Data centre operators are investing electrical energy and money into UPS batteries due to the fact that they protect highly sensitive ICT equipment against blackouts and power disturbances. In turn, UPS’s rely on a source of energy that is capable of storing power when there is a good mains supply and then deliver to the critical load from the UPS when the main fails or its acceptable limits are transgressed. The requirement sometimes is for the sustained power to be able to keep a system functioning over an extended period of time. However, usually, only short-term support is necessary, either to enable the critical load to safely shut down or until the supply is taken over by a generator.
The initial purchase cost of VRLA batteries is less expensive. Li-ion types cost up to twice as much. VRLA batteries, compared with the open-vented earlier versions, are safer and more environmentally friendly, and less routine maintenance is needed. However, over time their impedance always does increase, which reduces power capacity and after a problem like over-charging can increase sharply. Therefore, they need to be checked on a regular basis for failing cels. Otherwise, a fixed battery monitoring system must be invested in.
VRLA batteries are highly temperature-dependent. Therefore, they must be stored inside a temperature-controlled environment. Operating at temperatures significantly over the 20°C recommended level can cause irreversible damage, and for every 10°C increase in the average operating temperature can cut life expectancy in half. Low temperatures have only a negligible impact on the battery’s service life, but performance is reduced. Depending on the ambient temperature and how many discharge cycles there are, a VRLA battery with a five-year design life may be expected to last 3-5 years, and a battery with a 10-year design life can be expected to last 7-8 years.
Lithium-ion chemistry and performance
Although they are more expensive to buy, Li-ion batteries have a longer operational life compared to VRLA batteries, with a 12-15 lifespan being realistic. Also, they are as much as 60% lighter and 70% smaller and are able to perform reliably at higher temperatures compared to VRLA batteries. Also, it is important to note that li-ion batteries have been designed specifically for UPS applications, in contrast to other types of batteries where combustion problems have occurred in mobile phones, which are optimised for safety instead of power density. Those factors mean that you can locate li-ion batteries within or next to the UPS enclosure instead of needing to have a separate battery room that has its very own environmental conditioning. Although they still need some maintenance, li-ion batteries require simpler procedures and at longer intervals compared to VRLA batteries.
The improved performance of li-ion batteries is derived from their chemistry. They have simpler and fewer chemical processes compared to VRLA batteries. While discharging or charging a lead-acid cell, first the active materials’ internal structure is dissolved. It is then re-built inside of the electrode structures. The chemical reaction also involves sulphuric acid electrolytes. Hydrogen gets decomposed into energy-wasting side reactions and water into oxygen.
VRLA batteries will continue to be popular due to their less expensive initial purchase price and their proven reliability. However, Li-ion batteries are becoming increasingly competitive with their reduced maintenance requirements and longer lifespan, while saving data centre owners space due to the batteries being smaller and also because temperature-maintained, separate rooms are not necessary. Further traction can be gained by li-ion if the prices become lowers, and owners accept that the UPS solutions of li-ion have been developed for maximising safety instead of maximum power density.