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A valve regulated lead–acid (VRLA) battery, commonly known as a sealed lead–acid (SLA) battery, is a type of
lead–acid battery The lead–acid battery is a type of rechargeable battery first invented in 1859 by French physicist Gaston Planté. It is the first type of rechargeable battery ever created. Compared to modern rechargeable batteries, lead–acid batteries have ...
characterized by a limited amount of electrolyte ("starved" electrolyte) absorbed in a plate separator or formed into a gel; proportioning of the negative and positive plates so that oxygen recombination is facilitated within the
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
; and the presence of a relief valve that retains the battery contents independent of the position of the cells. There are two primary types of VRLA batteries, absorbent glass mat (AGM) and gel cell (gel battery). The lead–acid gel batteries contain a mixture of sulfuric acid and finely divided silica. This mixture forms a thick paste or
gel A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still dif ...
, thereby giving the batteries the name - Gel Cell. Gel batteries can be made with either flat or tubular positive plates. AGM batteries feature
fiberglass mesh Fiberglass mesh is a neatly woven, crisscross pattern of fiberglass thread that is used to create new products such as tape and filters. When it is used as a filter, it is not uncommon for the manufacturer to spray a PVC coating to make it strong ...
or an ultra thin glass mat (called AGM separator) between the battery plates which serves to contain the electrolyte and separate the plates. This additional layer absorbs the electrolyte and makes the battery non-spillable. Both types of VRLA batteries offer advantages and disadvantages compared to flooded vented lead–acid (VLA) batteries or each other. The gel provides a better means of heat conduction from the plates to the cell walls than in AGM batteries. Since the heat produced on overcharge is lost more efficiently, they are suitable for deep cycle applications. AGM batteries, on the other hand, offer fastest charging rate, offer long service life, and are maintenance-free. Due to their construction, the gel cell and AGM types of VRLA can be mounted in any orientation, and do not require constant maintenance. The term "maintenance free" is a misnomer as VRLA batteries still require cleaning and regular functional testing. They are widely used in large portable electrical devices, off-grid power systems and similar roles, where large amounts of storage are needed at a lower cost than other low maintenance technologies like
lithium ion A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
.


History

The first lead–acid gel battery was invented by Elektrotechnische Fabrik Sonneberg in 1934. The modern gel or VRLA battery was invented by
Otto Jache Otto is a masculine German given name and a surname. It originates as an Old High German short form (variants ''Audo'', '' Odo'', '' Udo'') of Germanic names beginning in ''aud-'', an element meaning "wealth, prosperity". The name is recorded ...
of Sonnenschein in 1957. The first AGM cell was the Cyclon, patented by Gates Rubber Corporation in 1972 and now produced by
EnerSys EnerSys is a stored energy systems and technology provider for industrial applications. manufactures and distributes reserve power and motive power batteries, battery chargers, power equipment, battery accessories and outdoor equipment enclosu ...
. The cyclon is a spiral wound cell with thin lead foil electrodes. A number of manufacturers seized on the technology to implement it in cells with conventional flat plates. In the mid 1980s, two UK companies, Chloride and Tungstone, simultaneously introduced ten year life AGM batteries in capacities up to 400 Ah, stimulated by a British Telecom specification for batteries for support of new digital exchanges. In the same period, Gates acquired another UK company, Varley, specialising in aircraft and military batteries. Varley adapted the Cyclon lead foil technology to produce flat plate batteries with exceptional high rate output. These gained approval for a variety of aircraft including the BAE 125 and 146 business jets, the Harrier and its derivative the AV8B, and some F16 variants as the first alternatives to then standard nickel–cadmium (Ni-Cd) batteries.


Basic principle

Lead–acid cells consist of two plates of lead, which serve as
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
s, suspended in an electrolyte consisting of diluted sulfuric acid. VRLA cells have the same chemistry, except the electrolyte is immobilized. In AGM this is accomplished with a fiberglass mat; in gel batteries or "gel cells", the electrolyte is in the form of a paste like gel created by adding silica and other gelling agents to the electrolyte. When a cell discharges, the lead and diluted acid undergo a chemical reaction that produces lead sulfate and water. When a cell is subsequently charged, the lead sulfate and water are turned back into lead and acid. In all lead–acid battery designs, charging current must be adjusted to match the ability of the battery to absorb the energy. If the charging current is too great, electrolysis will occur, decomposing water into hydrogen and oxygen, in addition to the intended conversion of lead sulfate and water into lead dioxide, lead, and sulfuric acid (the reverse of the discharge process). If these gases are allowed to escape, as in a conventional flooded cell, the battery will need to have water (or electrolyte) added from time to time. In contrast, VRLA batteries retain generated gases within the battery as long as the pressure remains within safe levels. Under normal operating conditions the gases can then recombine within the battery itself, sometimes with the help of a catalyst, and no additional electrolyte is needed. However, if the pressure exceeds safety limits, safety valves open to allow the excess gases to escape, and in doing so regulate the pressure back to safe levels (hence "valve regulated" in "VRLA"). Ronald Dell, David Anthony James Rand, Robert Bailey, Jr., ''Understanding Batteries'',Royal Society of Chemistry, 2001, p. 101, pp.120-122


Construction

Each cell in a VRLA battery has a pressure relief valve which will activate when the battery starts building pressure of hydrogen gas, generally a result of being recharged. The cell covers typically have gas diffusers built into them that allow safe dispersal of any excess hydrogen that may be formed during
overcharge Overcharge is an economic term that refers to the difference between an observed market price and a price that would have been observed in the absence of collusion. The latter is often called a "but-for price" or a competitive "benchmark price". W ...
. They are not permanently sealed, but are designated to be maintenance free. They can be oriented in any manner, unlike normal lead–acid batteries, which must be kept upright to avoid acid spills and to keep the plates' orientation vertical. Cells may be operated with the plates horizontal (''pancake'' style), which may improve cycle life.


Absorbent glass mat (AGM)

AGM batteries differ from flooded lead–acid batteries in that the electrolyte is held in the glass mats, as opposed to freely flooding the plates. Very thin glass fibers are woven into a mat to increase the surface area enough to hold a sufficient amount of electrolyte on the cells for their lifetime. The fibers that compose the fine glass mat do not absorb and are not affected by the acidic electrolyte. These mats are wrung out 2–5% after being soaked in acids just prior to finish manufacturing. The plates in an AGM battery may be of any shape. Some are flat, whereas others are bent or rolled. Both deep cycle and starting type of AGM batteries, are built into a rectangular case according to Battery Council International (BCI) battery code specifications. AGM batteries are more resistant to self discharging than conventional batteries within a wide range of temperatures. As with lead–acid batteries, in order to maximize the life of an AGM battery, it is important to follow the manufacturer's charging specifications and the use of a voltage regulated charger is recommended. There is a direct correlation between the
depth of discharge Depth of discharge (DoD) is an important parameter appearing in the context of rechargeable battery operation. Two non-identical definitions can be found in commercial and scientific sources. The depth of discharge is defined as: # the maximum fra ...
(DOD) and the cycle life of the battery, with differences between 500 and 1300 cycles depending on DOD.


Gel battery

Originally a kind of gel battery was produced in the early 1930s for portable valve (tube) radio LT supply (2, 4 or 6 V) by adding silica to the sulfuric acid. By this time the glass case was being replaced by celluloid and later in 1930s other plastics. Earlier "wet" cells in glass jars used special valves to allow tilt from vertical to one horizontal direction in 1927 to 1931 or 1932. The gel cells were less likely to leak when the portable set was handled roughly. A modern gel battery is a VRLA battery with a gelated electrolyte; the sulfuric acid is mixed with
fumed silica Fumed silica (CAS number 112945-52-5), also known as pyrogenic silica because it is produced in a flame, consists of microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglom ...
, which makes the resulting mass gel like and immobile. Unlike a flooded wet cell lead–acid battery, these batteries do not need to be kept upright. Gel batteries reduce the electrolyte evaporation, spillage (and subsequent
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
problems) common to the wet cell battery, and boast greater resistance to shock and
vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, su ...
. Chemically they are almost the same as wet (non sealed) batteries except that the
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient t ...
in the lead plates is replaced by
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
, and gas recombination can take place.


Applications

Many modern motorcycles and
all-terrain vehicle An all-terrain vehicle (ATV), also known as a light utility vehicle (LUV), a quad bike, or simply a quad, as defined by the American National Standards Institute (ANSI); is a vehicle that travels on low-pressure tires, with a seat that is stra ...
s (ATVs) on the market use AGM batteries to reduce likelihood of acid spilling during cornering, vibration, or after accidents, and for packaging reasons. The lighter, smaller battery can be installed at an odd angle if needed for the design of the motorcycle. Due to the higher manufacturing costs compared with flooded lead–acid batteries, AGM batteries are currently used on luxury vehicles. As vehicles become heavier and equipped with more electronic devices such as navigation and
stability control Electronic stability control (ESC), also referred to as electronic stability program (ESP) or dynamic stability control (DSC), is a computerized technology that improves a vehicle's stability by detecting and reducing loss of traction ( skiddi ...
, AGM batteries are being employed to lower vehicle weight and provide better electrical reliability compared with flooded lead–acid batteries. 5 series BMWs from March 2007 incorporate AGM batteries in conjunction with devices for recovering brake energy using
regenerative braking Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy into a form that can be either used immediately or stored until needed. In this mechanism, the electric traction mo ...
and computer control to ensure the alternator charges the battery when the car is decelerating. Vehicles used in
auto racing Auto racing (also known as car racing, motor racing, or automobile racing) is a motorsport involving the racing of automobiles for competition. Auto racing has existed since the invention of the automobile. Races of various sorts were organise ...
may use AGM batteries due to their vibration resistance. Deep-cycle AGMs are also commonly used in
off-grid Off-the-grid or off-grid is a characteristic of buildings and a lifestyle designed in an independent manner without reliance on one or more public utilities. The term "off-the-grid" traditionally refers to not being connected to the electrical g ...
solar power Solar power is the conversion of energy from sunlight into electricity, either directly using photovoltaics (PV) or indirectly using concentrated solar power. Photovoltaic cells convert light into an electric current using the photovolta ...
and
wind power Wind power or wind energy is mostly the use of wind turbines to generate electricity. Wind power is a popular, sustainable, renewable energy source that has a much smaller impact on the environment than burning fossil fuels. Historically ...
installations as an energy storage bank and in large-scale amateur robotics, such as the
FIRST First or 1st is the ordinal form of the number one (#1). First or 1st may also refer to: *World record, specifically the first instance of a particular achievement Arts and media Music * 1$T, American rapper, singer-songwriter, DJ, and rec ...
and
IGVC The Intelligent Ground Vehicle Competition (IGVC) is an annual international robotics competition for teams of undergraduate and graduate students. Teams design and build an autonomous ground vehicle capable of completing several difficult challe ...
competitions. AGM batteries are routinely chosen for remote sensors such as ice monitoring stations in the
Arctic The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenland), Finland, Iceland, N ...
. AGM batteries, due to their lack of free electrolyte, will not crack and leak in these cold environments. VRLA batteries are used extensively in power wheelchairs, as the extremely low gas and acid output makes them much safer for indoor use. VRLA batteries are also used in the
uninterruptible power supply An uninterruptible power supply or uninterruptible power source (UPS) is an electrical apparatus that provides emergency power to a load when the input power source or mains power fails. A UPS differs from an auxiliary or emergency power syste ...
(UPS) as a backup when the electrical power goes off. VRLA batteries are also the standard power source in sailplanes, due to their ability to withstand a variety of flight attitudes and a relatively large ambient temperature range with no adverse effects. However, charging regimes must be adapted with varying temperature. VRLA batteries are used in the US Nuclear Submarine fleet, due to their power density, elimination of gassing, reduced maintenance, and enhanced safety. AGM and gel-cell batteries are also used for recreational marine purposes, with AGM being more commonly available. AGM deep-cycle marine batteries are offered by a number of suppliers. They typically are favored for their low maintenance and spill-proof quality, although generally considered a less cost effective solution relative to traditional flooded cells. In telecommunications applications, VRLA batteries that comply with criteria in
Telcordia Technologies iconectiv is a supplier of network planning and network management services to telecommunications providers. Known as Bellcore after its establishment in the United States in 1983 as part of the break-up of the Bell System, the company's name ...
requirements documen
GR-4228
''Valve-Regulated Lead–Acid (VRLA) Battery String Certification Levels Based on Requirements for Safety and Performance,'' are recommended for deployment in the Outside Plant (OSP) at locations such as Controlled Environmental Vaults (CEVs), Electronic Equipment Enclosures (EEEs), and huts, and in uncontrolled structures such as cabinets. Relative to VRLA in telecommunications, the use of VRLA Ohmic Measurement Type Equipment (OMTE) and OMTE-like measurement equipment is a fairly new process to evaluate telecommunications battery plants. The proper use of ohmic test equipment allows battery testing without the need to remove batteries from service to perform costly and time-consuming discharge tests.


Comparison with flooded lead–acid cells

VRLA gel and AGM batteries offer several advantages compared with VRLA flooded lead–acid and conventional lead–acid batteries. The battery can be mounted in any position, since the valves only operate on over-pressure faults. Since the battery system is designed to be recombinant and eliminate the emission of gases on overcharge, room ventilation requirements are reduced, and no acid fume is emitted during normal operation. Flooded cell gas emissions are of little consequence in all but the smallest confined areas, and pose very little threat to a domestic user, so a wet cell battery designed for longevity gives lower costs per kWh. In a gel battery, the volume of free electrolyte that could be released on damage to the case or venting is very small. There is no need (or ability) to check the level of electrolyte or to top up water lost due to electrolysis, thus reducing inspection and maintenance requirements. Wet-cell batteries can be maintained by a self-watering system or by topping up every three months. The requirement to add distilled water is normally caused by overcharging. A well-regulated system should not require top-up more often than every three months. An underlying disadvantage with all lead–acid batteries is the requirement for a relatively long recharge cycle time arising from an inherent three-stage charging process: bulk charge, absorption charge, and (maintenance) float charge stages. All lead–acid batteries, irrespective of type, are quick to bulk charge to about 70% of capacity during which the battery will accept a large current input, determined at a voltage setpoint, within a few hours (with a charge source capable of supplying the design
C-rate A battery charger, recharger, or simply charger is a device that stores energy in a Electric battery, battery by running an electric current through it. The charging protocol (how much voltage or Current source, current for how long, and what to d ...
bulk stage
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
for a given Ah battery). However, they then require a longer time spent in the current-tapering off intermediate absorption charge stage after the initial bulk charge, when the LA battery charge acceptance rate gradually reduces, and the battery will not accept a higher C-rate. When the absorption stage voltage setpoint is reached (and charge current has tapered off), the charger switches to a
float voltage Float voltage is the voltage at which a battery is maintained after being fully charged to maintain that capacity by compensating for self-discharge of the battery. The voltage could be held constant for the entire duration of the cell's operation ( ...
setpoint at a very low C-rate to maintain the battery's fully charged state indefinitely (the float stage offsets the battery's normal
self-discharge Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes or any external circuit. Self-discharge decreases the shelf life Shelf life ...
over time). If the charger fails to supply a sufficient absorption stage charge duration and C-rate (it 'plateaus' or times out, a common fault of cheap solar chargers), and a suitable float charge profile, the battery's capacity and longevity will be dramatically reduced. To ensure maximum life, a lead–acid battery should be fully recharged as soon after a discharge cycle as possible to prevent
sulfation Sulfation is the chemical reaction that entails the addition of SO3 group. In principle, many sulfations would involve reactions of sulfur trioxide (SO3). In practice, most sulfations are effected less directly. Regardless of the mechanism, the ...
, and kept at a full charge level by a float source when stored or idle (or stored dry new from the factory, an uncommon practice today). When working a discharge cycle, a LA battery should be kept at a DOD of less than 50%, ideally no more than 20-40% DOD; a true LA
deep-cycle battery A deep-cycle battery is a battery designed to be regularly deeply discharged using most of its capacity. The term is traditionally mainly used for Lead–acid battery, lead–acid batteries in the same form factor as automotive batteries; and con ...
can be taken to a lower DOD (even an occasional 80%), but these greater DOD cycles always impose a longevity price. Lead–acid battery lifetime cycles will vary with the care given, with best care they may achieve 500 to 1000 cycles. With less careful use, a lifetime as few as 100 cycles might be expected (all dependent upon the use environment too). Because of calcium added to its plates to reduce water loss, a sealed AGM or gel battery recharges more quickly than a flooded lead–acid battery of either VRLA or conventional design.(stating sealed battery plates are hardened with calcium to reduce water loss which "raises the batteries' internal resistance and prevents rapid charging.") Compared to flooded batteries, VRLA batteries are more vulnerable to
thermal runaway Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...
during abusive charging. The electrolyte cannot be tested by hydrometer to diagnose improper charging that can reduce battery life. AGM automobile batteries are typically about twice the price of flooded-cell batteries in a given BCI size group; gel batteries as much as five times the price. AGM and gel VRLA batteries: * Have shorter recharge time than flooded lead–acid batteries. * Cannot tolerate overcharging: overcharging leads to premature failure. * Have shorter useful life, compared to properly maintained wet-cell battery. * Discharge significantly less hydrogen gas. * AGM batteries are by nature, safer for the environment, and safer to use. * Can be used or positioned in any orientation.


See also

*
Gaston Planté Gaston Planté (22 April 1834 – 21 May 1889) was a French physicist who invented the lead–acid battery in 1859. This type battery was developed as the first rechargeable electric battery marketed for commercial use and it is widely used in aut ...
*
List of battery types This list is a summary of notable electric battery types composed of one or more electrochemical cells. Three lists are provided in the table. The primary (non-rechargeable) and secondary (rechargeable) cell lists are lists of battery chemistry. ...
* Rechargeable battery * Sand battery


References


Further reading


Books and papers

* Valve-Regulated Lead-Acid Batteries. Edited by Patrick T. Moseley, Jurgen Garche, C.D. Parker, D.A.J. Rand
p202
* Vinal, G.W. (1955 Jan 01) Storage batteries. A general treatise on the physics and chemistry of secondary batteries and their engineering applications. Energy Citations Database (ECD)
Document #7308501
* John McGavack
The Absorption of Sulfur Dioxide by the Gel of Silicic Acid
Eschenbach Print. Company, 1920.


Patents

* – Treatment Of Porous Pots For Electric Batteries. Erhard Ludwig Mayer and Henry Liepmann * – Solid Acid Storage Battery Electrolyte.
Alexander Koenig Alexander Ferdinand Koenig (20 February 1858 – 16 July 1940) was a German naturalist and zoologist. Koenig was born at St Petersburg, Russia where his father was a successful merchant. He grew up in Bonn. Koenig became interested in nat ...
et al. * – Composite battery plate grid * – Lead acid battery plate with starch coated glass fibers * – Method of making a lead storage battery and lead storage battery made according to this method. Otto Jache's and Heinz Schroeder


External links


Why do I need a special battery for the automatic start-stop system?
published by Varta
Pros and cons of AGM batteries
published by Lifeline {{Galvanic cells Lead Rechargeable batteries Sulfuric acid