Uninterruptible power supply (UPS) systems are central to the design and implementation of a standby power supply strategy. At the heart of the UPS system is the rechargeable battery, an element as important as the design of the system itself and a possible ‘weak link’ unless maintained and serviced regularly.
This is the view of Jack Ward, managing director of energy consulting company Powermode. He says correct battery care will help ensure the reliability of the UPS system in the event of a power outage.
“In addition to regular maintenance, batteries must be sized correctly and properly installed to ensure they meet their maximum expected life spans, which, in the case of modern units, is up to 10 years or around 260 cycles at 100 per cent discharge,” he says.
However, certain conditions negatively affect performance and durability, warns Ward.
“One of the most import factors is the ambient temperature. For every five degrees above 25 degrees Celsius battery life is halved. So when batteries are exposed to large swings in temperature life expectancy can vary considerably.
This is why modern batteries, although marketed as ‘maintenance free’, must be regularly checked and analysed to prevent premature failures.”
He says regular checks will identify and rectify issues such as damaging ripple currents which can also cause premature aging.
“A ripple current is the root mean square value of alternating current flowing through the UPS’ capacitor. This current causes an internal temperature rise due to power losses within the capacitor which raises the overall battery temperature with consequent loss of performance and longevity,” he explains.
“On certain UPS systems this is caused by a temporary high loading of the inverter or it can be the result of a gradual failure of the direct current smoothing capacitors in the rectifier circuit.”
Ward says in typical South African hot climate conditions, batteries with a stated 10-year lifespan batteries generally require replacement every seven or eight years. “While many industry specialists believe it is good practice to replace a complete bank of batteries – in a multi-battery installation – and not mix new with old, there is a substantial cost involved.
“By performing battery analysis and providing base-lining over time, this generally applied ‘rule of thumb’ can be ignored. Substantial savings can result by replacing individual batteries when necessary.”
He says life expectancy and system reliability often depend on the type of battery or batteries supplied with the UPS system. “For example, internal battery plate structures play an important role in energy density ratios which, in turn, have an impact on the levels of current delivery in emergency, high discharge situations.
“It is therefore important to specify high quality batteries, correctly certified for the application and deal with a supplier capable of providing full battery installation services and periodic risk assessment surveys meeting industry-approved engineering standards,” he adds.
