Taking Charge

Alan Luscombe of Kohler Uninterruptible Power (KUP) compares the attributes of valve regulated lead acid (VRLA) and lithium-ion (Li-ion) battery teachnologies.

Data centre UPSs provide battery back-up to sensitive ICT equipment during power blackouts. Sometimes the requirement is to keep systems operational over an extended time, but typically just short-term support is needed, either to shut the critical load down safely or for a generator to take over.

Compare and contrast

VRLA battery types have been the most popular UPS solution for decades. They offer competitive pricing, and, if managed carefully, many years of reliable service. However, Li-ion batteries are now challenging this dominance. After being available for years, the technology has recently improved and prices have dropped, so it is an increasingly competitive contender for static UPS applications.

Much of the information below has been provided by Yuasa Battery Europe, based on its ongoing experience of supplying both technologies.

VRLA Considerations

VRLA batteries’ initial purchase cost is lower, with Li-ion types still being up to twice as expensive. However, VRLA impedance always increases over time, reducing power capacity, and can increase sharply following problems like overcharging. Accordingly, they must be regularly checked for failing cells, or a fixed battery monitoring system must be used.

VRLA batteries are very temperature dependent, and must be kept within a temperature controlled environment. Depending on the number of discharge cycles experienced and ambient temperature, a five year design life VRLA battery may last 3-4 years, while a 10-year design life battery may yield 7-8 years.

VRLA batteries also exhibit a voltage depression effect during initial discharge, which can cause unpredictable UPS tripping. This means that VRLA batteries should be specified for a minimum autonomy of five minutes or more.

VRLA batteries require continuous charging to replace self-discharge losses, while over 90 per cent of float charging current can be wasted as heat. However, float-charging VRLA batteries can be easily maintained in a balanced-charge condition. Li-ion batteries require expensive electronic battery management systems (BMSs) to achieve the same balancing effect. However, BMSs are also sometimes used with VRLA systems supporting critical loads.

Li-ion Performance

Li-ion batteries, although more expensive to purchase, operate for longer than VRLA variants and 12-15 years is realistic. They are also up to 70 per cent smaller, 60 per cent lighter, work reliably at higher temperatures than VRLA batteries, and can be sited by the UPS instead of requiring their own managed environment. Li-ion battery maintenance is easier and less frequent than for VRLA.

Li-ion’s improved performance stems from its chemistry, which is simpler and more predictable than VRLA’s. This desirable Li-ion predictability extends to ageing effects. VRLA failures occur primarily through positive electrode corrosion during the final weeks or months of operational life. Conversely, Li-ion exhibits a steady, predictable loss of capacity. For example, if a cell’s performance diminishes to 80 per cent of original value in 10 years, it will take another 10 years, under the same conditions, to drop to 60 per cent.

In Summary

VRLA batteries will remain popular because of their proven reliability and lower initial purchase price. However, Li-ion is increasingly offering a competitive total cost of ownership with longer life and reduced maintenance, while saving space for data centre owners – both because the batteries are smaller, and because they don’t need separate, temperature maintained rooms. Li-ion can gain further traction if prices drop more, and owners accept that Li-ion UPS solutions have been developed to optimise safety over maximum power density.

Posted by: Inside Networks Dec 2017, http://insidenetworks.clikpages.co.uk/dec17/

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