Battery (Electricity)
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Battery (electricity) A Voltaic pile, the first known battery. A battery is a device made of one or more electrochemical cells, which store chemical energy and make it available in an electrical form. There are many types of electrochemical cells, including galvanic cells, electrolytic cells, fuel cells, flow cells, and voltaic cells.[1] Formally, an electrical "battery" is an array of similar voltaic cells ("cells") connected in series. However, in many contexts it is common to call a single cell a battery.[2] A battery's characteristics may vary due to many factors including internal chemistry, current drain, and temperature. There are two types of batteries, primary (disposable) and secondary (rechargeable), both of which convert chemical energy to electrical energy. Primary batteries can only be used once because they use up their chemicals in an irreversible reaction. Secondary batteries can be recharged because the chemical reactions they use are reversible; they are recharged by running a charging current through the battery, but in the opposite direction of the discharge current.[3] Rechargeable batteries can be charged hundreds of times before wearing out; and even after wearing out some can be recycled.[4] Although an early form of battery may have been used in antiquity, the modern development of batteries started with the Voltaic pile, invented by the Italian physicist Alessandro Volta in 1800. Since then, batteries have gained popularity as they became portable and useful for many purposes.[5] The widespread use of batteries has created many environmental concerns, such as toxic metal pollution.[6] Many reclamation companies recycle batteries to reduce the number of batteries going into landfills.[7] Generally, battery life can be prolonged by storing the battery in a cool place and using it at an appropriate current.[8] History Main article: History of the battery The first known artifacts that may have served as batteries are the Baghdad Batteries, which existed some time between 250 BC and 640 AD, although this is unproven and controversial. The modern story of the battery begins in the 1780s with the discovery of "animal electricity" by Luigi Galvani, which he published in 1791.[9] He created an electric circuit consisting of two different metals, with one touching a frog's leg and the other touching both the leg and the first metal, thus closing the circuit. In modern terms, the frog's leg served as both electrolyte and detector, and the metals served as electrodes. He noticed that even though the frog was dead, its legs would twitch when he touched them with the metals.[10] By 1791, Alessandro Volta realized that the frog's moist tissues could be replaced by cardboard soaked in salt water, and the frog's muscular response could be replaced by another form of electrical detection. He already had studied the electrostatic phenomenon of capacitance, which required measurements of electric charge and of electrical potential. Building on this experience Volta was able to detect electric current flow through his system, now called a voltaic cell, or cell for short. The terminal voltage of a cell that is not discharging is called its electromotive force (emf), and has the same unit as electrical potential, named (voltage) and measured in volts, in honor of Volta. In 1799, Volta invented the battery by placing many voltaic cells in series, literally piling them one above the other. This Voltaic Pile gave a greatly enhanced net emf for the combination,[11] with a voltage of about 50 volts for a 32-cell pile.[12] In many parts of Europe batteries continue to be called piles. Unfortunately, Volta did not appreciate that the voltage was due to chemical reactions. He thought that his cells were an inexhaustible source of energy, and that the associated chemical effects (e.g., corrosion) were a mere nuisance, rather than, as Michael Faraday showed around 1830, an unavoidable consequence of their operation. Banks of cells came to be called 'batteries', presumably after the earlier use by Benjamin Franklin to describe Leyden jars (capacitors) in series and in parallel, which vaguely resembled artillery batteries.[13] Although early batteries were of great value for experimental purposes, their limitations made them impractical for large current drain. Later, starting with the Daniell cell in 1836, batteries provided more reliable currents and were adopted by industry for use in stationary devices, particularly in telegraph networks where they were the only practical source of electricity, since electrical distribution networks did not exist then.[14] These wet cells used liquid electrolytes, which were prone to leaks and spillage if not handled correctly. Many used glass jars to hold their components, which made them fragile. These characteristics made wet cells unsuitable for portable appliances. Near the end of the 19th century, the invention of dry cell batteries, which replaced liquid electrolyte with a paste made portable electrical devices practical. The battery has since become a common power source for many household and industrial applications. According to a 2005 estimate, the worldwide battery industry generates US$48 billion in sales annually.[15] How batteries work Main article: Electrochemical cell A battery is a device that converts chemical energy directly to electrical energy.[16] It consists of one or more voltaic cells. Each voltaic cell consists of two half cells connected in series by a conductive electrolyte. One half-cell is the positive electrode (cathode), and the other is the negative electrode (anode). The electrodes do not touch each other but are electrically connected by the electrolyte, which can be either solid or liquid.[17] In many cells the materials are enclosed in a container, and a separator, which is porous to the electrolyte, prevents the electrodes from coming into contact. Each half cell has an electromotive force (or emf), with the net emf of the battery being the difference between the emfs of the half-cells, a fact first recognized by Volta.[18] Thus, if the electrodes have emfs , then the net emf is . (Hence, two identical electrodes and a common electrolyte give zero net emf.) The electrical potential difference, or across the terminals of a battery is known as its terminal voltage, measured in volts.[19] An ideal battery would have a potential difference of , meaning that to produce a potential difference of 1.5 V, chemical reactions inside would do 1.5 J of work for a charge of 1 C;[19] however, the terminal voltage is less due to internal resistance.[20] The terminal voltage of a battery that is neither charging nor discharging is called the open-circuit voltage, and gives the emf of the battery. The terminal voltage of a battery that is discharging is smaller in magnitude than the open-circuit voltage, and the terminal voltage of a battery being charged is greater than the open-circuit voltage.[21] The voltage developed across a cell's terminals depends on the chemicals used in it and their concentrations. For example, alkaline and carbon-zinc cells both measure about 1.5 volts, due to the energy release of the associated chemical reactions.[22] Because of the high electrochemical potential changes in the reactions of lithium compounds, lithium cells can provide as much as 3 volts or more.[23] Classification of batteries Disposable and rechargeable Various batteries (clockwise from bottom left): two 9-volt, two "AA", one "D", a handheld ham radio battery, a cordless phone battery, a camcorder battery, one "C" and two "AAA" From top to bottom: Two button cells, a 9 volt PP3 battery, a AAA battery, a AA battery, a C battery, a D battery, a large 3R12 Batteries are usually divided into two broad classes: • Primary batteries irreversibly transform chemical energy to electrical energy. When the initial supply of reactants is exhausted, energy cannot be readily restored to the battery by electrical means.[24] • Secondary batteries can be recharged, that is, have their chemical reactions reversed by supplying electrical energy to the cell, restoring their original composition.[25] Historically, some types of primary batteries used, for example, for telegraph circuits, were restored to operation by replacing the components of the battery consumed by the chemical reaction. Secondary batteries are not indefinitely rechargeable due to dissipation of the active materials, loss of electrolyte, and internal corrosion. From a user's viewpoint, at least, batteries can be generally divided into two main types: non-rechargeable (disposable) and rechargeable. Each type is in wide usage, as each has its own advantages.[26] Disposable batteries are also called primary cells, are intended to be used once and discarded. These are most commonly used in portable devices with either low current drain, only used intermittently, or used well away from an alternative power source. Primary cells were also commonly used for alarm and communication circuits where other electric power was only intermittently available. Primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms. Battery manufacturers recommend against attempting to recharge primary cells, although some electronics enthusiasts claim it is possible to do so using a special type of charger.[27] By contrast, rechargeable batteries or secondary cells can be re- charged by applying electrical current, which reverses the chemical reactions that occur in use. Devices to supply the appropriate current are called chargers or rechargers. The oldest form of rechargeable battery still in modern usage is the "wet cell" lead-acid battery.[28] This battery is notable in that it contains a liquid in an unsealed container, requiring that the battery be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen gas produced by these batteries during overcharging.