Cheap UPS Batteray
The batteries within an uninterruptible power supply (UPS) are the heart and lungs of its ability to ride through short duration interruptions, keep loads up and running whilst another source or back up power (such as diesel generator) starts up, or to power through automatic shutdown routines.
UPS batteries must be sized to provide a specified runtime, usually measured in minutes or hours, at a specific Watts, kW or MV load. These load-specific factors are incorporated into a battery sizing algorithm that also takes into account a number of criteria such as ambient temperature and the inverter and system efficiencies of the uninterruptible power supply to which they are applicable.
Whilst battery manufacturers provide discharge curves and tables to assist selection, expert UPS manufacturers utilise bespoke battery sizing software programs designed around their own products' characteristics. Battery packs also tend to be uniform, based around particular types and ranges of battery and manufacturers' own battery cabinets or stands.
The uninterruptible power supply and battery set combination in a solution will be the nearest matching the calculated load. For example, for a total load (including UPS losses) of 20kW, the nearest matching UPS is a 30kVA/24kW system. Provision of a runtime of 30 minutes in this scenario would require the nearest battery option to be, say, a 38Ah battery set, which would provide 25 minutes at 24kW or 33 minutes at 20kW.
Several factors must be considered when sizing a battery set for an uninterruptible power supply. In particular, site power history is important in determining the depth, frequency and likelihood of mains power failures. The size of critical loads needs to be determined. If there is a maintained standby generator on site, this needs to be taken into consideration. How much runtime is needed to cover load shedding and/or an orderly shutdown? Thought must be be given to the immediate environment; how much available floor and rack space is there, for example, and is it a hot, humid environment or a cold, dry one? Cost implications also have to be factored in, particularly in a tight economic climate where cost-reduction is a must. What is the comparative cost, for example, of installing a number of battery strings to achieve particular runtime duration verses an individual standby generator or fuel cell?
Comparisons between runtime performances of different uninterruptible power supply manufacturers can be misleading. Firstly, comparing battery runtime performance between them is difficult without a common basis. The best solution is to ensure all runtimes are quoted using the same formula - load/Watt ratings and load power factors. An example could be: a 10kVA UPS supporting an 8kW load at 0.8pF, the runtime from a 12Ah internal battery set would be 14 minutes. However, at 75 percent load (6kW) and 0.6pF, the same UPS would provide almost 20 minutes of runtime.
It is possible to estimate battery set sizing, which may be useful for budgetary purposes at the specification stage of a power protection project, but as it is so critical to the overall performance and resilience of the uninterruptible power supply system, battery set sizing at the design stage should be carried out by industry professionals to achieve accurate and reliable ratings.
Robin Koffler is the co-author of The Power Protection Blog and the General Manager of Riello UPS Ltd, the UK subsidiary of Riello UPS (RPS S.p.A). For more information visit the Riello UPS - uninterruptible power supplies website.
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