battery

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 galvanic cells capable of such energy conversion, it is commonly applied to a single cell of this kind.

Every battery (or cell) has a cathode, or positive plate, and an anode, or negative plate. These electrodes must be separated by and are often immersed in an electrolyte that permits the passage of ions between the electrodes. The electrode materials and the electrolyte are chosen and arranged so that sufficient electromotive force (measured in volts) and electric current (measured in amperes) can be developed between the terminals of a battery to operate lights, machines, or other devices. Since an electrode contains only a limited number of units of chemical energy convertible to electrical energy, it follows that a battery of a given size has only a certain capacity to operate devices and will eventually become exhausted. The active parts of a battery are usually encased in a box with a cover system (or jacket) that keeps air outside and the electrolyte solvent inside and that provides a structure for the assembly.

Commercially available batteries are designed and built with market factors in mind. The quality of materials and the complexity of electrode and container design are reflected in the market price sought for any specific product. As new materials are discovered or the properties of traditional ones improved, however, the typical performance of even older battery systems sometimes increases by large percentages.

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Batteries are divided into two general groups: (1) primary batteries and (2) secondary, or storage, batteries. Primary batteries are designed to be used until the voltage is too low to operate a given device and are then discarded. Secondary batteries have many special design features, as well as particular materials for the electrodes, that permit them to be reconstituted (recharged). After partial or complete discharge, they can be recharged by the application of direct current (DC) voltage. While the original state is usually not restored completely, the loss per recharging cycle in commercial batteries is only a small fraction of 1 percent even under varied conditions.

Principles of operation

The anode of an electrochemical cell is usually a metal that is oxidized (gives up electrons) at a potential between 0.5 volt and about 4 volts above that of the cathode. The cathode generally consists of a metal oxide or sulfide that is converted to a less-oxidized state by accepting electrons, along with ions, into its structure. A conductive link via an external circuit (e.g., a lamp or other device) must be provided to carry electrons from the anode to the negative battery contact. Sufficient electrolyte must be present as well. The electrolyte consists of a solvent (water, an organic liquid, or even a solid) and one or more chemicals that dissociate into ions in the solvent. These ions serve to deliver electrons and chemical matter through the cell interior to balance the flow of electric current outside the cell during cell operation.

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Battery usefulness is limited not only by capacity but also by how fast current can be drawn from it. The salt ions chosen for the electrolyte solution must be able to move fast enough through the solvent to carry chemical matter between the electrodes equal to the rate of electrical demand. Battery performance is thus limited by the diffusion rates of internal chemicals as well as by capacity.

The voltage of an individual cell and the diffusion rates inside it are both reduced if the temperature is lowered from a reference point, such as 21 °C (70 °F). If the temperature falls below the freezing point of the electrolyte, the cell will usually produce very little useful current and may actually change internal dimensions, resulting in internal damage and diminished performance even after it has warmed up again. If the temperature is raised deliberately, faster discharge can be sustained, but this is not generally advisable, because the battery chemicals may evaporate or react spontaneously with one another, leading to early failure.

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The fundamental relationship of electrochemical cell operation, put forth by the English physicist-chemist Michael Faraday in 1834, is that for every ampere that flows for a period of time, a matching chemical reaction or other change must take place. The extent of such changes is dependent on the molecular and electronic structure of the elements constituting the battery electrodes and electrolyte. Secondary changes may also occur, but a primary pair of theoretically reversible reactions must take place at the electrodes for electricity to be produced. The actual energy generated by a battery is measured by the number of amperes produced × the unit of time × the average voltage over that time. For a cell with electrodes of zinc and manganese dioxide (e.g., the common flashlight dry cell), one finds that a chemical equivalent of zinc weighs 32.5 grams (1.4 ounces) and that of manganese dioxide about 87 grams (3.1 ounces). The discharge of one equivalent weight of each of these electrodes will cause 32.5 grams of zinc to dissolve and 87 grams of manganese dioxide to change into a different oxide containing more hydrogen and zinc ions. Some of the electrolyte also will be consumed in the reaction. One chemical equivalent of each electrode produces one faraday, or 96,485 coulombs of current equal to 26.8 amperes per hour. If the cells operate at an average of 1.2 volts, this would yield 32.2 watt-hours of DC energy. Expressed another way, Energy (joules) = nFV,where n equals the number of chemical equivalents discharged, F is the Faraday constant (9.6485 × 10⁴ coulombs per mole), V is the average (not necessarily constant) voltage of the cell for the period of the discharge, and 1 joule ≅ 2.78 × 10⁻⁴ watt-hours.

There are a large number of elements and compounds from which to select potentially useful combinations for batteries. The commercial systems in common use represent the survivors of numerous tests where continued use depends on adequate voltage, high current-carrying capacity, low-cost materials, and tolerance for user neglect. Better sealing technology and plastics are making further development of all cell systems possible, particularly those using very active lithium for the anode. This situation has yielded commercial cells with as much as 3.9 volts on load and very high current-carrying capability.

Battery manufacturers have designed many different sizes, voltages, and current loads for different specialized applications. In the case of common household batteries, standard sizes and electrical properties have been established by the American National Standards Institute (ANSI) and the International Electrotechnical Commission (IEC). The most common sizes, given in the form ANSI (IEC), are AAA (R03), AA (R6), C (R14), D (R20), and 9V (6F22).