Lithium-ion Batteries and UPS Energy Storage

When uninterruptible power supplies (UPS) were developed for the oil and emerging computer industries in the 1950s their primary purpose was to provide both ‘uninterruptible power’ and condition the power which could be mains derived or supplied by a local generating set. Early computer power supplies where less reliable than they are today, with smaller input voltage and frequency windows and were very susceptible to power pollution: sags, surges, brownouts, spikes, transients and electrical noise.

The power supplies within your servers are today far more robust. Whilst server power supplies undoubtedly benefit from a UPS conditioned supply (reduced ‘wear and tear’), the purpose of a UPS in most modern datacentres and comms rooms is to provide a ‘uninterruptible power’.

Today the modern UPS system can be thought of more as an energy storage device for the site it is protecting. With advancements in distributed energy storage management, UPS could provide a way for the National Grid to gain access to distributed energy storage in much the same way as their Short Term Operating Reserve (STOR) program couples on-site generators to the grid.

The STOR program pays owners of generating sets to be connected to the grid via aggregating companies. When the National Grid needs more power generation capacity, the aggregators contact their clients to see if they can run their on-site generators and take the power into the grid. Everyone benefits through a Feed-In-Tariff (FIT) like program.

There are thousands of UPS systems installed across the UK ranging from small systems to some fairly large installations above 1MW or more. What is to stop larger UPS owners form also signing up to a STOR-type program? Quite simply, the standard battery technology used.

Driven by the datacentre world, UPS manufacturers have focused on energy efficiency, compactness and modularity. Little development has gone into the primary ‘energy storage’ component within UPS systems; namely the battery set.

Today’s dominant battery technology is the VRLA – valve regulated lead-acid battery. Normally sealed and maintenance free, UPS battery technology is not far removed from that of a standard car battery. Lead-acid technology is ideal for periodic cyclic charge/discharge cycles i.e. non-continuous use. Of course a UPS battery will provide a runtime (from several minutes to hours) but the point is that the discharges (to cover mains power cuts) are fairly infrequent.

Whilst Nickel-Cadmium batteries fell foul of environmental changes (and wisely so), they did offer some benefits in terms of capacity and their charge/discharge cycle capabilities, all be it with their infamous ability to remember their last point of recharge (recall your early camcorder battery problems).

Lithium-ion battery technology has emerged rapidly as the only viable alternative for applications requiring rapid recharges and frequent discharge rates. These are the batteries most commonly found in smart mobile phones, tablets and laptops and for those pioneering few, their electric vehicles.

For a UPS system to perform as part of an energy storage system and fulfill a role within the National Grid’s STOR program it must be capable of using Lithium-ion technology or some other battery capable of frequent charge/discharge cycles and there’s the rub. Car manufacturers such as BMW, Tesla and Toyota are harnessing lithium-ion technology for their electric vehicles but few UPS manufacturers have launched lithium-ion enabled back-up systems. Yet, Lithium-ion provides several advantages over traditional VRLA batteries.

Firstly, this battery technology is more suited to the rapid and frequent charge/discharge cycles required for energy storage. A downside is battery cost and the complex charging system required to optimise charging current and prevent thermal runaway.

Lithium-ion batteries are more compact than an equivalent Ah-rated lead-acid battery set, with potential footprint and volume reductions of about 60%. Weight may also be reduced by a similar factor leading to less physical space requirements within a datacentre plant room or white space server rack.

Lithium-ion batteries are more temperature tolerant than lead-acid batteries. Lead-acid requires an operating ambient of 20-25°C and for every 10% ambient temperature increase above 30°C the design life can halve. Lithium-ion batteries are less susceptible to temperature variation. This has implications on cooling within datacentres and where a UPS and its battery can be installed; important as the ASHRAE standard pushes up white space operating temperatures, in a bid to reduce cooling energy consumption.

Lithium-ion batteries may also be more predictable in terms of working life. For lead-acid batteries the problem has always been one of how to measure the ‘health’ of an individual battery block. Battery testing technologies are developing and conductance battery testing is considered to be superior to the traditional impedance testing variety. Whilst impedance testing measures whether a battery has failed, conductance testing measures health and regular testing can help to identify failing cells more accurately.

Lithium-ion batteries may prove to be more reliable long-term and less prone to erratic failure, with working lives for a Lithium-ion battery set of 10 years or more as standard. Compare this to the 3-4 and 7-8 year typical working life spans of 5-year and 10-year design life batteries and the economic argument for Lithium-ion batteries becomes even stronger. As with any UPS system and critical power investment, one thing that clients always consider is the Total Cost of Ownership (TCO). Lithium-ion batteries typically carry a 30-40% premium over lead-acid and their capacity range, whilst increasing still limits them in terms of the kVA/kW size of UPS they can be used with.

This can be offset against the ‘extra’ life the batteries should provide over lead-acid leading to fewer battery set replacements. Other potential cost benefits to take into the calculation include reduced cooling if the Lithium-ion enabled UPS is to operate within a higher ambient environment.

Finally, UPS with lithium-ion batteries could be connected to the grid, they could receive an annual connection fee and then generation tariff based on usage. Not only would this help to provide a quicker payback. It would also help to boost the capacity reserve on the grid itself.