Although Lithium-Ion batteries are steadily increasing in popularity, Lead-Acid – particularly the modern Valve Regulated Lead Acid (VRLA) type – currently remain predominant in data centre UPSs. They are more environmentally friendly, are self-contained and safe, and can be stored or used in any orientation.
Designing a battery installation starts with sizing the battery – and battery size depends on the load size and autonomy required. Some critical applications require the battery to keep the load running even for extended power breaks, while others just want enough autonomy to allow the load to shut down gracefully, or for a generator to start up and synchronise its output. The load/autonomy requirement translates into battery capacity in Ah, the number of battery blocks required and their dimensions.
If possible, the batteries are installed into the UPS cabinet; clearly the neatest and most space-efficient solution. For larger requirements, UPS manufacturers supply additional cabinets which match the UPS. These may need to be made to order. Yet larger installations may require separate battery racks of cladded or open type. Open racks with batteries must usually be installed in a separate battery room with controlled access.
A basic battery arrangement comprises a string of blocks connected serially, or ‘end-to-end’. The overall battery voltage is the sum of the individual block voltages, and must match the UPS’s float voltage setting. To increase the battery’s Ah rating, or to build in resilience against a string failure, two or more serial strings can be connected in parallel. The total battery capacity equals the sum of all the strings’. Increasing the number of parallel strings beyond six increases the risk of equalisation problems, where different batteries draw unequal charge.
The battery installation should include capabilities for monitoring, reporting and managing battery status at a remote location via a network or the internet. The monitoring system should sequentially measure each block’s internal resistance, temperature and voltage, then correct the charging voltage as required to ensure a balanced charging condition across the entire battery installation. This constant monitoring and management ensures that all blocks in the battery are kept at their optimal voltage and guarantees battery availability at all times.
Other benefits accrue from using this on-line battery management. Equalisation prevents unnoticed overcharging of individual batteries with gassing, dry-out or thermal runaway. It equally prevents unnoticed undercharging leading to sulphation and capacity loss. These problems can be spotted early through associated increases in impedance and temperature.
Equalising, by keeping all batteries within their ideal voltage window, can extend battery service life by up to 30%. Maintenance is also made easier as remote monitoring supplies a constant stream of real time data and battery history at all times, without the need to visit the UPS or disconnect any batteries.
Battery life can also be optimised by carefully keeping the battery installation’s ambient conditions and operating temperature close to the manufacturer’s recommendation – usually around 20°C. Excessive temperatures will reduce battery operating life, and in extreme cases, cause irrecoverable damage due to thermal runaway. Low temperatures will have little effect on battery life, but will reduce performance.
It is important that system planners understand the realities of battery design life. Manufacturers quote figures for this, but unfortunately these are often based on unrealistic assumptions about how the battery will be used and its environment. In practice, the ambient temperature is unlikely to match the manufacturer’s specifications, and the frequency and depth of discharge will be determined by the site mains voltage’s quality. Accordingly, the battery’s actual working life is invariably less than the design life quoted by the manufacturer.