Uninterruptible Power Supply Extended Runtime Options

From an entry-level desktop or wall-mounted unit to an 80kVA n+1 installation supporting a major data centre, batteries are at the heart of every UPS (uninterruptible power supply) system. They provide the required runtime and a source of dc supply to the inverter in the event of a mains power failure; when more runtime is needed, it can be extended by adding extra battery packs or battery extension packs.

For a given UPS battery installation the following factors ensure that, when needed, a battery will give its rated runtime: correct operation (following the UPS manufacturer's instructions), a suitable operating environment (especially ambient temperature), regular battery maintenance and timely battery replacement.

Ideally, a UPS battery will be sized to provide a specified runtime at a specific load, with the runtime being sufficient to do the following: keep loads running, allow start-up of standby generators (automatic and manual if necessary) and allow sufficient time for completion of automatic server shutdown routines.

Battery runtime and orderly system shutdown. Whatever the rated battery runtime, there should be sufficient duration for protected systems to be shutdown in an orderly manner using appropriate proprietary software such as Riello UPS's PowerShield.

To ensure a required battery runtime, battery size is dictated by load size (measured in kW) and the amount of back-up time required in the event of a mains failure. Most UPS are designed to offer up to ten minutes runtime - enough to cover the most common power interruptions and safely shutdown non-critical loads.

Determining required battery runtime. The required UPS battery runtime, even the decision to specify battery extension packs, will be determined by factors including the following: site power history and existing UPS equipment, availability of a maintained standby generator on site, length of time required for load-shedding and orderly system shutdown, environmental aspects (space, temperature and humidity) and overall cost of long battery runtime versus a standby generator or fuel cell.

Correct operation, maintenance and battery replacement can ensure that a given UPS battery installation performs to the best of its ability. From the smallest UPS (typically used for power protection of small to medium sized file servers, PBX systems and point-of-sale or back-office systems), to the largest industrial series power protection units, UPS battery runtime can be extended by simply adding more batteries.

Battery extension packs for extended runtime. At their simplest, plug-in UPS battery extension packs can extend the shortest (typically ten minutes) internal battery runtime to an hour or more. On larger uninterruptible power supplies, where an inverter is rated for continuous operation, the UPS can be used to support large loads for long runtimes up to and exceeding several hours. This is normally achieved with battery extension packs that connect to the rear panel connector on the UPS (and/or a generator) in series, a configuration also referred to as 'daisy chaining'.

Designed to match the UPS aesthetically. UPS up to around 30kVA have capacity for internal battery sets. Where this is not the case, or where additional power protection runtime is required, external battery extension cabinets or rackmount battery trays are used; as far as is reasonably practical, these are designed to match the UPS aesthetically.

An alternative is to use clad or unclad battery stands; this is more common for large UPS battery sets due to their size and weight. They can also provide better access for testing.

Few line-interactive UPS below 2kVA support extension packs. In terms of extended runtimes, few line-interactive UPS below 2kVA have the capability for external battery extension packs. Some 2kVA UPS, have a battery extension pack connector and can be used to support loads (without oversizing) for up to several hours. Others don't, and must be oversized for the load at the UPS system design stage if longer runtime is required.

Major UPS manufacturers bring you a wealth of experience. Whether the answer to runtime requirements lies in UPS oversizing or the specification of UPS battery extension packs, one thing is certain. It is always best to deal with manufacturers of UPS equipment, such as Riello UPS, who have a wealth of experience as well as the range of battery extension products that can help users get the maximum battery runtime out of their UPS protection.

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