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A VRLA battery (valve-regulated lead–acid battery) more commonly known as a sealed battery is a lead–acid rechargeable battery. Because of their construction, VRLA batteries do not require regular addition of water to the cells, and vent less gas than flooded lead-acid batteries. The reduced venting is an advantage since they can be used in confined or poorly ventilated spaces. But sealing cells and preventing access to the electrolyte also has several considerable disadvantages as discussed below.
VRLA batteries are commonly further classified as:
An absorbed glass mat battery has the electrolyte absorbed in a fiber-glass mat separator. A gel cell has the electrolyte mixed with silica dust to form an immobilized gel.
While these batteries are often colloquially called sealed lead–acid batteries, they always include a safety pressure relief valve. As opposed to vented (also called flooded) batteries, a VRLA cannot spill its electrolyte if it is inverted. Because AGM VRLA batteries use much less electrolyte (battery acid) than traditional lead–acid batteries, they are sometimes called an "acid-starved" design.
The name "valve regulated" does not wholly describe the technology. These are really "recombinant" batteries, which means that the oxygen evolved at the positive plates will largely recombine with the hydrogen ready to evolve on the negative plates, creating water and preventing water loss. The valve is a safety feature in case the rate of hydrogen evolution becomes dangerously high. In flooded cells, the gases escape before they can recombine, so water must be periodically added.
These batteries have a pressure relief valve which will activate when the battery is recharged at high 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 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.
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. Both AGM and Gel cells are commonly used in powered aerobatic aircraft, for the same reasons.
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.
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: