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Lessons on: Stored Energy

In Parts 1 & 2 we looked at UPS systems topologies and configurations. This time we’ll look at the stored energy devices, most commonly batteries, that make the UPS uninterruptible.

There are two main stored energy technologies that have evolved to support the critical loads during short term power outages; batteries and flywheels. We’ll deal with the most common first then look at how flywheel technology has evolved.


Mission critical backup systems have been around since the PSTN (plain old telephone system) was developed nearly a century ago. Central telephone switches were actually powered directly by 48V DC batteries. The batteries were kept charged by AC to DC converters, the precursor to today’s UPS systems. When UPS systems were developed in the late 70s and early 80s to handle large mainframe computers, the most common industrial grade batteries available were those made for the telephone industry and consisted of large glass jars containing the electrodes and electrolyte, called “wet cells.”

Today, the wet cell battery is still a mainstay for large mission critical UPS installations where reliability, high capacity, long life and ability to handle sudden deep discharges are crucial to trouble-free uptime. Wet cell batteries, however, need to be located in a room designed for class 1 hazardous environments, and require exhaust fans to create a negative-pressure environment. Some locales require hydrogen gas detectors.

As the technology evolved for sealed-cell maintenance free batteries in the automotive industry, the critical power industry began offering what is known as Valve-Regulated Lead-Acid batteries (or VRLA for short). This technology is less expensive and requires less maintenance than wet cell technology, but does not last as long, nor does it perform as well in a frequent-discharge environment, such as occurs in industrial parks where power anomalies take place regularly throughout the day, due to large motor load switching. Still, VLRA batteries are ideal where they must be located in the data center itself (such as in a UPS battery cabinet), or an area not compliant with Class-1 hazardous environment design.

With either technology, the best way to keep the batteries running long and sure is to couple them to a rotary UPS system. Rotary systems protect the batteries from frequent discharges due to power anomalies and result in longer battery life.


While batteries are a stable technology that have performed well over the years, there are a number of factors that are creating demand for better-performing and more cost-effective stored energy solutions. Batteries are messy, hazardous, and – in the case of wet cells – expensive to house. One alternative that has emerged is flywheel technology.

There are several ways to incorporate flywheels as a battery alternative. One way is to simply replace the battery string with a DC flywheel. This solution provides from 30-90 seconds of stored energy. This will work where the existing backup generator can start within this time frame. Another way is to couple the flywheel to the backup generator as a single system to ensure reliable starting of the generator within the timeframe of the flywheel capacity.

Both systems have their benefits and drawbacks. The battery replacement method can be effective where there is an existing UPS and generator that are being re-used, and neither are paralleled. A system with parallel connected units will take up to 30 seconds to synchronize output, much too close to the flywheel capacity for comfort. Where new systems are being installed, the close-coupled flywheel/generator combination would be a good alternative. These units are designed to start quickly and can be paralleled for large loads and greater flexibility.


Although batteries are messy and expensive to maintain, they continue to garner a larger share of UPS installations because they are a known evil – and because they allow for a greater margin of error should generators fail to start for some reason.

If you would like an analysis of stored energy technologies for your data center, contact Reliable Resources.