cells. Thin strips of wood, rubber, or plastic in between the plates keep them from touching. A well-built storage battery can be discharged and recharged as many as 500 times before finally becoming worn out. It gives back from 75 to 85 per cent of the electrical energy used in charging it. The rest of the energy causes other changes. Some of it produces unwanted chemical changes, and some of it produces heat.
If properly taken care of, a storage battery lasts quite a while. Some of the water in the electrolyte evaporates, and some is decomposed into hydrogen and oxygen by the charging current. So the water that is lost must be replaced now and then. Only distilled water should be used for this purpose. It contains no minerals that would ruin the electrodes. To make sure that the battery is charged, the electrolyte in each cell should be tested with a hydrometer.