How and Why Batteries Work

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A battery is a device that produces electricity by means of chemical action. It consists of one or more units called electric cells, each of which has all the chemicals and parts needed to produce an electric current. The word battery actually means a group of connected cells, but the term generally refers to single cells, such as those that power flashlights and electric toys.

Kinds of Batteries

Manufacturers produce a wide variety of batteries, which may be classified according to their basic design. The design of a battery determines the amount of electricity provided. Some batteries, called primary batteries, stop working and must be discarded after one of their chemicals has been used up. Other batteries can be recharged and used again after they have discharged their electric energy; these are called secondary, or storage, batteries.

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

Batteries come in a wide range of sizes as well, from the tiny batteries that power hearing aids and watches, to the huge, one-ton batteries that power submarines. However, manufacturers produce most batteries in certain standard sizes, meaning that batteries made by different manufacturers can be used in the same clock, radio, flashlight, or other device.

Batteries also differ in voltage. A primary cell of the type used in a standard flashlight has 1½ volts. Most secondary batteries for automobiles consist of six 2-volt cells connected in a series.

How Dry Primary Batteries Work

Dry cell batteries are the most widely used type of primary cell. Such batteries differ in various ways, but all have certain basic parts. Every dry primary battery has two structures called electrodes. Each electrode consists of a different kind of chemically active material. An electrolyte between the electrodes causes one of them, called an anode, to become negatively charged and the other, called a cathode, to become positively charged. The electrolyte helps promote the chemical reactions that occur at the electrodes.

The major types of dry primary batteries are (1) carbon-zinc cells, (2) alkaline cells, and (3) mercury cells.

Carbon-Zinc Cells are the general-purpose batteries used in flashlights, toys, cameras, etc. Also called Leclanché dry cells, these cells are contained within a zinc "can," which 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. The actual cathode material, however, 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 separator is a sheet of porous material, such as paper or cardboard, soaked with electrolyte that prevents the electrode materials from mixing together and reacting when a battery is not being used. Without a separator the zinc anode could wear away prematurely and reduce the life of the battery.

The chemical process that produces electricity inside a carbon-zinc cell begins when the zinc atoms at the surface of the anode oxidize (give up their oxygen electrons). The zinc atom then becomes an ion (electrically charged atom) with a positive charge. The zinc ions move away from the anode, leaving their electrons behind on the anode's surface. The anode thus gains an excess of electrons and becomes more negatively charged than the cathode.

If the cell is connected to an external circuit, the zinc anode's excess electrons flow through the circuit to the carbon rod. This movement of electrons forms an electric current. As 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 oxice 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 which the negative hydroxide ions combine with positive ammonium ions that form when ammonium chloride, producing molecules of ammonia and molecules of water.

The chemical reactions that produce electricity inside a carbon-zinc cell continue until the manganese dioxide wears away. Once this cathode material has been "used up," the cell can no longer provide useful energy and is "dead."

A carbon-zinc cell cannot be recharged efficiently, but a battery charger can extend the life of a cell for a short time. The charger partially restores the cell's ability to produce electricity 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, and undergo similar chemical reactions. But the two types differ in several important ways.

An alkaline cell has a highly porous zinc anode that oxides 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 power source for electric shavers, portable TVs, walkie talkies, and portable radios. 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.

Mercury cells have certain advantages over carbon-zinc and alkaline cells. 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.

How Secondary Batteries Work

Secondary batteries are made so that their chemical reactions can be reversed. This feature enables them to recharged efficiently after they have delivered their electric energy. The most common types of secondary batteries are (1) lead-acid storage batteries and (2) nickel-cadmium batteries.

Lead-Acid Storage Batteries consist of a plastic or hard-rubber container that holds three or six cells, each of which 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.

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.

The current-producing process decreases and dilutes the electrolyte of sulfuric acid by using up sulfate ions and by adding water molecules to the solution. The battery becomes discharged when so little sulfuric acid remains that the necessary chemical reactions can no longer occur.

A lead-acid battery can be recharged by means of a battery charger, which forces electrons through the battery in a direction opposite to that of the discharge process. This action reverses the chemical reactions that occur at the electrodes 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. Once recharged, a lead-acid battery can again produce electricity.

Lead-acid storage batteries produce energy for the electrical systems of automobiles. They also power submarines and provide emergency electricity for hospitals, sanitation plants, etc.

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 most portable tools, cellular phones, etc. Most space satellites also use these batteries.

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