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"). 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. In contrast, VRLA batteries retain generated gases within the battery as long as the pressure remains within safe levels. 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. 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). In all lead–acid battery designs, charging current must be adjusted to match the ability of the battery to absorb the energy. When a cell is subsequently charged, the lead sulfate and water are turned back into lead and acid. When a cell discharges, the lead and diluted acid undergo a chemical reaction that produces lead sulfate and water. 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. VRLA cells have the same chemistry, except the electrolyte is immobilized. Lead–acid cells consist of two plates of lead, which serve as electrodes, suspended in an electrolyte consisting of diluted sulfuric acid. Basic principle Ĭutaway view of a 1953 automotive battery 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. Varley adapted the Cyclon lead foil technology to produce flat plate batteries with exceptional high rate output. In the same period, Gates acquired another UK company, Varley, specialising in aircraft and military batteries. 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. A number of manufacturers seized on the technology to implement it in cells with conventional flat plates. The cyclon is a spiral wound cell with thin lead foil electrodes. The first AGM cell was the Cyclon, patented by Gates Rubber Corporation in 1972 and now produced by EnerSys. The modern gel or VRLA battery was invented by Otto Jache of Sonnenschein in 1957. The first lead–acid gel battery was invented by Elektrotechnische Fabrik Sonneberg in 1934. 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. The term "maintenance free" is a misnomer as VRLA batteries still require cleaning and regular functional testing. ĭue to their construction, the gel cell and AGM types of VRLA can be mounted in any orientation, and do not require constant maintenance. Both types of VRLA batteries offer advantages and disadvantages compared to flooded vented lead–acid (VLA) batteries or each other. AGM (absorbent glass mat) batteries feature fiberglass mesh between the battery plates which serves to contain the electrolyte and separate the plates. Gel cells add silica dust to the electrolyte, forming a thick putty like gel. There are two primary types of VRLA batteries, absorbent glass mat ( AGM) and gel cell ( gel battery). A 12V VRLA battery, typically used in small uninterruptible power supplies and emergency lampsĪ valve regulated lead–acid ( VRLA) battery, commonly known as a sealed lead–acid ( SLA) battery, is a type of lead–acid battery 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 and the presence of a relief valve that retains the battery contents independent of the position of the cells.
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