The ability to provide backup power during a mains failure or voltage excursion beyond acceptable limits is fundamental to the role of any UPS system installation. Currently, lead-acid batteries are typically used as this power source within UPS systems ranging in size from data centre installations to desktop units for small office environments.
Modern valve-regulated lead-acid (VRLA) batteries are environmentally friendly, safe, self-contained and can be stored or used in any orientation. They can provide a reliable and cost-effective service life, however they are ultimately replaceable items whose quality and length of service is significantly affected by how well they are maintained as well as by their original design and installation.
Effective battery maintenance depends on regular, or ideally continuous, monitoring of battery condition, and, through battery monitoring, identifying preventative actions to take before potential problems occur. These include equalisation, overcharging, undercharging and sulphation.
Equalisation problems can arise because batteries for UPS and other applications comprise strings of individual battery cells connected in parallel to achieve the voltage and Ah rating requirements of individual applications; this creates the possibility of individual batteries charging unequally. If this leads to some batteries being consistently undercharged without being reported, sulphation – a build-up of lead sulphate crystals – can arise, causing a permanent inability to fully recharge. An open-circuit voltage reading lower than rated value is symptomatic of sulphation.
Conversely, if batteries are subjected to overcharging, possibly due to equalisation problems, this can cause a high overcharge current into the battery. Gas venting and positive plate material corrosion can result, reducing the battery’s operating life.
Temperature is also an important issue, with most manufacturers recommending a battery operating temperature of 20°C. Excessive temperatures will reduce battery life and may cause thermal runaway; this will permanently damage the batteries, which will have to be replaced. Low temperatures will have little effect on battery life but will reduce performance.
Another parameter important to batteries is their internal impedance. New batteries normally have a low impedance, measured in milliohms. This increases incrementally as the batteries age, due to normal internal corrosion. Other problems, such as overcharging, can also lead to increases in impedance. This does mean, however, that latent battery problems can be spotted early through monitoring their impedance.
In fact an advanced Web-based maintenance product, known as PowerNSURE and available from KUP. (see above image), can continuously monitor voltage, temperature and impedance levels across an entire UPS battery installation, checking each battery sequentially. Changes in any of these parameters are stored and displayed locally, or can be accessed via the Internet; the system can also generate warnings when attention is required.
These warnings allow users to identify and replace failing batteries before they compromise the UPS’s backup capability; overall battery availability is guaranteed. Additionally, PowerNSURE applies an equalisation process, ensuring all batteries receive charging voltages within their required operating range at all times. Gassing, dry-out and thermal runaway are prevented.
The modest investment required by a continuous on-line monitoring system such as PowerNSURE is rewarded in two key areas. Firstly, the equalisation process ensures that the maximum possible useful life is extracted from each battery, and secondly the ability to spot and act on battery problems before they become failures allows overall battery and UPS availability to be maintained at optimum levels.
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