Battery sizing criteria

Back-up power
White Paper Wed 12 Sep 2018

One of the key features of a UPS system is its energy storage system, as it will provide the load with immediate power if the main power supply becomes unavailable. The type and size of the energy storage system are chosen based on various factors such as the load characteristics, the quality of the power supply network, the electrical infrastructure in which the UPS is installed, and the environmental characteristics of the technical room.

Batteries are electrochemical devices that store energy chemically and convert it into electricity. In UPS systems, several batteries are connected in series (a battery string) until the required DC voltage is obtained. Several battery strings can then be connected in parallel to increase the battery system’s runtime in the event of a mains outage, and / or for redundancy.

Batteries can either be installed within the UPS (usually only for small UPS systems), or they can be assembled in external cabinets or on shelving. The different types of batteries used as part of a UPS system include: 

  • normal / long life VRLA batteries in flame-retardant containers,
  • long life open-vented lead batteries in flame-retardand containers,
  • long life nickel-cadmium (NiCd) batteries for special applications,
  • Lithium-Ion (Li-Ion) batteries with integrated battery monitoring and equalisation systems.