BATTERY

in electricity and electrochemistry, any of a class of devices that convert chemical energy directly into electrical energy. Although the term battery, in strict usage, designates an assembly of two or more voltaic cells capable of such energy conversion, it is commonly applied to a single cell of this kind. The mechanism by which a battery generates an electric current involves the arrangement of constituent chemicals in such a manner that electrons are released from one part of the battery and made to flow through an external circuit to another part. The part of the battery at which the electrons are released to the circuit is called the anode, or the negative electrode; the part that receives the electrons from the circuit is known as the cathode, or the positive electrode. (In a device that consumes current-e.g., electroplating cell, electron tube, etc.-the term anode is often applied to the positive electrode, while the negative electrode is called the cathode.) The first battery appears to have been constructed about 1800 by Alessandro Volta, a professor of natural philosophy at the University of Pavia in Italy. This device, later known as the voltaic pile, was composed of a series of silver and zinc disks in pairs, each of which was separated with a sheet of pasteboard saturated in salt water. A current was produced when the uppermost disk of silver was connected by a wire to the bottom disk of zinc. In 1836 the English chemist John Daniell developed what is considered the classic form of the voltaic cell. A voltaic cell is composed of two chemicals with different electron-attracting capabilities that are immersed in an electrolyte and connected to each other through an external circuit. These two chemicals are called an electrochemical couple. In a zinc-acid cell, for example, the electrochemical couple is a zinc-hydrogen ion couple. The reaction that occurs between an electrochemical couple in a voltaic cell is an oxidation-reduction reaction. At rest, a voltaic cell exhibits a potential difference (voltage) between its two electrodes that is determined by the amount of chemical energy available when an electron is transferred from one electrode to the other and is thus subject to the chemical nature of the materials used in the electrode. The current that flows from a cell is determined by the resistance of the total circuit, including that of the cell itself. A low-resistance cell is required if extremely large currents are desired. This can be achieved by the utilization of electrodes with large areas. In short, the maximum current that can be drawn from a cell is dependent on the area of the electrodes. Whenever a current flows, the voltage of a cell decreases because of internal resistance of the cell and the slowness of the chemical process at the electrodes. A voltaic cell has a limited energy content, or capacity, which is generally given in ampere-hours and determined by the quantity of electrons that can be released at the anode and accepted at the cathode. When all of the chemical energy of the cell has been consumed-usually because one of the electrodes has been completely exhausted-the voltage falls to zero and will not recover. The capacity of the cell is determined by the quantity of active ingredients in the electrode. There are two major types of voltaic cells: primary batteries and secondary, or storage, batteries. (The latter are sometimes also called accumulators.) Primary cells are constructed in such a way that only one continuous or intermittent discharge can be obtained. Secondary devices, on the other hand, are constructed so that they can be discharged and then recharged to approximately their original state. The charging process is the reverse of the discharge process; therefore, the electrode reactions in these batteries must be reversible. in electricity and electrochemistry, any of a class of devices that convert chemical energy directly into electrical energy. Although the term battery, in strict usage, designates an assembly of two or more galvanic cells capable of such energy conversion, it is commonly applied to a single cell of this kind. Additional reading Stanley W. Angrist, Direct Energy Conversion, 4th ed. (1987), provides a historical introduction and overview. Colin A. Vincent et al., Modern Batteries: An Introduction to Electrochemical Power Sources (1984), is written for the nonspecialist. David Linden (ed.), Handbook of Batteries and Fuel Cells (1984), provides comprehensive information on types and applications. Brooke Schumm, Jr.