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  definition of VRLA Battery

VRLA stands for valve regulated lead-acid and is the designation for low maintenance lead-acid batteries, also called recombinant batteries.[1] VRLA batteries are commonly further classified as:
* Absorbent glass mat battery
* Gel battery
These batteries are often colloquially called sealed lead-acid batteries, but this term is misleading: a sealed battery would be a safety hazard due to overpressure risks when overcharging, and there is always a safety valve present, hence the name valve-regulated. Sealed is opposed to vented (also called flooded). Because VRLA batteries use much less electrolyte (battery acid) than traditional lead-acid batteries, they are also occasionally referred to as 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--thus preventing water loss. The valve is strictly a safety feature in case the rate of hydrogen evolution becomes dangerously high.
One result of this design is a much higher ratio of power to "floorspace" than large, flooded type battery systems; another is a high-rate power capacity, though of relatively short duration. As a result, VRLA batteries are frequently employed in UPS (uninterruptible power supply) or other high-rate applications. Long duration applications such as telecommunications are not as well-suited to VRLA batteries, as there are other, more applicable battery designs available.

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, a 12 volt battery has 6 cells). Valve activation allows some of the active material to escape thus decreasing the overall capacity of the battery.The lids (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; and 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.
At high 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. Charging with a constant voltage (called the float charge voltage; 2.26 V per cell for a lead-acid chemistry) can cause a rapid initial current, so therefore it is suggested to begin with a constant current, using constant voltage only for the final portion of the charging. However, the float charge voltage should not be exceeded by much for typical usage, so the switch between the two modes typically occurs when the float voltage is needed to sustain the charging current through the battery's internal resistance (as per Ohm's Law). The easiest way to implement this is to use a constant voltage device with a current limiter.

Comparison with flooded lead-acid cells
Compared with flooded lead-acid cells, VRLA batteries offer several advantages. 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. 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.

Absorbent glass mat (AGM)
Absorbent glass mat (AGM) is a class of VRLA battery in which the electrolyte is absorbed into a mat of fine glass fibers. The plates in an AGM battery may be flat like a wet cell lead-acid battery in a rectangular case. In cylindrical AGMs, the plates are thin and wound, like most consumer disposable and rechargeable cells, into spirals so they are also sometimes referred to as spiral wound. Their unique (for lead acid chemistries) construction also allows for the lead in their plates to be purer as they no longer need to support their own weight as in traditional cells. Their internal resistance is lower than traditional cells due to plate proximity and the pure lead plates have lower resistivity, they handle higher temperatures better, and self discharge more slowly.
Their specific power is very good and they can be charged and discharged quite rapidly, however their specific energy tends to be lower than traditional flooded batteries. They are often used in high performance electric vehicles due to their high power density.
In high grade and military spec AGMs the plates are thin and hand-wrapped in envelopes to discourage short circuiting due to vibration2.
AGM, rechargeable batteries usually come in large sizes.

Gel battery
A gel battery (also known as a "gel cell") is a VRLA battery with a gelified electrolyte; the sulfuric acid is mixed with a silica fume, which makes the resulting mass gel-like and immobile. Unlike a traditional wet-cell lead-acid battery, these batteries do not need to be kept upright (though they cannot be charged inverted). In addition, gel batteries virtually eliminate the electrolyte evaporation, spillage (and subsequent corrosion issues) common to the wet-cell battery, and boast greater resistance to extreme temperatures, shock, and vibration. These batteries are often colloquially referred to as sealed lead-acid (SLA) batteries due to their non-leaking containers, but they are not completely sealed; the valve regulation system allows for gas to be expelled. Chemically they are the same as wet (non-sealed) batteries except that the antimony in the lead plates is replaced by calcium. This preserves the mechanical characteristics but renders the construction far less prone to gassing. The battery type is often referred to as a lead-calcium battery.

Many modern motorcycles on the market utilize AGM or factory-sealed AGM batteries for the combined benefits of reduced likelihood of acid-spilling during accidents, and for packaging reasons (lighter, smaller battery to do the same job; battery can be installed at odd angles if needed for the design of the motorcycle).
Due to the higher manufacturing costs compared with flooded lead acid batteries, AGM technology is currently being used on premium vehicles. As vehicles become heavier and equipped with greater 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.[1]
NASCAR, IHRA, SCORE, and other auto racing leagues are continually seeing AGM products introduced into their vehicles. This is due in large part to the fact that these batteries are vibration resistant.
Deep-cycle AGMs are also commonly used in large-scale amateur robotics. Two examples are the FIRST and IGVC competitions.
AGM batteries are routinely chosen for remote sensors such as the AMIGO Ice monitoring stations in the Arctic. AGM batteries due to their lack of "FREE" electrolyte will not crack and leak in these cold environments3.

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