An electric battery consists of one or more electrochemical cells that produce power by means of a chemical reaction. A battery can be either primary or secondary: the primary type is normally regarded as unrechargeable whereas a secondary-cell, or storage, battery can be recharged.
There are some indications that batteries may have been used in the third century B.C.E. by the Parthians, a tribe in what is now Iran, for electroplating jewelery. The work that led to modern batteries, however, began with a discovery in the early 19th century. An Italian, Alessandro Volta, found that he could cause an electric current to pass through a wire by immersing two different metals in a salt solution.
All batteries, be they primary or secondary types, produce electricity through chemical reactions in which electrons are deposited on one metal or carbon electrode, the negative terminal, while they are removed from a second, the positive terminal. When the battery is connected to a circuit, the electrons from the negative terminal move to the positive through the circuit, while the chemical reactions restore the original charges.
The outer layer of an atom is composed of electrons, tiny particles each carrying a negative electrical charge. These particles are not all permanently attached to their atoms. In all but a few elements (the rare gases), there are loosely bound electrons that can be exchanged between atoms during chemical reactions.
When an atom gains an electron, it gains an extra negative charge and so becomes negatively charged as a whole. When it loses one, on the other hand, it becomes positively charged. Atoms or groups of atoms in this charged state are known as ions. Positive and negative ions are attracted to each other and, when circum- stances allow, will move together and combine to form compounds. Ions with similar charges repel each other.
The chemical reactions that provide electricity in cells involve either atoms leaving one of the metal electrodes to become positive ions and leaving their electrons behind, or ions or atoms accepting electrons from an electrode, or atoms and ions giving up electrons to an electrode. At least two such processes must occur in the cell: one is adding electrons to one electrode and the other is taking electrons away from the other electrode. A solution called an electrolyte is used to provide ions and atoms for such reactions to take place. Electrolytic solutions can be acids such as sulfuric acid; alkalis, such as caustic soda; or salts formed by the interaction of an acid and a base.
Electricity is generated in cells because when any of these chemical substances is dissolved in water, its molecules break up and become electrically charged ions. A good example is sulfuric acid, H2SO4, the molecules of which consist of two atoms of hydrogen, one of sulfur, and four of oxygen. When dissolved in water, the molecules split into three parts; the two atoms of hydrogen separate, and in the process, each loses an electron, becoming a positively charged hydrogen ion (represented by the sign H+). The sulfur atom and the four atoms of oxygen remain together as a sulfate group (SO4) and acquire the two electrons lost by the hydrogen atoms, thus becoming negatively charged (written SO42–). These groups can combine with others of opposite charge to form other compounds.
If one plate, or electrode, of zinc and one of either copper or carbon is dipped into a sulfuric acid electrolyte and each is externally connected to a load such as a light bulb, a current will flow through the bulb, lighting it. This is because one of the chemical elements chosen for the electrodes is electrically positive (that is, has a tendency to lose electrons and acquire a positive charge) with respect to the other, and when they are electrically connected, the chemical equilibrium of the cell is upset and reactions start at both plates. Under these circumstances, the atoms of zinc each give up two electrons, which flow through the external circuit and form the current. The positively charged zinc atoms left behind dissolve into the electrolyte, and each one combines with one of the negatively charged sulfate ions. The result is a neutral zinc sulfate molecule. The two electrons originally given up by each zinc atom travel around the external circuit and reach the other plate. There they combine with and neutralize the positive charges on two hydrogen atoms from the electrolyte. These two neutral hydrogen atoms then combine to form a molecule of hydrogen gas, and gas bubbles are produced at that plate or electrode.
In theory, the chemical reaction would go on, and electric current would continue to flow until all the zinc on the zinc plate (known as the negative electrode, or cathode) has been used up. But in the simple cell, a film of hydrogen bubbles begins to form on the copper or carbon plate (known as the positive electrode, or anode), and as hydrogen has a much higher electrical resistance than the electrolyte proper, the internal resistance of the cell increases, reducing the current that can flow in the external circuit.
If the external circuit is disconnected, the hydrogen bubbles will gradually disappear and the cell can be used again, but the same thing will reoccur. In all modern batteries, this effect, which is known as polarization, is greatly reduced by surrounding the positive electrode with a material known as the depolarizer. This works either by reacting with the hydrogen to form water or by taking over from the hydrogen the task of accepting the electrons as they arrive from the external circuit.
The Leclanché cell
The "dry" batteries used in flashlights and so on, which have a depolarizer, are of a type known as Leclanché with modifications to make the liquid electrolyte a semisolid. In its original form, the Leclanché cell was entirely "wet" with an electrolyte consisting of a strong solution of ammonium chloride. A zinc plate was used for the negative electrode, and a carbon rod packed into a porous pot containing crushed carbon and manganese dioxide (to accept the electrons) formed the positive electrode and its depolarizer. Similar materials are used in a modern dry Leclanché cell. The electrolyte is not, in fact, dry but is made up in the form of a moist paste or jelly.
Additives include mercuric chloride, introduced to inhibit what is known as local action, the name given to the chemical reactions that take place between zinc atoms and carbon and iron atoms, which occur as impurities in the zinc plate. It can be overcome by a process known as amalgamation in which the mercury forms an amalgam, or alloy, with the zinc, preventing it from reacting with its impurities. Other additives include potassium dichromate, which inhibits the corrosion of the zinc—an effect that would otherwise reduce the shelf life of the battery, that is, the length of time a battery can be stored without deterioration.
In the cylindrical Leclanché cell used in flash-lights, the zinc forms the outer casing and is also the negative electrode. The positive electrode consists of a mixture of graphite (carbon) and manganese dioxide depolarizer around a graphite rod.
Dry cells are supplied singly or in groups of two, three, or more connected in series to give higher voltages. In a series connection, the positive electrode of one cell is connected to the negative electrode of the next. High-voltage batteries, in which sixty or more individual cells are connected in series, are available but are very heavy and cumbersome. A lighter and more compact construction, where large numbers of cells are to be connected in series, is the flat, or layer, type. Batteries of this type consist of alternate thin, flat layers of zinc electrolyte and the materials making up the positive electrode and its respective depolarizer.
