Choosing the right battery technology
(Fig.1 VRLA battery internal and external components)
While batteries based on lead-acid technologies remain the most popular choice for UPS users, chemistries such as lithium-ion and nickel cadmium are offering increasingly attractive alternatives.
There is no single 'best' battery technology; each approach has its own strengths and weaknesses, and it is up to each data centre operator to choose the one that best meets their requirements.
LA batteries remain popular because they have a long-proven track record for reliability. They are the most economical choice for larger power applications where weight is of little concern. They provide excellent performance and efficiency with low internal impedance, high tolerance to improper treatment and attractive purchasing costs.
LA batteries use an electrolyte comprising water and sulphuric acid, and plates made up of sponge lead (negative electrode) and lead oxide (positive). The two main LA battery types are VRLA (valveregulated lead-acid) also known as `sealed' or 'maintenance-free' (shown in Fig.i) and flooded, also called 'vented' or `open.'
VRLA batteries are sealed and can be mounted in any orientation. The battery case is equipped with a valve that vents any gas build-up to atmosphere. They normally require no direct maintenance - no water top-ups are necessary, 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; and Gel, which has an electrolyte gel made from a mixture of sulfuric acid and silica.
UPS applications normally use the VRLA AGM type because of its lower internal resistance, high specific power and efficiency, low selfdischarge and lower purchasing costs. AGM batteries also charge faster and can deliver high current of short duration.
Flooded LA batteries have plates that are immersed in an acid electrolyte. Since they are not sealed, the hydrogen generated during operation escapes directly into the environment, so ventilation systems must be more powerful than those for VRLA and sized adequately. In most cases, the battery banks are accommodated in a dedicated 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. LA battery rooms must be kept at a reasonably constant temperature (2o-25°C) to avoid reducing service life or even causing damage.
In li-ion batteries like the examples in Fig.2, the `cathode' is usually a metal oxide, while the anode is usually porous carbon graphite. Both are immersed in a liquid electrolyte made of lithium salt and organic solvent.
Various Ii-ion chemistries exist, which can be simplified into six types: Lithium cobalt oxide (LCO); lithium manganese oxide (LMO); lithium-nickel manganese cobalt oxide (NMC); lithium iron phosphate (LFP); nickel cobalt alumina (NCA); lithium titanium oxide (LTO). Choosing between these depends on several factors and precise comparison is not possible, since many aspects such as mechanical form, cell size and active material Leadmix, play a significant role in performance.
Li-ion batteries are becoming an increasingly attractive alternative to LA. In data centre environments, where power availability has absolutely the highest priority, they offer higher reliability than VRLA solutions. Not only is each individual cell inherently more safe and stable, but each battery module has an electronic controller that continuously checks every cell for any sign of change in performance.
Single cell temperature, current, voltage and charge status are all monitored at the cabinet level to provide a clear overview of current battery status and to predict future runtime and performance.
Lithiumion batteries can be charged much more quickly than conventional batteries, offer more discharge/ recharge cycles than traditional batteries and provide higher power density and efficiency, especially under heavy discharge rates. This eliminates battery oversizing, while
much less real estate is needed for the battery installation. Although VRLA initial purchase prices are lower, li-ion battery operating life is at least double that of VRLA equivalents, so reducing the overall capital cost. The labour costs associated with battery removal and replacement are also reduced. Liion produces less waste heat, which in turn lowers cooling costs and creates a smaller carbon footprint.
Nickel-cadmium (NiCd) battery electrodes comprise nickel hydroxide (positive plate) and cadmium hydroxide (negative plate). NiCd batteries provide a very long calendar life (up to 20 years) and can cope with temperature extremes (-zo°C to +40°C.) They also offer a high cycle life and have good tolerance to deep discharges. Other benefits relate to the low internal resistance, which offers high power density combined with fast-charging capability. NiCd batteries offer long storage times, and provide high protection against improper treatment.
(Fig. 3 Shows LA alongside Li-ion)
However, NiCd batteries cost much more than traditional VRLA equivalents. Furthermore, as both nickel and cadmium are toxic, battery disposal/recycling processes are costly. NiCd batteries also require maintenance in the form of topping up with water - especially in high-cycle applications, or under heavy charging rates with some charging methodologies.
From an article by Alex Emms , Operations Director at Uninterruptible Power Supplies
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