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Batteries

In science and technology, a battery is a device that stores chemical energy and makes it available in an electrical form. Batteries consist of electrochemical devices such as one or more galvanic cells, fuel cells or flow cells. The earliest known artifacts that may have been batteries are the Baghdad Batteries, from some time between 250 BCE and 640 CE. The modern development of batteries started with the Voltaic pile developed by the Italian physicist Alessandro Volta in 1800. The worldwide battery industry generates US$48 billion in sales annually (2005 estimate).

History

There is some evidence—in the form of the Baghdad Batteries from some time between 250 BCE and 640 CE (while Baghdad was under Parthian and Sassanid dynasties of ancient Persia) of galvanic cells having been used in ancient times. Such ancient knowledge in the history of electricity bears no known continuous relationship to the development of modern batteries. The hypothesis that these devices had an electrical function, while plausible, remains unproven, as with devices discovered in Egyptian digs that are alleged to be batteries as well. There are no writings or drawings or other evidence such as wires or electroplated objects to substantiate the proposed use of the objects as electrical cells.

In 1748, Benjamin Franklin, engaged in fundamental electrical researches, employed the term battery to describe an array of charge storage devices, or capacitors, known at that time in the form of the Leyden jar. Daniel Gralath had been the first to combine several Leyden jars in parallel to obtain a larger stored charge. The word battery had been in use to describe arrays of cannon on land and at sea, which could more effectively batter, or beat, a foe.

In 1786, while studying the biological effects of electricity, Luigi Galvani discovered a device which could produce an electric current by chemical means far greater than the current produced by earlier electrostatic generators, although at a lower voltage-- the galvanic cell. This was a circuit consisting of two dissimilar metals in contact, their other ends exposed to salt water. (Two identical metals in contact will produce no electrochemical effect.) The nature of galvanic cells (often called voltaic cells, or electrochemical cells) was partly elucidated by Volta in the 1790's. In 1800 Volta piled up a series array of galvanic cells to invent the Voltaic pile. Many Europeans still use the word pile to describe a voltaic pile -- what in English is now called a battery. (The term battery has come into disuse in the context of capacitors; rather we speak of a bank of capacitors.) In 1801, Volta demonstrated the Voltaic cell to Napoleon Bonaparte (who later ennobled him for his discoveries).

The scientific community at this time called this battery either a pile (because Volta had simply piled one cell upon another), or an accumulator (because it stored charge), or an artificial electrical organ. All electrochemical cells produce a current of electrons that flow only in one direction, known as direct current.

What follows is a brief history of different types of batteries and some of their uses.

The dry pile was a high voltage low current semi-permanent battery developed in the early 1800s and constructed from silver foil, zinc foil, and paper. Foil disks of about 2cm dia. were stacked up several thousand thick and then either compressed in a glass tube with endcaps and a screw assembly, or simply stacked between three glass rods with wooden endplates. It is a type of Voltaic pile, with an output potential in the kilovolt range. In effect it was a electrostatic battery. It was referred to as a dry pile because no electrolyte other than atmospheric humidity was present.

In 1800 William Nicholson and Anthony Carlisle used a battery to decompose water into hydrogen and oxygen. Sir Humphry Davy researched this chemical effect at the same time. Davy researched the decomposition of substances (called electrolysis). In 1813 he constructed a 2,000-plate paired battery in the basement of Britain's Royal Society, covering 889 ft² (83 m²). From this experiment, Davy deduced that electrolysis was the action in the voltaic pile that produced electricity. In 1820 the British researcher John Frederic Daniell improved the voltaic cell. The Daniell cell consisted of copper and zinc plates and copper and zinc sulfates. It was used to operate telegraphs and doorbells. Some early battery researchers called the Daniell cell a gravity cell because gravity kept the two sulfates separated. The name crowfoot cell was also commonly used because of the shape of the zinc electrode used in the batteries. Between 1832 and 1834 Michael Faraday conducted experiments with an iron ring, a galvanometer, and a connected battery. When the battery was connected or disconnected, the galvanometer deflected. Faraday also developed the principle of ionic mobility in chemical reactions of batteries. In 1839 William Robert Grove developed the first fuel cell, which produced electrical energy by combining hydrogen and oxygen. Grove developed another form of voltaic cell using zinc and platinum electrodes. These electrodes were exposed to two acids separated by a diaphragm.

In the 1860s Georges Leclanché of France developed the carbon-zinc battery. It was a wet cell, with electrodes plunged into a body of electrolyte fluid. Rugged and easily manufactured, it also had a reasonable shelf life. An improved version called a dry cell was later made by sealing the cell and changing the fluid electrolyte to a wet paste. The Leclanché cell is a type of primary (non-rechargeable) battery. Also in the 1860s, Raymond Gaston Planté invented the lead-acid battery. He immersed two thin solid lead plates separated by rubber sheets in a dilute sulfuric acid solution to make a secondary (rechargeable) battery. However, the original invention had a short shelf life. Around 1881 Émile Alphonse Faure, with his colleagues, developed batteries using a mixture of lead oxides for the positive plate electrolyte. These had faster reactions and higher efficiency. In 1878 the air cell battery was developed. In 1897 Nikola Tesla researched a lightweight carbide cell and an oxygen-hydrogen storage cell. In 1898 Nathan Stubblefield received a patent (US600457) for a cell made of cloth-insulated copper wire and iron wire wound in a coil, which was to be buried in damp earth: this electrolytic coil is referred to as an "earth battery". Thomas Edison also got into the act, in 1900 developing the nickel storage battery, and in 1905 developing the nickel-iron battery.

During World War II Samuel Ruben and Philip Rogers Mallory developed the mercury cell, and in the 1950s Ruben improved the alkaline manganese battery. In the early 1950s Russell S. Ohl developed a wafer of silicon that produced free electrons, and in 1954 Gerald L. Pearson, Daryl M. Chapin, and Calvin S. Fuller produced an array of several such wafers, making the first solar battery or solar cell. In 1956 Francis Thomas Bacon developed the hydrogen-oxygen fuel cell. In 1959 Lewis Urry developed the small alkaline battery at the Eveready Battery Company laboratory in Parma, Ohio. In the 1960s German researchers invented a gel-type electrolyte lead-acid battery.

The Clark cell, invented by Josiah Latimer Clark, was for many years used for as standard cell to provide a voltage standard. It was replaced by the Weston cell in 1905, which was employed until 1972. Since that time, the United States standard of voltage has been set by the Josephson junction voltage standard, which requires the use of superconductors (and thus low temperatures). Here, measurement of the frequency of current oscillation across a junction leads to a determination of the voltage difference across that junction.

Battery concepts

A battery consists of one or more voltaic cells. In the figure to the right, the battery consists of two or more voltaic cells in series. (The conventional symbol does not represent the number of voltaic cells.) The positive terminals or electrodes are the longer horizontal lines. Real voltaic cells have ion-carrying electrolyte, made of solid or liquid, separating their terminals. Thus their terminals are not in direct electrical contact. The figure shows no line connecting the negative terminal of the top cell to the positive terminal of the bottom cell, but in a real cell they would be in direct electrical contact.

The electrolyte contains ions that can react with chemicals in the electrode. Chemical energy is converted into electrical energy by chemical reactions that transfer charge between the electrode and the electrolyte at their interface. Such reactions are called faradaic, and are responsible for current flow through the cell. Ordinary, non-charge-transferring (non-faradaic) reactions also occur at the electrode-electrolyte interfaces. Non-faradaic reactions are one reason that voltaic cells (particularly the lead-acid cell of ordinary car batteries) "run down" when sitting unused.

Around 1800, Alessandro Volta studied the effect of different electrodes on the net electromotive force (emf) of many different types of voltaic cells. (Emf is equivalent to what was called the internal voltage source in the previous section.) He showed that the net emf (E) is the difference of the emfs Ε1 and Ε2 associated with the two electrolyte-electrode interfaces. Hence identical electrodes yield Ε=0 (zero emf). Volta did not appreciate that the emf 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 by-product of their operation.

Electromotive force (emf) is measured in units of volts. Voltaic cells, and batteries of voltaic cells, are normally rated in terms of volts. The voltage across the terminals of a battery is known as the terminal voltage. The terminal voltage of a battery that is neither charging nor discharging equals its emf. The terminal voltage of a battery that is discharging is less than the emf, and that of a battery that is charging is greater than the emf.

Most voltaic cells are rated at only about 1.5 volts, because of the nature of the chemical reactions inside. Because of the high electrochemical potentials of lithium compounds, Li cells can provide as many as 3 or more volts. However, lithium compounds can also be hazardous.

The conventional model for a voltaic cell, as drawn above, has the internal resistance drawn outside the cell. This is a correct Thevenin equivalent for circuit applications, but it oversimplifies the chemistry and physics. In a more accurate (and more complex) model, a voltaic cell can be thought of as two electrical pumps, one at each terminal (the faradaic reactions at the corresponding electrode-electrolyte interfaces), separated by an internal resistance largely due to the electrolyte. Even this is an oversimplification, since it cannot explain why the behavior of a voltaic cell depends strongly on its rate of discharge. For example, it is well known that a cell that is discharged rapidly (but incompletely) will recover spontaneously after a waiting time, but a cell that is discharged slowly (but completely) will not recover spontaneously.

The simplest characterization of a battery would give its emf (voltage), its internal resistance, and its capacity. In principle, the energy stored by a battery equals the product of its emf and its capacity.

Battery capacity

The capacity of a battery to store charge is often expressed in ampere hours (1 A·h = 3600 coulombs). If a battery can provide one ampere (1 A) of current (flow) for one hour, it has a capacity of 1 A·h. If it can provide 1 A for 100 hours, its capacity is 100 A·h. The more electrolyte and electrode material in the cell, the greater the capacity of the cell. Thus a tiny AAA cell has much less capacity than a much larger D cell, even if both rely on the same chemical reactions (e.g. alkaline cells), which produce the same terminal voltage. Because of the chemical reactions within the cells, the capacity of a battery depends on the discharge conditions such as the magnitude of the current, the duration of the current, the allowable terminal voltage of the battery, temperature, and other factors.

Battery manufacturers use a standard method to determine how to rate their batteries. The battery is discharged at a constant rate of current over a fixed period of time, such as 10 hours or 20 hours, down to a set terminal voltage per cell. So a 100 ampere-hour battery is rated to provide 5 A for 20 hours at room temperature. The efficiency of a battery is different at different discharge rates. When discharging at low rate, the battery's energy is delivered more efficiently than at higher discharge rates. This is known as Peukert's Law. See also Battery (electricity)#Common Battery Capacities

Battery lifetime

Even if never taken out of the original package, disposable (or "primary") batteries can lose two to twenty-five percent of their original charge every year. This rate depends significantly on temperature, since typically chemical reactions proceed more rapidly as the temperature is raised. This is known as the "self discharge" rate and is due to non-faradaic (non-current-producing) chemical reactions, which occur within the cell even if no load is applied to it. Batteries should be stored at cool or low temperatures to reduce the rate of the side reactions. For instance, some people make a practice of storing unused batteries in their refrigerators or freezers to extend battery lifetime. Extreme high or low temperatures also reduce battery performance.

Some information on the care and disposal of alkaline batteries can be found here and here.

Rechargeable batteries self-discharge more rapidly than disposable alkaline batteries; up to three percent a day (depending on temperature). Due to their poor shelf life, they shouldn't be left in a drawer and then relied upon to power a flashlight or a small radio in an emergency. For this reason, it's a good idea to keep a few alkaline batteries on hand. Ni-Cd Batteries are almost always "dead" when you get them, and must be charged before first use.

Most NiMH and NiCd batteries can be charged several hundred times.

Automotive lead-acid rechargeable batteries lead a much harder life. Because of vibration, shock, heat, cold, and sulfation of their lead plates, few automotive batteries last beyond six years of regular use.

Special "reserve" batteries intended for long storage in emergency equipment or munitions keep the electrolyte of the battery separate from the plates until the battery is activated, allowing the cells to be filled with the electrolyte. Shelf times for such batteries can be years or decades. However, their construction is more expensive than more common forms.

Battery explosion

A battery explosion is caused by the misuse or malfunction of a battery, such as attempting to recharge a primary battery, or short circuiting a battery. With car batteries, explosions are most likely to occur when a short circuit generates very large currents. In addition, car batteries liberate hydrogen when they are overcharged (because of electrolysis of the water in the electrolyte). Normally the amount of overcharging is very small, as is the amount of explosive gas developed, and the gas dissipates quickly. However, when "jumping" a car battery, the high current can cause the rapid release of large volumes of hydrogen, which can be ignited by a nearby spark (for example, when removing the jumper cables).

When a non-rechargeable battery is recharged at a high rate, an explosive gas mixture of hydrogen and oxygen may be produced faster than it can escape from within the walls of the battery, leading to pressure build-up and the possibility of an explosion. In extreme cases, the battery acid may spray violently from the casing of the battery and cause injury.

Additionally, disposing of a battery in fire may cause an explosion as steam builds up within the sealed case of the battery.

Overcharging -- that is, attempting to charge a battery beyond its electrical capacity -- can also lead to a battery explosion, leakage, or irreversible damage to the battery. It may also cause damage to the charger or device in which the overcharged battery is later used.

Common battery types

From a user's viewpoint, at least, batteries can be generally divided into two main types—rechargeable and non-rechargeable (disposable). Each is in wide usage.

Disposable batteries, 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. Battery manufacturers don't recommend attempting to recharge primary cells.

By contrast, rechargeable batteries or secondary cells can be re-charged after they have been drained. This is done by applying externally supplied 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. 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 deal with the hydrogen gas which is vented by these batteries during overcharging. The lead-acid battery is also very heavy for the amount of electrical energy it can supply. Despite this, its low manufacturing cost and its high surge current levels make its use common where a large capacity (over approximately 10Ah) is required or where the weight and ease of handling are not concerns.

A common form of lead-acid battery is the modern wet-cell car battery. This can deliver about 10,000 watts of power for a short period, and has a peak current output that varies from 450 to 1100 amperes. An improved type of lead-acid battery called a gel battery (or "gel cell") has become popular in automotive industry as a replacement for the lead-acid wet cell. The gel battery contains a semi-solid electrolyte to prevent spillage, electrolyte evaporation, and out-gassing, as well as greatly improving its resistance to damage from vibration and heat. Another type of battery, the Absorbed Glass Mat (AGM) suspends the electrolyte in a special fiberglass matting to achieve similar results. More portable rechargeable batteries include several "dry cell" types, which are sealed units and are therefore useful in appliances like mobile phones and laptops. Cells of this type (in order of increasing power density and cost) include nickel-cadmium (NiCd), nickel metal hydride (NiMH), and lithium-ion (Li-Ion) cells.

Disposable

Non-rechargeable - sometimes called "primary cells".

* Zinc-carbon battery - low cost - used in light drain applications
* Zinc-chloride battery - similar to zinc carbon but slightly longer life
* Alkaline battery - alkaline/manganese "long life" batteries widely used in both light drain and heavy drain applications
* Silver-oxide battery - commonly used in hearing aids
* Lithium battery - commonly used in digital cameras. Sometimes used in watches and computer clocks. Very long life (up to ten years in wristwatches) and capable of delivering high currents but expensive
* Mercury battery - commonly used in digital watches
* Zinc-air battery - commonly used in hearing aids
* Thermal battery - high temperature reserve. Almost exclusively military applications.

Rechargeable

Also known as secondary batteries or accumulators.

* Lead-acid battery - commonly used in vehicles, alarm systems and uninterruptible power supplies. Used to be used as an "A" or "wet" battery in valve/vacuum tube radio sets. The major advantage of this chemistry is its low cost - a large battery (e.g. 70Ah) is relatively cheap when compared to other chemistries. However, this battery chemistry has lower energy density than other battery chemistries available today
o Absorbed glass mat
o Gel battery
* Lithium ion battery - a relatively modern battery chemistry that offers a very high charge density (i.e. a light battery will store a lot of energy) and which does not suffer from any "memory" effect whatsoever. Used in laptops (notebook PCs), modern camera phones, some rechargeable MP3 players and most other portable rechargeable digital equipment.
* Lithium ion polymer battery - similar characteristics to lithium-ion, but with slightly less charge density. This battery chemistry can be used for any battery to suit the manufacturer's needs, such as ultra-thin (1 mm thick) cells for the latest PDAs
* NaS battery
* Nickel-iron battery
* Nickel metal hydride battery
* Nickel-cadmium battery - used in many domestic applications but being superseded by Li-Ion and Ni-MH types. This chemistry gives the longest cycle life (over 1500 cycles), but has low energy density compared to some of the other chemistries. Ni-Cd cells using older technology suffer from memory effect, but this has been reduced drastically in modern batteries.
* Sodium-metal chloride battery
* Nickel-zinc battery
* Molten salt battery

Homemade cells

Almost any liquid or moist object that has enough ions to be electrically conductive can serve as the electrolyte for a cell. As a novelty or science demonstration, it is possible to insert two electrodes into a lemon, potato, glass of soft drink, etc. and generate small amounts of electricity. As of 2005, "two-potato clocks" are widely available in hobby and toy stores; they consist of a pair of cells, each consisting of a potato (lemon, etc.) with two electrodes inserted into it, wired in series to form a battery with enough voltage to power a digital clock. Homemade cells of this kind are of no real practical use, because they produce far less current—and cost far more per unit of energy generated—than commercial cells, due to the need for frequent replacement of the fruit or vegetable.

Traction batteries

Traction batteries (secondary batteries or accumulators) are designed to provide power to move a vehicle, such as an electric car or tow motor. A major design consideration is power to weight ratio since the vehicle must carry the battery. While conventional lead acid batteries with liquid electrolyte have been used, the electrolyte in traction batteries is often gelled to prevent spilling. The electrolyte may also be embedded in a glass wool which is wound so that the cells have a round cross-sectional area (AGM-type).

Battery types used in electric vehicles:

* Conventional lead-acid batteries with liquid electrolyte.
* AGM-type (Absorbed Glass Mat)
* Zebra NiNaCl (or NaNiCl) battery operating at 270 °C requiring cooling in case of temperature excursions
* NiZn battery (higher cell voltage 1.6 V and thus 25% increased specific energy, very short lifespan)

Lithium-ion batteries are now pushing out NiMh-technology in the sector while for low investment costs the lead-acid technology remains in the leading role.

Flow batteries

Flow batteries are a special class of battery where additional quantities of electrolyte are stored outside the main power cell of the battery, and circulated through it by pumps or by movement. Flow batteries can have extremely large capacities and are used in marine applications and are gaining popularity in grid energy storage applications.

Zinc-bromine and vanadium redox batteries are typical examples of commercially-available flow batteries.

Common battery sizes

Disposable cells and some rechargeable cells come in a number of standard sizes, so the same battery type can be used in a wide variety of appliances. Some of the major types used in portable appliances include the A-series (AA, AAA, AAAA), B, C, D, F, G, J, and N, 3R12, 4R25 and variants, PP3 and PP9, and the lantern 996 and PC926. These and less common types are included in the list of battery sizes.

Common Battery Capacities

Information on the ampere-hour capacities of rechargeable batteries is normally readily available, but can be much more difficult to obtain for primary batteries. Some primary battery capacities can be found at for Energizer and for Duracell

Environmental considerations

Since their development over 250 years ago, batteries have remained among the most expensive energy sources, and their manufacture consumes many valuable resources and often involves hazardous chemicals. For this reason many areas now have battery recycling services available to recover some of the more toxic (and sometimes valuable) materials from used batteries.

Cell vs. battery

Strictly, an electrical "battery" is an interconnected array of one or more similar voltaic cells ("cells"). That distinction, however, is considered pedantic in most contexts (other than the expression dry cell), and in current English usage it is more common to call a single cell used on its own a battery than a cell.

The cells in a battery can be connected in parallel, series, or in both. A parallel combination of cells has the same voltage as a single cell, but can supply a higher current (the sum of the currents from all the cells). A series combination has the same current rating as a single cell but its voltage is the sum of the voltages of all the cells. Most practical electrochemical batteries, such as 9 volt flashlight (torch) batteries and 12 V automobile (car) batteries, have several cells connected in series inside the casing. Parallel arrangements suffer from the problem that, if one cell discharges faster than its neighbor, current will flow from the full cell to the empty cell, wasting power and possibly causing overheating. Even worse, if one cell becomes short-circuited due to an internal fault, its neighbor will be forced to discharge its maximum current into the faulty cell, leading to overheating and possibly explosion. Cells in parallel are therefore usually fitted with an electronic circuit to protect them against these problems. In both series and parallel types, the energy stored in the battery is equal to the sum of the energies stored in all the cells.

A battery can be simply modeled as a perfect voltage source (i.e. one with zero internal resistance) in series with a resistor. The voltage source depends mainly on the chemistry of the battery, not on whether it is empty or full. When a battery runs down, its internal resistance increases. When the battery is connected to a load (e.g. a light bulb), which has its own resistance, the resulting voltage across the load depends on the ratio of the battery's internal resistance to the resistance of the load. When the battery is fresh, its internal resistance is low, so the voltage across the load is almost equal to that of the battery's internal voltage source. As the battery runs down and its internal resistance increases, the voltage drop across its internal resistance increases, so the voltage at its terminals decreases, and the battery's ability to deliver power to the load decreases.


batteries, camera battery, digital batteries