Battery

Batteries serve as a convenient source of electricity. They power such portable equipment as radios, tape recorders, and television sets. In an automobile, a battery provides power to start the engine. Batteries also supply electricity in spacecraft and submarines. During power failures, batteries provide an emergency supply of electricity for telephones, fire alarms, and hospitals and other essential buildings.

Batteries also can be classified according to the general makeup of their electrolyte, the chemical substance that conducts electric current inside a cell. Many primary batteries have a jellylike or pastelike electrolyte. Batteries that contain such nonspillable material are known as dry cells. A few types of primary batteries, called wet cells, contain liquid chemicals. Most secondary batteries have a liquid electrolyte.

Batteries are manufactured in a wide range of sizes. For example, the tiny batteries used in electric watches weigh only about 1/20 of an ounce (1.4 grams). The huge batteries that power submarines weigh up to 1 short ton (0.91 metric ton). However, manufacturers produce most batteries in certain standard sizes. Therefore, batteries made by different manufacturers can be used in the same clock, radio, or other device.

A dry cell consists of a zinc container filled with substances that produce an electric current by reacting chemically with one another. The container itself is the negative terminal of the cell. A carbon rod in the center serves as the positive terminal. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagram.

Batteries also differ in voltage. A primary cell of the type used in a flashlight has 1 1/2 volts. Most secondary batteries for automobiles are 12-volt batteries consisting of six 2-volt cells connected in a series.

A 9-volt battery has six individual dry cells. Each cell produces 1 1/2 volts of electricity. The cells are connected together in series (one after another) so that their total voltage equals 9 volts. Such batteries power transistor radios. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagram.

There are three major dry primary batteries. They are (1) carbon-zinc cells, (2) alkaline cells, and (3) mercury cells.

Carbon-zinc cells are the general-purpose batteries used in flashlights, photoflash units, and toys. These cells, also called Leclanche dry cells, are contained in a zinc can. The can serves both as a container for the parts of the cell and as the anode. A carbon rod in the center of the cell functions as the cathode current-collector. But the actual cathode material is a mixture of manganese dioxide and carbon powder packed around the rod. The electrolyte is a paste composed of ammonium chloride, zinc chloride, and water.

The anode and the cathode are separated by a sheet of porous material, such as paper or cardboard, soaked with the electrolyte. This thin layer, called a separator, prevents the electrode materials from mixing together and reacting when a battery is not being used. Such action could cause the zinc anode to wear away prematurely and reduce the life of the battery.

The chemical process that produces electricity begins when the atoms of zinc at the surface of the anode oxidize. A zinc atom oxidizes when it gives up both its electrons. It then becomes an ion (an electrically charged atom) with a positive charge. The zinc ions move away from the anode. As they do so, they leave their electrons behind on its surface. The anode thus gains an excess of electrons and becomes more negatively charged than the cathode.

If a cell is connected to an external circuit, the zinc anode's excess electrons flow through the circuit to the carbon rod. The movement of electrons forms an electric current. After the electrons enter the cell through the rod, they combine with molecules of manganese dioxide and molecules of water. As these substances are reduced (gain electrons) and react with one another, they produce manganese oxide and negative hydroxide ions.

This reaction makes up the second half of the cell's discharge process. It is accompanied by a secondary reaction. In the secondary reaction, the negative hydroxide ions combine with positive ammonium ions that form when ammonium chloride is dissolved in water. The secondary reaction produces molecules of ammonia and molecules of water.

The various chemical reactions by which a carbon-zinc cell produces electricity continue until the manganese dioxide wears away. After this cathode material has been "used up," the cell can no longer provide useful energy and is dead.

Dead cells should be removed immediately. Even after a cell stops working, its electrolyte continues to eat away at the container and may puncture it. If the electrolyte leaks out, it can damage the equipment.

A carbon-zinc cell, like most primary batteries, cannot be recharged efficiently. But a device called a battery charger may extend the life of a cell for a short time. It partially restores the cell's ability to produce electricity. A battery charger functions by passing a current through the cell in a direction opposite to that of the flow of electricity during discharge.

Alkaline cells resemble carbon-zinc cells. Both have the same anode and cathode materials, which undergo similar chemical reactions. But these two types of dry primary cells differ in several ways.

An alkaline cell has a highly porous zinc anode that oxidizes more readily than that of a carbon-zinc cell. Its electrolyte is a strong alkali solution called potassium hydroxide. This compound conducts electricity inside the cell better than does the solution of ammonium chloride and zinc chloride in a carbon-zinc cell. Such features enable an alkaline cell to deliver sustained high currents more efficiently than a carbon-zinc cell.

Alkaline cells serve as an excellent source of energy for bicycle lights, electric shavers, portable TV's, and walkie-talkies. In electric toys that require much current, they are more economical than zinc-carbon cells because they last from five to eight times as long.

Mercury cells have an anode of zinc, a cathode of mercuric oxide, and an electrolyte of potassium hydroxide. During discharge, the zinc changes to zinc oxide and the mercuric oxide becomes mercury. The potassium hydroxide remains unchanged.

A mercury cell has certain advantages over a carbonzinc cell and an alkaline cell. For example, the voltage of a mercury cell remains constant, but that of the other primary cells drops during use. This feature makes mercury cells suitable for sensitive devices, such as hearing aids and scientific instruments.

Lead-acid storage batteries consist of a plastic or hard-rubber container that holds three or six cells. Each cell has two sets of latticelike electrodes or plates. The frames of these structures, called grids, are made of a lead-antimony alloy. The meshes (open spaces) of the negative electrode are filled with a mass of pure lead in spongy form. The meshes of the positive electrode contain lead dioxide, a compound of lead and oxygen. An electrolyte of sulfuric acid and water surrounds the electrodes.

Most lead-acid storage batteries have six cells. Each cell contains two sets of lead electrodes called plates. The plates are separated by plastic or rubber sheets. A solution of sulfuric acid, called the electrolyte, surrounds the plates. Each terminal post on the outside of the battery is connected to one set of plates. Vent holes in the case allow water to be added to the electrolyte and also permit gases produced in the cell to escape. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagram.

During the discharge process, chemical reactions take place between the electrode materials and the electrolyte. At the negative electrode, atoms of pure lead react with negative sulfate ions of the electrolyte. The negative sulfate ions, along with positive hydrogen ions, form when sulfuric acid dissolves in water. As the lead atoms combine with the sulfate ions, each lead atom loses two electrons and becomes a molecule of lead sulfate.

The electrons lost by the lead atoms flow from the negative electrode to the positive electrode through a device using the electric current. At the positive electrode, they are captured by molecules of lead dioxide, which in turn combine with the hydrogen and sulfate ions of the electrolyte. This reaction produces lead sulfate and water.

Adding together the positive and negative electrode reactions yields a combined discharge reaction. Thus, sulfuric acid is consumed and water is produced during battery use. Eventually the sulfuric acid becomes so diluted that the necessary chemical reactions can no longer occur.

After a lead-acid battery loses its ability to supply electricity, it can be recharged by means of a battery charger. The battery charger forces electrons through the battery in a direction opposite to that of the discharge process. This action reverses the chemical reactions that occur when a battery discharges.

The reversed reactions of the charging process restore the electrode materials to their original form. They also increase the amount of sulfuric acid in the electrolyte to a satisfactory level. After a battery has been charged, it can again produce current.

Lead-acid batteries furnish energy for the electrical systems of cars and trucks. They also power submarines and provide emergency electricity for such vital facilities as hospitals and sanitation plants.

Nickel-cadmium storage batteries operate on the same general principles as lead-acid batteries but use different chemical substances. In a nickel-cadmium battery, the negative electrode is made of cadmium and the positive electrode of nickel oxide. A solution of potassium hydroxide serves as the electrolyte.

The chemical composition of a nickel-cadmium battery allows the battery container to be sealed airtight, which prevents the corrosive electrolyte from leaking. Because of this advantage, nickel-cadmium batteries are used in drills, garden tools, and other portable equipment. Most space satellites use these batteries.

In 1836, John F. Daniell, an English chemist, introduced a more efficient primary cell. The Daniell cell had two liquid electrolytes and produced a steadier current than Volta's device. In 1859, the French physicist Gaston Plante invented the first secondary battery, the lead-acid storage battery. During the 1860's, another French scientist, Georges Leclanche, invented a type of primary cell from which the modern dry cell was developed.

Through the years, scientists have designed smaller but increasingly powerful batteries for the growing number of portable electric devices. For example, a lithium cell is so tiny that it is often called a button battery. But it produces voltages higher than any other single cell. It uses lithium metal as the negative electrode and any one of several oxidizing agents as the positive electrode. Lithium cells are used mainly in calculators, cameras, pacemakers, and watches.

Researchers have also developed a lead-acid storage battery that does not require the periodic addition of water. This battery, called a maintenance-free battery, is completely sealed except for a safety valve for venting gases. It lasts much longer than the standard lead-acid battery because its grids are made of lead-calcium-tin alloys. These substances, unlike lead-antimony alloys, do not cause a battery to discharge when it is not in use.

Contributor: Stanley D. James, Ph.D., Research Specialist in Electrochemistry, Naval Surface Warfare Center.

See also Electric Circuit; Electricity; Electrolysis; Fuel cell.

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