The right fit

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Mark Coughlin, applications manager for reserve power at EnerSys, highlights the importance of choosing the right battery for your UPS.

Today’s data centres depend on uninterruptible power supplies (UPS) to provide clean, continuous power throughout the facility’s entire operational life. While the mains supply is available, the UPS protects the data centre’s sensitive information and communication technology (ICT) equipment from any power aberrations appearing on the incoming power feed. If the supply fails, the UPS battery must take over instantly, supporting the load until either the mains is restored, or a generator can be started, or the data centre systems can be shut down safely.

A UPS battery’s ability to store energy reliably and efficiently during normal operation, making it immediately available to the load during a mains failure, is critical to data centre security. In this article, we review the key battery technologies currently available, to inform commercial and technical specifiers responsible for battery selection.

We will first look at how data centre evolution is affecting the demands placed on batteries. Then, compare lead-acid, the pre-dominant battery chemistry used within data centres, with Lithium-ion (Li-ion), which has been generating rising interest in recent years. Finally, we will examine why data centre operators should consider advanced Thin Plate Pure Lead (TPPL) technology to optimise their UPS and power system performance. 

The evolving data centre landscape

Data centres today experience rising numbers of power outages and grid fluctuations while, simultaneously, their workload is expanding. These factors increase pressure for ‘best in class’ technologies and reliable power.

UPS batteries are directly impacted by reduced autonomy times, now typically between 30 seconds and five minutes, compared with historical averages of around 15 minutes. This is because of the shorter times needed to start up generators and switch loads. Fast recharge times are also desirable, quickly readying batteries to support further power outages. 

Energy efficiency now concerns all data centres, not just because of costs, but also through pressure to reduce carbon footprint. There is also growing interest in using UPS battery assets for revenue-generating energy storage applications.

Battery technologies and trends

The two UPS backup battery chemistries that currently dominate the data centre industry are lead-acid, which represents over 90% of the UPS market share, and Li-ion, which is attracting increasing interest due to its purported performance benefits and high visibility through its use in electric vehicles. 

Compared to traditional VRLA equivalents, Li-ion offers a high cycle life, together with significant size and weight reduction. Li-ion batteries also have high charge efficiency, with excellent partial state of charge tolerance – in fact, partial charge is preferred for long cycle life while operation in float conditions at full state of charge is avoided. Li-ion self-discharge rate is also low, resulting in prolonged storage shelf life. Finally, it has good high and low temperature performance, and no gas emissions. 

However, Li-ion has challenges along with its benefits. Despite historical cost reductions, pricing remains a barrier for many users. Furthermore, although space-saving may be important within data centres, weight reduction, which Li-ion batteries offer, is seldom critical. Similarly, the high cycling capability of Li-ion isn’t essential to UPS applications, where batteries mostly float at near full state of charge.

While considered a safe technology, any Li-ion solution, unlike lead-acid, must include a battery management system (BMS) to ensure safe charging and discharging. This increases complexity, and requires users to thoroughly understand Li-ion technology. However, the BMS provides built-in diagnostics, which identify most problems and allow minimal maintenance.

Additionally, the MTBF (Mean Time Before Failure) of the electronic components factored into Li-ion calendar lifetime calculations must be considered. 15-year lifetimes are claimed, but service life is not field-proven. By comparison, advanced TPPL, with 12+ years design life, provides eight to 10 years’ service life, while traditional VRLA 10-year design life batteries typically provide five to six years’ service life.

To fast-charge Li-ion, higher charging capacity, with increased cost, may be required. Also, the charging architecture would often need replacing or changing to support different Li-ion battery charger voltages, so two different UPS rectifier types would be required across a data centre deploying both Li-ion and lead-acid batteries. 

For delivery, Li-ion faces legislative shipping restrictions, while lead-acid batteries, including AGM and TPPL, are classified as non-hazardous for all transportation modes. Then, at end-of-life, lead-acid has inherent value, and is about 95% recyclable by a very well-established network of smelters; this possibility, however, is not mature for Li-ion.

TPPL: Optimised performance without the drawbacks

While Li-ion has been slow to replace lead-acid in data centres, advanced TPPL technology offers the best of both chemistries.

As a lead-acid based battery technology, TPPL is reliable, well-proven, and easy to transport, handle and recycle. Crucially, advanced TPPL technology significantly improves energy efficiency, by providing up to 43% energy reduction compared with traditional VRLA batteries through reducing float current requirements. Further energy savings accrue as it can operate, within warranty, at elevated temperatures, reducing air-conditioning requirements.

Meanwhile, advanced TPPL battery technology reduces data centre vulnerability to multiple mains blackouts, through very short recharge times and time to repeat duty. For example, with 0.4C10 A charging current using fast charge methodology, TPPL can be fully recharged, following a one-minute discharge to 1.6 Vpc, in 2.5 hours, and ready to repeat duty in 22 minutes.

Battery replacement costs are also reduced through low internal corrosion rates, yielding a service life 25% longer than for traditional VRLA. Additionally, storage life is increased from six to 24 months due to low self-discharge rates.

Advanced TPPL technology is used today in many demanding critical applications, including UPSs within data centres. Autonomies of under five minutes are supported.

What of the future?

Lead-acid technology is expected to dominate the market for at least the next few years, although Li-ion activity will continue to grow. In particular, applications requiring high cycling will be seeking advanced TPPL or Li-ion solutions.

Depending on the application, Li-ion could be the preferred battery type. Nevertheless, before opting for Li-ion, a full consideration of the requirements should be undertaken. This should reflect the Total Cost of Ownership, with the benefits and challenges of Li-ion compared against other available technologies, including TPPL.

Irrespective of the technology chosen, battery monitoring systems will become increasingly popular, due to the battery condition visibility and predictive maintenance opportunities that they provide. This will also bring UPS applications into the Internet of Things (IoT) environment, making them visible as components of the larger data centre infrastructure.