Currently, lead-acid batteries remain the most popular standby UPS power source used in data centres, offices and other commercial, retail and industrial environments. Although they successfully fulfil all the requirements for backup UPS power provision, they have a finite operational life that will be reduced without a carefully planned and implemented UPS monitoring and maintenance strategy.
Such strategies involve several activities, an important one of which is load bank testing. This provides information on a battery’s real rather than theoretical capacity, and should be used together with impedance testing to determine overall battery condition. The procedure depends on using a load bank, which is an accurately-specified electrical load available for hire or rent in a range of sizes and configurations. It allows a variety of tests to be conducted, from full load testing to checking the UPS’s response to a step load change or a reactive load. Most load banks, however, are resistive and comprise heating elements and fans for cooling. They should be sited where their generated heat can be safely dissipated.
Although valuable as being the only practical way of truly establishing a battery’s capacity and therefore autonomy, the approach has some disadvantages and should be used sparingly. Therefore, it’s important to understand both the full value and the issues that should be considered before setting up a test programme.
Load bank testing proves the capacity of the UPS batteries at the time of the test, as well as the integrity of all the electrical interconnections. Any problems or shortfall in capacity can be spotted and rectified immediately, rather than waiting for them to reveal themselves under real critical load conditions.
Testing is particularly useful if conducted one week after commissioning a new UPS battery installation, as this ensures that the voltages across the battery blocks have had the opportunity to equalise and the batteries are fully charged. Another useful time for testing is at two-thirds of the UPS battery’s expected useful life, to confirm predicted capacity. If this shows the battery to be healthy, there is no reason to suspect that it will not perform correctly in an emergency, so no further load tests will be necessary.
As the battery under test comprises a string of battery cells, load testing can be used to identify if any individual cell is undercharged, allowing it to be replaced before it fails entirely under real load conditions.
However setting up for load bank testing means disconnecting the real load from the UPS, and making arrangements for its continued supply and protection. It is also necessary to check that the site’s mains supply and power network has sufficient capacity to support the real load and the load bank simultaneously.
Also, completely discharging a VRLA lead-acid battery will mean that for the succeeding short-term it will be incapable of supporting a load fully until it is recharged. Sometimes, batteries can recharge unevenly, with some cells undercharging while others overcharge, leading to possible problems. There are also the practical considerations that load banks can be large and expensive to hire or purchase, and transportation and positioning on site create non-trivial challenges.
In summary it can be said that load bank testing is an important part of a UPS battery maintenance strategy as it provides the only practical way of assessing a UPS battery’s performance under real load – yet it should be used sparingly because of the cost and disruption it causes, as well as using up some of the batteries’ life cycle.
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