How To Size A UPS Battery
The battery within a UPS must be sized to provide a specified runtime, in minutes or hours, at a specific load. Load-specific factors are incorporated into a general battery sizing algorithm that will also take into account a number of other issues relating to ambient temperature and the inverter and system efficiencies of the UPS being considered.
Whilst battery manufacturers provide discharge curves and tables to assist selection, UPS manufacturers generally have bespoke battery sizing programs designed around their own UPS characteristics. Their battery packs also tend to be somewhat uniform, based around battery ranges and their source of battery cabinets or stands.
For any given load, the runtime period must be sufficient to keep the loads running, allow the start-up of standby generators (both automatic and manual if necessary) and sufficient time for the completion of automatic load shedding or shutdown routines. When sizing a battery set for a UPS the primary factors to consider are:
- Site power history, and in particular, the duration of previous mains power supply failures
- Critical load sizes
- If there is a maintained standby generator on site
- The length of time required for load shedding and/or an orderly shutdown
- The available environment, for example, space, temperature and humidity
- Cost versus space requirements for longer runtime systems