Accurate Prediction of Backup Time in an Ups and Its Display on User Computer Monitor Screen

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IEEE 1999 International Conference on Power Electronicsand Drive Systems, PEDS’99, July 1999, Hong Kong.

Accurate Prediction of Backup Time in an U P S and its Display on User Computer Monitor Screen Pratik Mukberjee, Biswajit Majumdar, FazaI k Talukdar, StUdentMember, IEEE Ashim GanguIy* and Sujit K. Biswas, Sr. Member, E E E Department of Electrid Engineerins JadavpurUmve.rsily, Calcutta 700032,N I A . *TataConsultancyServices calartta700 091, INDJA

E-maik p a i - u h r e @ t a l a rtkmkejebmi.m

Abstract -In an UPS the battery acts as a resetvoir of energy. However the available energy is dependent upon several parametels like the c h a w time, charging rate, self discharge, aging of the cells and the permitted voltage upto which discharge is to be allowed. Thus the stored eneqy content must be kmwn 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 on-line calculation o the f 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 tme estimate of backup t m ie Thii information is sent to the computer via a serial port and is displayed at one corner of the e r monitor screen,Such a hcility helps the user to save and close all the running programs before the shut down of the UPS

L INTRODUCTION
The computer interfaced Ups has a greater impact on the user as the system can make the user aware about the status of the battery backup directly via the computer monitor. As a result, during a power outage, information on the backup time left can help the user to save and close all the running software on the computer before the shutdown of the UPS due to low battery condition. In UPS, the battery acts as an energy reservoir. When the power line condition is good, it takes energy from the charger and when the power line condition is abnormal, it release the enera through the inverter to the critical load Hence the charge as well as discharge characteristic of the stationary battery is very important and the backup time of the UPS depends upon the charge condition. Another serious problem of a storage battery is the aging: as the time goes on, the battery cannot supply the full backup time even it is attempted to be cbarged m . So it is a y challenging task to any UPS manufacturer to predict the real backup time left a any instant, including the previous charge t condition, selfdischarge aad the aging of the battery. The warning indicator or the alarm attached to the U P S cabinet is not always usefid as the UPS may be far away from the computer and its user. Also, t i type of system disturbs hs the attention of the user, especially when the backup time is low. Hence an onscreen display is higbly appreciated

In a popular approac4 t o e UPS system which bave the hs facility to orderly shut down the c m u e ,a few dedicated optr softwares are available for t i purpose, one of them is hs POWERMON II [l]. When power fails, a massage is broadcast to the user that the computer is nmning on battery power. A user programmable timer is also activated If power is restored, the shut down timer is reset and a massage is broadcast to the user indicatingthat the computer is once again running on utility power. If power is not restored, the preiixed time of the timer will expire, the monitoring system will broadcast the shut down massage before initiating an orshutdown. Again if the system receives a low battery signal, the pre-programmed timer is ovemdden and the s s e begins ytm an orderly shut down to prevent any possible data corruption. This is a good system for data protection, but the user cannot avail the full utilization of the battery backup time, as before shutdown of the U P S due to low battery, the user may not have an exact idea about how long...
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