The right fit

To select the right size of the battery you require for your UPS, you will need the following information:

• Total connected load to the UPS z UPS DC bus voltage
• UPS inverter efficiency and load power factor
• Minimum DC bus voltage permitted for UPS operation z Back up time required
• Battery wattage table. Consider the following example to understand this better : The normal loads in an IT set up are PC nodes (with CRT/TFT monitors). Servers, routers, peripherals like printers and some power points. Consider a set up having 5 computers with 14 inch monitors and one laser printer.

This translates to an approximate load of 200 VA per computer and 150 VA for the printer, thereby totalling up to 1150 VA. Considering future expansion etc, a UPS of 1500 VA (1.5 KVA) capacity is selected. This Online UPS operates at 85% inverter efficiency and the load power factor is considered as 0.8. The UPS DC bus voltage is 24 Volts and the back up time required is 30 minutes. The lowest operational DC Bus voltage is 20.4 Volts. Armed with this information, the battery can be sized as below. As the UPS DC Bus voltage is 24V DC, 2 nos of 12V Battery modules, each capable of delivering a minimum of 706 Watts need to be selected. The lowest DC bus voltage corresponding to each battery will be 10.2V (10.2 V x 2 = 20.4V DC). The battery wattage look up table is now required to select the right AH capacity. The look up table provides information on the lowest DC bus voltage (also called as End Module Voltage (EMV), the various back up times, battery Ampere Hour (AH) capacity and the associated power wattage deliverability of the battery.