Selecting and Sizing Batteries for UPS Backup
Battery selection for UPS backup is normally carried out in the following order.
First, establish the DC bus voltage. The battery string voltage is set by the UPS model: with 2V cells the number in series equals the bus voltage divided by 2, and with 12V blocks it is divided by 12. The number of cells in series must match the bus voltage and cannot be varied freely.
Second, establish the backup time and discharge power. The backup time comes from the user requirement; the discharge power equals the UPS output power divided by inverter efficiency and then by power factor. Note that actual load power must be used, not the UPS nominal rating.
Third, look up the power table for the required backup time rather than dividing energy by voltage. This is where mistakes are most common: short backup times such as 15 minutes are high-rate duty, and the usable capacity is well below the 10-hour nominal figure. Sizing linearly from the nominal Ah rating results in an undersized battery and a backup time shorter than required.
Fourth, check ambient temperature and float voltage. Available battery capacity falls as temperature drops; ratings are usually given at 25℃, so capacity must be increased for low-temperature sites. Float voltage must be temperature-compensated, reduced at higher temperatures to avoid overcharge and accelerated water loss.
Finally, confirm installation conditions: floor or rack loading, ventilation, maintenance space and terminal connection method. Large 2V cells are heavy, so rack load capacity and handling routes need to be considered at the design stage.
