Accurate prediction of backup time in an UPS and its display on user computer monitor screen

In an UPS, the battery acts as a reservoir of energy. However the available energy is dependent upon several parameters like the charging time, charging rate, self discharge, aging of the cells and the permitted voltage up to which discharge is to be allowed. Thus the stored energy content must be known accurately to predict the backup operation time left in the event of abnormal mains supply, after which the UPS has to be shut down. This paper presents a method of online calculation of the battery backup time depending on the actual rate of discharge and its terminal voltage. This technique takes into account the charge condition of the battery, the self-discharge aging and change in load current to give a true estimate of backup time. This information is sent to the computer via a serial port and is displayed at one corner of the user monitor screen. Such a facility helps the user to save and close all the running programs before the shut down of the UPS.