Battery Options for Uninterruptible Power Supplies
- Tuesday, 01 May 2012
Valve Regulated Lead-Acid
The largest number of UPS types sold are the small, portable varieties. Typically, their power ratings range from 100VA up to 10kVA. As such, the largest bulk of the batteries used in the UPS industry are the VRLA types. The VRLA battery is ideal for portable applications as they incorporate a pressure relief vent that seals the battery’s cells and prevents the escape of the electrolyte. They incorporate re-sealable vents that are normally closed to prevent the entrance of oxygen, and just as important, retain hydrogen gas to keep their recombination process in balance.
Within the VRLA battery family, the electrolyte is immobilized using two differing methods. The first is through the use of a gelled electrolyte and is often referred to as a Gel Cell. The gel is impregnated into a sheet of micro - porous material that acts as the separator between the positive and negative electrodes. Like with flooded batteries, the gel electrolyte contains water and sulfuric acid. The second design is referred to as Absorptive Glass Mat (AGM). The design is almost identical to the gel type, except the separator material is AGM made from glass microfibers.
With a conventional flooded battery, hydrogen and oxygen are lost to the outside of the battery. Hydrogen and oxygen balance is not a problem, as the battery can have lost water added. However, in a VRLA battery, the closely spaced electrode plates are separated by a porous glass mat material. The cell is filled with only enough electrolytes to cover the surface of the electrode plates and the individual glass strands of the separator material. This creates a starved-electrolyte condition, and allows for the homogeneous gas transfer between the plates, which facilitates the recombination of hydrogen and oxygen gasses back into water when the battery is recharged. Under normal recharge conditions, the cell vent remains closed, and maintains cell pressure, assisting in the process.
VRLA battery electrode topology is typically in two forms: cylindrically wound or flat-plate prismatic. Cylindrical VRLA batteries can withstand higher internal pressures. As such, their vents have been designed to open at 25 to 40 psi. However, flat-plate prismatic type batteries can have vent relief pressures as low as 1 to 2 psi.
When stored, both flooded and VRLA batteries will self-discharge over time and, if not recharged, will become excessively discharged. Further, the rate of self-discharge is accelerated as the storage temperature is elevated above the ideal 25 ºC temperature. Battery manufacturers recommend the batteries be fully charged prior to storage, and recharged at specified intervals dependent on the ambient temperature in which the batteries are stored. As an example, a manufacturer may recommend a recharge be performed every six months if stored at 25 ºC, while storage in a 60 ºC environment may require a recharge every two weeks.
Extreme Environment Operation
Operating batteries in high-temperature environments will greatly reduce their service life. At the higher temperatures, the chemistry inside the battery can become very active and increase the rate of grid corrosion and internal deterioration. The typical service life of a prismatic VRLA battery is three to five years in a 25 ºC operating environment. Most VRLA battery manufacturers state the battery life will be less than a year at 50 ºC. Cold temperatures below 0 ºC typically affect a battery’s capacity. As the operating temperature is reduced, the internal resistance of the battery increases. The internal resistance of a typical lead-acid battery may increase by 50% between 30 and -18 ºC.
There are other factors that affect battery life. Lead-acid batteries have a cycle life rating (Figure 1). Increasing the number of discharge and recharge cycles over a year will shorten the battery’s life. Even when used within the ideal temperature range, most prismatic VRLA batteries suffer from a slow loss of hydrogen and oxygen gasses, which adversely affects the recombination process, causing the battery cells to dry out over time. However, metalencased cylindrical VRLA designs incorporate higher-pressure vents and ultrapure lead, thereby lengthening their life. Some manufacturers offer cylindrical VRLA batteries with an advertised 10- year service life at 25 ºC. The stated operational temperature range is a remarkable -60 to 80 ºC.
The China Storage Battery Company (CSB) has recently started offering their XTV line of VRLA prismatic batteries (Figure 2) with an advertised service life of 12 years (at 25 ºC). Even more remarkable, CSB states the batteries will have a four-year service life when used in a 50 ºC (122 ºF) environment. CSB is keeping their XTV battery design details under wraps, and no further details are available.
As UPS manufacturers are making great strides in designing systems that will provide many years of reliable operation, in many systems, the batteries are often the weakest link. In some cases, they can fail without warning if not maintained and tested periodically. A good understanding of the battery’s suitability for a specific application is essential.
This article was written by Michael A. Stout, Vice President of Engineering at Falcon Electric, Inc., Irwindale, CA. For more information, visit http://info.hotims.com/40433-121.