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PowerPedia:Electric Battery

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For the directory of battery resources, see Directory:Batteries; this includes resources on theory, research and development, and market devices that improve the trend toward clean, renewable energy.
Various batteries(clockwise from bottom left): two 9-volt, two "AA", one "D", a cordless phone battery, a camcorder battery, a 2-meter handheld ham radio battery, and a button battery, one "C" and two "AAA", plus a U.S. quarter, for scale
Various batteries(clockwise from bottom left): two 9-volt, two "AA", one "D", a cordless phone battery, a camcorder battery, a 2-meter handheld ham radio battery, and a button battery, one "C" and two "AAA", plus a U.S. quarter, for scale

In science and technology, a battery is a device that stores energy and makes it available in an electrical form. Usually, batteries consist of electrochemical devices such as one or more galvanic cells, fuel cells or flow cells. There are various conventions on the use of "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.

Contents

Brief early history

Main article: Electric battery history

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).

battery.gif
Baghdad Battery
(Image from Why the electric battery was forgotten)

The Baghdad Battery is the common name for a number of artifacts apparently discovered in the village of Khuyut Rabbou'a (near Baghdad, Iraq) in 1936. 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. These artifacts came to wider attention in 1938, when Wilhelm König, the German director of the National Museum of Iraq, found the objects in the museum's collections, and (in 1940, having returned to Berlin due to illness) published a paper speculating that they may have been galvanic cells, perhaps used for electroplating gold onto silver objects.

It is interesting that a Discovery Channel program, called MythBusters, determined that it was indeed plausible for ancient peoples to have used the Baghdad Battery for electroplating or electrostimulation. However, the batteries which they reproduced did not produce a substantial amount of energy and had to be connected in series in order to test the myths. On Mythbusters' 29th episode (which aired on March 23, 2005), the Baghdad battery "myth" was put to the test. Ten hand-made terracotta jars were fitted to act as batteries. Lemon juice was chosen as the electrolyte to activate the electrochemical reaction between the copper and iron. (Oddly enough, it was discovered that a single lemon produced more voltage than one of the batteries). When all of the batteries were linked together, they produced upwards to 4 volts.

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. 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). 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 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". One of the earliest examples of an earth battery was built by Alexander Bain in 1841 in order to drive a prime mover. Bain buried plates of zinc and copper in the ground about one meter apart and used the resulting voltage, of about one volt, to operate a clock.

Concepts

A battery is a device in which chemical energy is directly converted to electrical energy. Circuit symbol for a battery; simplified electrical model; and more complex but still incomplete model (the series capacitor has an extremely large value and, as it charges, simulates the discharge of the battery).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.

Common battery sizes and capacities

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.
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.

9-volt batteryDisposable 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. A good cross-references of different manufacturer's battery and cell designations can be useful. Information on the ampere-hour capacities of rechargable batteries is normally readily available, but can be much more difficult to obtain for primary batteries.Some primary battery capacities can be found at Energizer and Duracell webpage datasheets.

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.

An ampere-hour is a unit of electric charge. The SI unit of electric charge is the coulomb. One ampere-hour is equal to 3600 coulombs. It is a common measurement of how long a battery will last (or in the case of a rechargeable battery, how long it will last when fully charged). The ampere is the SI unit of electric current. One ampere-hour is equal to 3600 coulombs (ampere-seconds), and indicates the amount of electric charge that passes either terminal of the battery when it provides one ampere of current flow for one hour. The commonly-seen milliampere-hour (mA·h) is equal to 3.6 coulombs.

However, in reality, the available capacity of a battery depends on the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected. Therefore, a battery rated at 100 Ah (360000 coulomb) will deliver 20 A (20 coulombs per second) over a 5 hour period, but if it is instead discharged at 50 A (50 coulombs per second), it will run out of charge before the theoretically expected 2 hours. For this reason, a battery capacity rating is always related to an expected discharge time, which is typically 5 or 20 hours.

The relationship between current, discharge time and capacity is expressed by Peukert's law. In general, the higher the ampere-hour rating, the longer the battery will last for a certain device. Installing batteries with different Ah ratings will not affect the operation of a device rated for a specific voltage. The Ah rating of a battery is related to, but not the same as, the amount of energy it stores when fully charged. If two batteries have the same nominal voltage, then the one with the higher Ah rating stores more energy. It would also typically take longer to recharge.

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.

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 electrical component

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 neighbour, 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 neighbour 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 modelled 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.

Rechargeable and disposable batteries

50_AH_Battery.jpg
Future Battery
(Image from http://www.grc.nasa.gov/WWW/Electrochemistry/doc/batteries.html)

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.

Rechargeable batteries are batteries that can be restored to full charge by the application of electrical energy. They come in many different designs using different chemistry. They are also called storage battery, secondary cell or accu/akku (short for accumulator). Attempting to recharge non-rechargeable batteries may lead to a battery explosion. Some types of rechargeable batteries are susceptible to damage due to reverse charging if they are fully discharged; other types need to be fully discharged occasionally in order to maintain the capacity for deep discharge. There exist fully integrated battery chargers that optimize the charging current.

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 fibreglass 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 types

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 types

Four double-A (AA) rechargeable batteries
Four double-A (AA) rechargeable batteries

In the order of improving energy per weight ratios there are:

  • Nickel-iron battery
  • 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. 70 Ah) is relatively cheap when compared to other chemistries. However, this battery chemistry has lower energy density than other battery chemistries available today.
    • Absorbed glass mat
    • Gel battery - a type of lead-acid 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.
  • Nickel metal hydride 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 and a greater life cycle degradation rate. 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.

Other rechargable batteries (out of order):

  • Absorbed glass mat - another type of lead-acid battery
  • NaS battery
  • Sodium-metal chloride battery
  • Nickel-zinc battery
  • Molten salt battery

Recharging

The energy used to recharge rechargeable batteries mostly comes from mains electricity using an adapter unit. Recharging from solar panels is also attractive. Recharging from the 12V battery of a car is also possible. Use of a hand generator is also possible, but it is not clear if such devices are commercially made. For uses like radios and torches, rechargeable batteries may be replaced by clockwork mechanisms or dynamos.

Reverse charging

Reverse charging is when a rechargeable battery is recharged with its polarity reversed. Reverse charging can occur under a number of circumstances. The two most important being:

  • When a battery is incorrectly inserted into a charger
  • When multiple batteries are used in series in a device. When one battery completely discharges ahead of the rest, the other batteries in series may force the discharged battery to discharge to below zero voltage.

Reverse charging may lead to explosion, leakage, damage to the battery and/or to the device or charger. Old and new batteries and batteries of varying types or brands should not be mixed in the same circuit.

Homemade cells and earth batteries

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.

A simple homemade cell is the earth battery. It consist of conductive plates from different locations in the electropotential series, buried in the ground so that the soil acts as the electrolyte in a voltaic cell. As such, the device acts as a rechargeable battery. Operating only as electrolytic devices, the devices were not continuously reliable owing to drought condition. These devices were used by early experimenters as energy sources for telegraphy. However, in the process of installing long telegraph wires, engineers discovered that there were electrical potential differences between most pairs of telegraph stations, resulting from natural electrical currents (called telluric currents) flowing through the ground. Some early experimenters did recognise that these currents were, in fact, partly responsible for extending the earth batteries' high outputs and long lifetimes. Later, experimenters would utilize these currents alone and, in these systems, the plates became polarized.

It had been long known that continuous electric currents flowed through the solid and liquid portions of the Earth and the collection of current from an electrically conductive medium in the absence of electrochemical changes (and in the absence of a thermoelectric junction) was established by Lord Kelvin. Lord Kelvin's "sea battery" was not a chemical battery. Lord Kelvin observed that such variables as placement of the electrodes in the magnetic field and the direction of mediums's flow affected the current output of his device. Such variables do not affect battery operation. These metal plates were immersed in a flowing medium and created a magneto-hydrodynamic generator. In the various experiments, metal plates were symmetrically perpendicular to the direction of the medium's flow and were carefully placed with respect to a magnetic field which differentially deflected electrons from the flowing stream. The electrodes can be assymmetrically oriented with respect to the source of energy, though.

To obtain the natural electricity, experimenters would thrust two metal plates into the ground at a certain distance from each other in the direction of a magnetic meridian, or astronomical meridian. The stronger currents flow from south to north. This phenomenon possesses a considerable uniformity of current strength and voltage. As the Earth currents flow from south to north, electrodes are positioned, beginning in the south and ending in the north, to increase the voltage at as large a distance as possible. In many early implementations, the cost was prohibitive because of an overreliance on extreme spacing between electrodes.

It has been found that all the common metals behave relatively similarly. The two spaced electrodes, having a load in an external circuit connected between them, are disposed in an electrical medium, and energy is imparted to the medium in such manner that "free electrons" in the medium are excited. The free electrons then flow into one electrode to a greater degree than in the other electrode, thereby causing electric current to flow in the external circuit through the load. The current flows from that plate whose position in the electropotential series is near the negative end (such as palladium). The current produced is highest when the two metals are most widely separated from each other in the electropotential series and that the material nearer the positive end is to the north, while that at the negative end is towards the south. The plates, one copper and another iron or carbon, are connected above ground by means of a wire with as little resistance as possible. In such an arrangement, the electrodes are not appreciably chemically corroded, even when they are in earth saturated with water, and are connected together by a wire for a long time.

It had been found that to strengthen the current, it was most advantageous to drive the northerly electropositive electrode deeper into the medium than the southerly electrode. The greatest currents and voltages were obtained when the difference in depth was such that a line joining the two electrodes was in the direction of the magnetic dip, or magnetic inclination. When the previous methods were combined, the current was tapped and utilized in any well-known manner.

In some cases, a pair of plates with differing electrical properties, and with suitable protective coatings, were buried below the ground. A protective or other coating covered each entire plate. A copper plate could be coated with powered coke, a processed carbonaceous material. To a zinc plate, a layer of felt could be applied. To use the natural electricity, earth batteries fed electromagnets, the load, that were part of a motor mechanism.

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.

A Flow Battery is a form of battery in which electrolyte containing one or more dissolved electroactive species is flowed through a power cell / reactor in which chemical energy is converted to electricity. Additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor (although gravity feed systems are also known). Fuel cells are generally defined as electrochemical devices for converting chemical energy to electricity in which the reactants are flowed through a power cell/ reactor from an external source (tank, cylinder or surrounding environment).

Under these definitions it may be concluded that the flow battery is a special type of fuel cell. However, what is rarely explicitly stated is that the electrolyte in a fuel cell remains at all times within the reactor (in the form of an ion-exchange membrane, for example). What is flowed into the reactor is only the electroactive substances, which are non-conducting (e.g. hydrogen, methanol, oxygen, etc.) This is in stark contrast to a flow battery in which at least some of the electrolyte (generally the majority in weight and volume terms) is flowed through the reactor.

Flow batteries are also distinguished from fuel cells by the fact that the chemical reaction involved is often reversible, i.e. they are generally of the secondary battery type and so they can be recharged without replacing the electroactive material. In this sense fuel cells, as sources of electricity, and flow batteries, for storage of electricity, may be seen as complementary in achieving a hydrogen economy. To add to the confusion the European Patent Organisation classes redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).

Radioactive batteries

Radioactive battery or nuclear batteries operate on the continuous radioactive decay of certain elements. These theoretical batteries last a long time. Betavoltaics is an alternative energy technology that promises vastly extended battery life and power density over current technologies. Whether betavoltaics will replace current battery technologies altogether remains to be seen. Recent developments however, are promising. The following is meant to provide a basic introduction to betavoltaics in general and the current state of the art in betavoltaic technology -

Patents

American patents

See also

External articles, references and resources

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Further readings
  • Lamont, J. V., "Der Erdstrom und der Zusammen desselben mit dem Erdmagnetismus". Leopold-Voss-Verlag, Leipzig und Muenchen, 1862. (Tr., Telluric currents and their relationship to geomagnetism)
  • Weinstein, "Electrotechnische Zeitshrift". 1898, pg., 794. (Tr., Electrotechnic magazine)
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