Sealed lead acid battery

Wednesday, 03 January 2007

Lead-acid batteries have been known for a substantially long period of time and have been employed commercially in a relatively wide variety of applications. Lead-acid batteries have been used for many diverse applications. Such applications include use as a starting, lighting and ignition power source for vehicles (SLI), use in marine batteries for starting, lighting and other auxiliary power requirements, as a motive power source for use in golf carts and other vehicles and other applications of this sort. Lead-acid batteries have been employed in a variety of stand-by power applications to provide a power source when the main power source becomes inoperable. Lead-acid batteries have also been employed in many other applications, e.g., uniform power distribution, power damping applications, and even for small electronic devices such as video cameras and cassette players. In general, a lead acid battery is a battery which has a stable characteristic at a lower cost as a secondary battery and which is used widely as an electric power source of a movable type for use in a portable electronic apparatus, a starter for automobile or a golf cart and a stationary type for use in a back up electric source for computer. The lead-acid electrochemical system has provided a reliable energy source which is capable of being manufactured in automated production while providing acceptable quality. Based on the quantity of acid present in a battery, lead-acid batteries are classified as either flooded or starved electrolyte batteries. In flooded batteries, acid is in excess which is also referred to as free acid. A conventional flooded lead-acid cell produces stoichiometric quantities of hydrogen and oxygen during the recharging process. Hydrogen and oxygen gases generated as a result of the electrolytic decomposition of the water in the cell eventually lead to significant water loss and the need for cell maintenance. Whereas, in starved electrolyte batteries there is no excess or free acid. Starved batteries are designed such that all the acid in the battery is immobilized in the plates and in either the separators or in gel form. Where the acid is immobilized in the separators, the battery is referred to as an absorbed glass matrix (AGM) battery. Where the acid is immobilized in the gel form, the battery is referred to as a gel type battery. Lead-acid batteries include sulfuric acid as an electrolyte. This electrolyte readily wets the surfaces of internal lead components of the battery or cell, inasmuch as sulfuric acid has a very low surface tension. Sulfuric acid and can readily pass through small orifices and cracks. Without an effective seal between the interior of the battery or cell and the ambient atmosphere, sulfuric acid would readily migrate out to the environment, potentially causing corrosion of the terminals or exterior equipment. A seal also is necessary to prevent gases, such as oxygen or hydrogen, from leaking into or out of the battery or cell.

A sealed lead acid battery, sometimes termed VRLA cell and battery (valve regulated lead acid), has a structure in which separators and plates are arranged in stack in sealed cells, and an electrolyte in the battery is retained without flowing in the pores the separators and both. A sealed lead acid battery comprises separators and plates stacked within a container, in which an electrolyte is retained in pores of the separators and both of positive and negative electrode plates so as not to flow. The sealed lead acid battery has an advantageous feature of excellent liquid leakage proofness, requiring no water supplement and causing less self discharge. A lead-acid battery can be sealed by utilizing a so-called "oxygen cycle" in which oxygen gas evolving at the positive plates by electrolysis of water during charging reacts at the negative plates with the charged active material and electrolyte to be converted into water. Beyond these basically passive functions, separators in lead-acid batteries can also actively affect the battery performance in many ways. Sealed lead-acid batteries that make use of oxygen cycles are available in two types, a retainer-type and a gel-type. In a retainer type sealed lead-acid battery, a mat separator (glass separator) made of fine glass fibers is inserted between a positive and a negative plate in order to retain the necessary amount of sulfuric acid electrolyte for charging and discharging the battery and for keeping the electrodes apart. Sealed lead-acid cells and batteries utilize a safety valve to maintain the desired internal pressure for an efficient oxygen recombination cycle. Sealed lead-acid batteries are normally sealed from the atmosphere by a valve designed to regulate the internal pressure within the cell so as to provide what is termed an effective "oxygen recombination cycle" (hence, the use of the terms "sealed" and "valve-regulated"). A key for the successful operation of a VRLA battery is the oxygen recombination reaction which prevents the water loss from a battery by recombining the oxygen and by suppressing hydrogen gas liberation at the negative electrode. Valve-regulated lead-acid (VRLA) batteries rely upon internal gas recombination to minimize electrolyte loss over the life of the battery, thereby eliminating the need for re-watering. Internal gas recombination is achieved by allowing oxygen generated at the positive electrode to diffuse to the negative electrode, where it recombines to form water and also suppresses the evolution of hydrogen. The starved acid design of the battery facilitates the oxygen recombination reaction. The recombination reactions are facilitated by the starved acid or electrolyte condition where the electrolyte is immobilized in glass separators disposed between the plates of the battery. In VRLA batteries oxygen, which is generated during charging at the positive electrode, is reduced at the negative electrode. Thus the battery can be charged and even be overcharged without water consumption and is therefore theoretically maintenance-free. The formation of hydrogen at the negative electrode is suppressed, for instance by using larger negative than positive plates in order to generate oxygen at the positive plate before the negative plate is fully charged. In valve regulated lead-acid (VRLA) batteries they additionally determine properties like oxygen transfer, electrolyte distribution and plate expansion. For VRLA batteries two technologies are predominant, i.e. batteries with an absorptive glassmat (AGM) and gel batteries. In batteries with AGM, the absorptive glassmat immobilizes the electrolyte and simultaneously functions as a separator. In gel batteries, the acid is immobilized by means of fumed silica and an additional separator is required to fix the plate distance and to prevent electronic shorts. The sealed lead-acid stationary cells and/or batteries used for industrial applications where the power requirements are high and quite demanding are typically comprised of from several to a large number of individual sealed lead-acid cells connected to one another to form a battery with the desired capacity and power requirements.

Sealed lead-acid batteries utilize highly absorbent separators, and the necessary electrolyte is absorbed in the separators and plates. Accordingly, such batteries may be used in any attitude without electrolyte spillage as would occur with a flooded electrolyte lead-acid battery. The advantages that are provided by sealed lead-acid cells and batteries in comparison to conventional, flooded lead-acid electrolyte batteries are substantial and varied. Sealed lead-acid technology thus offers substantial benefits by eliminating maintenance, expense, environmental concerns, and safety. Sealed lead-acid cells and batteries are widely used in commerce today for various applications that have widely differing requirements. In one type of application, generally termed as stationary applications, lead-acid cells and batteries are used for stand-by or operational power in a wide variety of applications, including telecommunications, utilities, for emergency lighting in commercial buildings, as stand-by power for cable television systems, and in interruptible power supplies for computer back-up power and the like. The sealed lead-acid stationary batteries used for industrial applications where the power requirements are high and quite demanding are typically comprised of from several to a large number of individual sealed lead-acid cells connected to one another to form a battery with the desired capacity and power requirements. Stationary batteries are usually maintained at a full-state-of-charge and in a ready-to-use condition typically by floating at a constant preset voltage. With the rapid progress of the society towards the information and intelligence-oriented one in the recent years, uninterruptible power supplies are being widely used as power sources for backing-up computers which are key components of such society. Uninterruptible power supplies are systems that back-up computers and communication networks. Sealed lead-acid cells and/or batteries may comprise the power source. The uninterruptible power source allows for the orderly shut down of computers when there is a sudden interruption in the primary electrical source, such as during a power outage and provides back-up power for communications networks. When the principal power supply to the electronic equipment or the like has been cut off, such as during a power outage, the sealed cells provide a source of reserve power to allow the telecommunication or computer system to remain operational until the principal power supply can be restored.

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