A sealed lead acid SLA , valve-regulated lead acid VRLA or recombining lead acid battery prevent the loss of water from the electrolyte by preventing or minimizing the escape of hydrogen gas from the battery. In a sealed lead acid SLA battery, the hydrogen does not escape into the atmosphere but rather moves or migrates to the other electrode where it recombines possibly assisted by a catalytic conversion process to form water.
Rather than being completely sealed, these batteries include a pressure vent to prevent the build-up of excess pressure in the battery. Sealed batteries require stringent charging controls to prevent the build-up of hydrogen faster than it can recombine, but they require less maintenance than open batteries.
Valve regulated lead acid VRLA batteries are similar in concept to sealed lead acid SLA batteries except that the valves are expected to release some hydrogen near full charge.
SLA or VRLA batteries typically have additional design features such as the use of gelled electrolytes and the use of lead calcium plates to keep the evolution of hydrogen gas to a minimum. Despite the range in battery types and applications, the characteristics particularly important in PV applications are the maintenance requirements of the battery and the ability to deep charge a battery while maintaining a long lifetime.
To promote long cycle life with deep discharge, deep cycle batteries may be either of the open-flooded type, with an excess of electrolytic solution and thick plates, or of the immobilized electrolytic type.
Sealed gelled batteries may be rated as deep cycle batteries, but they will usually withstand fewer cycles and lower discharges than the specially designed flooded plate or AGM batteries.
The stringent requirements for batteries used in photovoltaic systems have prompted several manufacturers to make batteries specifically designed for PV or other remote power systems. The batteries most commonly used in stand-alone photovoltaic systems are either deep-cycle lead acid types, or shallower cycle maintenance-free batteries.
Deep-cycle batteries may be open flooded batteries which are not maintenance-free or captive electrolyte AGM batteries which are maintenance-free but which do require care in regulator selection.
A long-life battery in an appropriately designed PV system with correct maintenance can last up to 15 years, but the use of batteries which are not designed for long service life, or conditions in a PV system, or are part of a poor system design can lead to a battery bank which fails after only a few years.
Starting, lighting ignition batteries SLI. These batteries are used in automotive applications and have high discharge and charge rates. Most often they use electrode plates strengthened with either lead antimony in a flooded configuration, or lead calcium in a sealed configuration.
These batteries have a good life under shallow-cycle conditions, but have very poor lifetime under deep cycling. SLI batteries should not be used in a PV system since their characteristics are not optimized for use in a renewable energy system because lifetime in a PV system is so low.
Traction or motive power batteries. Traction or motive batteries are used to provide electric power for small transport vehicles such as golf carts. Compared to SLI batteries, they are designed to have a greater ability to be deep-cycled while still maintaining a long lifetime. Although this feature makes them more suited to a PV system than one which uses SLI batteries, motive power batteries should not be used in any PV systems since their self discharge rate is very high due to the use of lead antimony electrodes.
A high self discharge rate will effectively cause high power losses from the battery and make the overall PV system inefficient unless the batteries experience large DOD on a daily basis.
The ability of these batteries to withstand deep cycling is also far below that of a true deep-cycle battery. Therefore, these batteries are not suited to PV systems. RV or marine batteries. These batteries are typically a compromise between SLI batteries, traction batteries and true deep-cycle batteries.
Although they are not recommended, both motive and marine batteries are used in some small PV systems. The lifetime of such batteries will be restricted to a few years at best, so that the economics of battery replacement mean that such batteries are typically not a long-term cost effective option.
Stationary batteries. Stationary batteries are often used for emergency power or uninterruptable power supply applications. They are shallow-cycle batteries intended to remain close to fully charged for the majority of their lifetime with only occasional deep discharges.
Deep-cycle Batteries. Wide differences in cycle performance may be experienced with two types of deep cycle batteries and therefore the cycle life and DOD of various deep-cycle batteries should be compared. A lead acid battery consists of electrodes of lead oxide and lead are immersed in a solution of weak sulfuric acid. Potential problems encountered in lead acid batteries include:. Gassing: Evolution of hydrogen and oxygen gas. Gassing of the battery leads to safety problems and to water loss from the electrolyte.
The water loss increases the maintenance requirements of the battery since the water must periodically be checked and replaced. Damage to the electrodes. The lead at the negative electrode is soft and easily damaged, particularly in applications in which the battery may experience continuous or vigorous movement. Stratification of the electrolyte.
Sulfuric acid is a heavy, viscous liquid. As the battery discharges, the concentration of the sulfuric acid in the elecotrolyte is reduced, while during charging the sulfiric acid concentratin increases. This cyclicing of sulfuric acid concentration may lead to stratification of the electrolyte, where the heavier sulfuric acid remains at the bottom of the battery, while the less concentrated solution, water, remains near the top. The close proximity of the electrode plates within the battery means that physical shaking does not mix the sulfuric acid and water.
However, controlled gassing of the electrolyte encourages water and sulfuric acid to mix, but must be carefully controlled to avoid problems of safety and water loss. Periodic but infrequent gassing of the battery to prevent or reverse electrolyte stratification is required in most lead acid batteries in a process referred to as "boost" charging. Sulfation of the battery. At low states of charge, large lead sulfate crystals may grow on the lead electrode as opposed to the finely grained material which is normally produced on the electrodes.
Lead sulphate is an insulating material. Spillage of the sulfuric acid. If sulfuric acid leaks from the battery housing it poses a serious safety risk. Gelling or immobilizing the liquid sulfuric acid reduces the possibility of sulfuric acid spills.
Freezing of the battery at low discharge levels. If the battery is at a low discharge level following the conversion of the whole electrolyte to water, then the freezing point of the electrolyte also drops. Loss of active material from the electrodes. The loss of active material from the electrodes can occur via several processes. The oxide is mixed with water, sulphuric acid and a mixer, and then mixed to form a paste. It is then integrated with the grid by extrusion to form a plate.
The paste is pressed by a machine into the interstices of the grid. They are partially dried, then stacked for curing. The curing process transforms the paste to a cohesive, porous solid. The simplest cell would consist of one cathode plate, one anode plate and a separator between them. In practice, most cells contain up to 30 plates with separators between.
The separators are usually cellulose, PVC, rubber, microporous polyethylene or non-woven polypropylene. The plates are stacked and welded together. The tabs that are fixed to the plates are cast, then punched on between the layers and welded together. The plates are suspended inside the case, which is filled with electrolyte in order to activate it. Flooded lead acid batteries must be periodically topped off with distilled water, which can be a cumbersome maintenance chore if your battery bays are difficult to get to.
AGM and gel cells though are truly maintenance free. Being maintenance free comes with a downside though — a flooded cell battery that is accidentally overcharged can often be salvaged by replacing the water that boiled off. A gel or AGM battery that is overcharged is often irreversibly destroyed. A fully charged volt lead acid battery starts off around An odd number of plates is usually used, with one more negative plate than positive.
Each alternate plate is connected. The paste contains carbon black, barium sulfate, and lignosulfonate. The barium sulfate acts as a seed crystal for the lead-to-lead sulfate reaction. The lignosulfonate prevents the negative plate from forming a solid mass during the discharge cycle, and instead enables the formation of long needle-like crystals.
Carbon black counteracts the effect of inhibiting formation caused by the lignosulfonates. The electrolyte loses much of its dissolved sulfuric acid and becomes primarily water. The discharge process is driven by the conduction of electrons from the negative plate back into the cell at the positive plate in the external circuit.
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