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A VRLA battery (valve-regulated lead–acid battery), more commonly known as a sealed battery or maintenance free battery, is a type of lead–acid rechargeable battery. Due to their construction, they do not require ventilation, can be mounted in any orientation, and do not require constant maintenance. The reduced venting is an advantage since they can be used in confined or poorly ventilated spaces. 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 li-ion.
There are two primary types of VRLA batteries, gel cells and AGM. Gel cells add silica dust to the electrolyte, forming a thick putty-like gel. These are sometimes referred to as "silicone batteries". AGM's, short for "Absorbed Glass Matt", forces fibreglass mesh between the battery plates with the same basic outcome. Both designs offer advantages and disadvantages compared to conventional batteries, as well as each other.
Lead-acid cells consist of two plates of lead suspended in an electrolyte consisting of an aqueous sulphuric acid solution with a low concentration (typically 29–32%, by weight). A lead-acid battery consists of multiple cells wired in series in a single container. The battery produces power by reducing (or oxidizing) the lead plates and turning them into lead-sulphur-oxide while simultaneously increasing the pH of the electrolyte. When the battery is charged, this reaction is reversed; the plates turn back into pure lead, and the acid is re-created. This cycle is not perfect - hydrogen gas often escapes the electrolyte before it can re-combine into water.
To prevent the flammable gas from building up to dangerous levels, venting is required, both in the battery and anything it is placed within. These vents generally mean that the electrolyte can spill out of the battery if it is tipped over, which presents a hazard during shipping and makes them unsuitable for many portable applications. Furthermore, the constant loss of hydrogen leads to a reduction of water in the electrolyte, which must be replaced by opening the battery and "topping off" the water. Many modern batteries include a small float behind a window to allow visual inspection of the electrolyte level, and pop-off caps for refilling it.
VRLA batteries attempt to avoid all of these problems by immobilizing the electrolyte. In doing so, the hydrogen is trapped near the plates, and is available for re-combining when the battery is re-charged. This dramatically reduces the water loss during repeated charge/discharge cycles, and generally makes them "maintenance free". It also reduces the amount of hydrogen build-up to the point that no venting is required, and the batteries can be completely sealed, which in turn allows them to be used in any orientation without fear of spilling.
During rapid recharging, the electrolyte may boil and pressurize the case, or gas build-up may be too rapid for recombination. These effects necessitate the "valve regulation". This is normally in the form of a one-way pop-off valve that only opens in the case of pressure build-up. Even if the valve pops, the immobilizing agent prevents rapid, or any, acid leakage. Most designs are so stable and contain so little electrolyte that they won't leak acid even if they are cut open.
The main downside to the VRLA design is that the immobilizing agent also immobilizes the chemical reactions creating power. For this reason, VRLAs have lower peak power ratings than conventional designs. This makes them less useful for roles like car starting batteries where usage patterns are brief high-current pulses (during starting) followed by long slow recharging cycles. VRLAs are mostly found in roles where the charge/recharge cycles are slower, such as power storage applications.
VRLA batteries have a pressure relief valve which will activate when the battery starts building pressure of hydrogen gas, generally a result of being recharged at excessive voltage, typically greater than 2.30 volts per cell. Valve activation allows some of the gas or electrolyte to escape, thus decreasing the overall capacity of the battery. Rectangular cells may have valves set to operate as low as 1 or 2 psi; round spiral cells, with metal external containers, can have valves set as high as 40 psi.
The cell covers typically have gas diffusers built into them that allow safe dispersal of any excess hydrogen that may be formed during overcharge. 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.
VRLA cells may be made of flat plates similar to a conventional flooded lead–acid battery, or may be made in a spiral roll form to make cylindrical cells.
At high overcharge currents, electrolysis of water occurs, expelling hydrogen and oxygen gas through the battery's valves. Care must be taken to prevent short circuits and rapid charging. Constant-voltage charging is the usual, most efficient and fastest charging method for VRLA batteries, although other methods can be used. VRLA batteries may be continually "float" charged at around 2.35 volts per cell at 25 °C. Some designs can be fast charged (1 hour) at high rates. Sustained charging at 2.7 V per cell will damage the cells. Constant-current overcharging at high rates (rates faster than restoring the rated capacity in three hours) will exceed the capacity of the cell to recombine hydrogen and oxygen.
The first AGM cell was the Cyclon, patented by Gates Rubber Corporation in 1972 and now produced by Enersys. The cyclon is a spirally wound cell with thin lead foil electrodes. A number of manufacturers seized on the technology to implement it in cells with conventional flat plates. The first manufacturer to achieve a significant market position was arguably Yuasa of Japan. Their low capacity lightweight batteries achieved rapid penetration in the alarm and emergency lighting markets by about 1980, and also some acceptance for UPS and PABX support.
In the mid-1980s two UK companies, Chloride and Tungstone, simultaneously introduced 10 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 the normal NiCd batteries.
Moves to higher capacity AGM batteries were led by GNB's Absolyte range extending to 3900 Ah. VRLA/AGM technology is now widespread in both stationary and vehicle batteries.
Absorbed glass mat
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 surface area enough to hold sufficient electrolyte on the cells for their lifetime. The fibers that compose the fine glass mat do not absorb nor are they affected by the acidic electrolyte. These mats are wrung out 2–5% after being soaked in acids, prior to manufacture completion and sealing.
The plates in an AGM battery may be any shape. Some are flat, others are bent or rolled. AGM batteries, both deep cycle and starting, are built in a rectangular case to BCI battery code specifications.
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A gel battery (also known as a "gel cell") is a VRLA battery with a gelified electrolyte; the sulfuric acid is mixed with silica fume, 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 problems) common to the wet-cell battery, and boast  shock, and vibration. Chemically they are almost the same as wet (non-sealed) batteries except that the antimony in the lead plates is replaced by calcium, and gas recombination can take place.
The modern gel formulation and large scale production was from Otto Jache's and Heinz Schroeder's <US Patent 4,414,302> assigned to the German Co. Accumulatorenfabrik Sonnenschein GmbH. With gel electrolyte the separator was no longer such a critical, hard-to-make component, and cycle life was increased, in some cases dramatically. With gel electrolyte, shedding of active material from the plates was reduced.
More importantly, real gas recombination was used to make batteries that were not "watered" and could be called maintenance-free. The one-way valves were set at 2 psi, and this was high enough to have full recombination take place. At the end of charge when oxygen was evolved from overcharge on the positive plate it traveled through the shrink cracks in the gel directly to the negative plate made from high surface area pure lead and "burned" up as fast as it was made. This oxygen gas and the hydrogen adsorbed on the surface of the sponge lead metal negative plate combined to make water that was retained in the cell.
This sealed, non-spill feature made it possible to make very small VRLA batteries (1 –12 Amp hr. range) that fit into the growing portable electronics market. A large market for inexpensive smaller sealed Pb/Acid batteries was generated quickly. Portable TV, light for news cameras, children's toy riding cars, emergency lighting, and the growing market for UPS systems for computer back-up, to name a few, were powered with small sealed VRLA.
Many modern motorcycles and 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 premium vehicles. As vehicles become heavier and equipped with more electronic devices such as navigation, stability control, and premium stereos, AGM batteries are being employed to lower vehicle weight and provide better electrical reliability compared with flooded lead–acid batteries.
New 5 series BMWs from March 2007 incorporate AGM batteries in conjunction with devices for recovering brake energy using regenerative braking and computer control to ensure the alternator charges the battery when the car is decelerating. Vehicles used in auto racing may use AGM batteries due to their vibration resistance.
AGM batteries are routinely chosen for remote sensors such as ice monitoring stations in the Arctic. 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 UPS (uninterruptible power supply) as a back up 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. 
AGM and Gell-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.
Comparison with flooded lead–acid cells
VRLA batteries offer several advantages compared with flooded lead–acid cells. The battery can be mounted in any position, since the valves only operate on overpressure 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. The volume of free electrolyte that could be released on damage to the case or venting is very small. There is no need (nor possibility) to check the level of electrolyte or to top up water lost due to electrolysis, reducing inspection and maintenance.
Because of calcium added to its plates to reduce water loss, a sealed battery recharges much quicker than a flooded lead acid battery. "From a standard car, 4WD or truck alternator they will recharge quickly from full use in about 2 to 3 hours. A deep cycle wet cell battery can take 8-12 hours to achieve only 70% to 80% of its potential charge."  Compared to flooded batteries, VRLA batteries are more vulnerable to thermal run-away during abusive charging. The electrolyte cannot be tested by hydrometer to diagnose improper charging, which 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 greater. VRLA batteries:
- Are less reliable[vague] than flooded lead acid 
- Have shorter recharge time than flooded lead-acid.
- Cannot tolerate overcharging: overcharging leads to premature failure.
- Have shorter useful life, compared to properly maintained wet-cell battery. 
- Discharge significantly less hydrogen gas.
- Books and papers
- Valve-Regulated Lead-Acid Batteries. Edited by Patrick T. Moseley, Jurgen Garche, C.D. Parker, D.A.J. Rand. p202
- The storage of electrical energy. By Gaston Planté.
- How to Make and Use the Storage Battery. By Percy B. Warwick. Bubier Publishing Company, 1896. (Flint glass p121)
- 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
- Storage Batteries: Their Theory, Construction and Use. By Arthur Eugene Watson. Bubier Publishing Company, 1911.
- John McGavack. The Adsorption of Sulfur Dioxide by the Gel of Silicic Acid. Eschenbach Print. Company, 1920.
- U.S. Patent 417,392 Treatment Of Porous Pots For Electric Batteries. Erhard Ludwig Mayer And Henry Liepmann
- U.S. Patent 3,271,199 Solid Acid Storage Battery Electrolyte. Alexander Koenig et al
- U.S. Patent 4,134,192 Composite battery plate grid
- U.S. Patent 4,238,557 Lead acid battery plate with starch coated glass fibers
- David Linden, Thomas B. Reddy (ed). Handbook Of Batteries 3rd Edition. McGraw-Hill, New York, 2002 ISBN 0-07-135978-8, Chapter 24
- Clark, M.S. (Steve) (2008). "Lead-Antimony, Lead-Calcium, Lead-Selenium, VRLA, NI-CD. What's In A Name?". www.battcom.com. Retrieved 1 February 2012.(comparing virtues and vices of sealed batteries versus flooded lead acid).
- John Devitt (1997). "An account of the development of the first valve-regulated lead/acid cell". Journal of Power Sources. doi:10.1016/S0378-7753(96)02516-5.
- Linden, Reddy (ed), Handbook of batteries, third ed, 2002
- Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07-020974-X pages 11–116
- Barre, Harold (1997). Managing 12 Volts: How to Upgrade, Operate and Troubleshoot 12 Volt Electrical Systems. Summer Breeze Publishing. p. 44. ISBN 0-9647386-1-9.(stating sealed battery plates are hardened with calcium to reduce water loss which "raises the batteries' internal resistance and prevents rapid charging.")
- Sterling, Charles (2009). "FAQ: What Is The Best Battery System to Use for an Auxiliary Charging System". Retrieved 2 February 2012.(discussing excessive cost and poor performance of newer sealed gel or AGM batteries versus regular lead-acid flooded batteries in leisure boats.)
- First Start. "Frequently Asked Questions". Retrieved 21 August 2013.(Discussing AGM Facts and Questions.)
- HandyBob (2004 revised 2010). "The RV Battery Charging Puzzle". Retrieved 1 February 2012. (noting that with sealed batteries, you "can’t check the electrolyte to monitor their condition" and they give you "less power in the same amount of space and weight.")
- Calder, Nigel (1996). Boatowner's Mechanical and Electrical Manual (2nd ed.). p. 11. ISBN 0-07-009618-X.