UPS Battery Runtime Choices

When you are looking to install an uninterruptible power supply one of the first to assess is the runtime time you want to build into your UPS installation. Runtime is the amount of time your UPS system will be expected to run for when the mains power fails. The amount of time relies on the choice of a suitably sized energy storage component.

The most common energy storage system is a Valve Regulated Lead Acid (VRLA) battery and either a 5-year or then year design life type. The most common VRLA battery for single phase UPS systems is a 5-year design life, with the then year design life battery more commonly found in three phase UPS systems. This is due to cost and size. To increase the design life of a UPS battery requires more lead plate and acid electrolyte.

Most UPS manufacturers will provide a range of battery runtime options for their UPS. The battery will be housed inside the UPS or provided in a matching battery cabinet. Batteries for larger UPS systems may be housed on battery stands, that may or may not be cladded. These larger battery sets are often installed with a separate plant or battery room and may be segregated off in a metal cage for safety. Dependent upon the battery recharge required (24 hours to 80% capacity is a good rule of thumb) and the size of the UPS battery charger, the external battery set may also have its own battery charger installed.

Battery runtimes vary and the most common period is around 5-10 minutes. Most UPS installations protect file servers, some of which can be shutdown within a short period of time (2-3 minutes). This can be automated by installing UPS monitoring software onto the server. When there is a mains failure, the UPS monitoring broadcasts an alarm message to server admins and users and waits for a preset runtime threshold before automatically beginning a shutdown script. If a server installation requires a longer period to shutdown a battery set with a longer runtime may be installed. Battery runtime periods can run to several hours. An alternative is to install a local standby power generator with a built-in fuel tank (diesel or LPG) which can run for 8-24 hours or longer.

It is sometimes difficult to achieve shorter battery runtimes without turning to alternative energy sources. One that is growing in popularity is Super Caps (super capacitors). SuperCaps can only store energy for several milliseconds (around 20-50mS typically) and this can be sufficient to cover very small breaks in the electrical supply. DC flywheels can also provide a similar energy source and similar runtime capabilities to super capacitors. The advantages of both super capacitors and DC flywheels is their recharge rate which is very short; several seconds.

Lithium-ion batteries perhaps provide the most flexibility for the future as they open up other options for UPS installations. VRLA batteries are designed for standby mode applications of which uninterruptible power supplies are a typical example. Li-on batteries can recharge far faster and also cope with more rapid and frequent charge/discharge cycles associated with solar energy storage applications and virtual power plants (VPPs).

Lithium-ion batteries are an alternative solution to VRLA batteries and especially for UPS installations. Cost is the main prohibitive factor however, with Li-ion batteries typically costing 30-40% more per UPS installation in terms of CAPEX and initial investment. However, over the lifetime of the UPS system, OPEX may be lower as lithium-ion batteries can have a ten or more years design life. If you install a UPS system with 5-year design life batteries, expect these to be replaced around years 3-4 and years 7-8 for ten-year design life batteries.

There are several choices when it comes to emergency battery energy sources to power your UPS system when the mains power fails. For complete resilience it is as important to consider both the technology required and how to initiate an orderly shutdown of the powered loads if the mains power failure outlasts the runtime (minutes/hours) available from your UPS batteries or other installed energy source.