lead-acid (LA) batteries are currently the most common technology. In specific instances with special requirements, nickel-cadmium or lithium-ion batteries are sometimes used. Lithium-ion is a rapidly growing battery technology, used where high-energy density and low weight are the primary requirements.

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The LA battery represents the most economical choice for larger power applications where weight is of little or no concern. Most UPS systems use LA batteries as they provide excellent performance, high power efficiency with low internal impedance, high tolerance to improper treatment, and attractive purchasing costs. LA batteries use an electrolyte that consists of water and sulfuric acid, and plates made up of sponge lead (negative electrode) and lead oxide (positive). The two main LA battery types are: • VRLA (valve-regulated lead–acid), also known as “sealed” or “maintenance-free” • Flooded, also called “vented” or “open”

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VRLA batteries VRLA batteries are sealed and can be mounted in any orientation. The battery case is equipped with a valve that vents any buildup of gas externally. Gas buildup is usually an exceptional event, which may occur, for example, when the battery is subject to a high charging rate or rapid discharge. VRLA batteries normally require no direct maintenance – they do not need to be topped up with water, as any hydrogen released during charging is recombined internally with oxygen to form water. There are two main VRLA types, distinguished by their electrolyte composition: • Absorbed glass material (AGM), where the electrolyte is held within a highly porous microfiber glass separator. • Gel, which has an electrolyte gel made from a mixture of sulfuric acid and silica. UPS applications normally work with the VRLA AGM type because of its lower internal resistance, high specific power and efficiency, low self-discharge, and lower purchasing costs. AGM batteries also charge faster and can deliver high current of short duration.

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Flooded batteries

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Flooded LA batteries, as the name suggests, have plates that are immersed in an acid electrolyte. Since they are not sealed, the hydrogen generated during operation escapes directly into the environment, meaning that ventilation systems must be more powerful than those for VRLA and, so, sized adequately. In most cases, the battery banks are accommodated in a dedicated room. DIN VDE 0510 Part 2, for example, sets out the provisions for equipping such a battery room. Flooded batteries must be kept and operated upright, and their water levels must be manually topped up. They provide a longer lifespan and higher reliability than sealed LA batteries.

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