Even my UPS's battery had become weak. So, I replaced 'em with new EXIDE battery. My PC was running fine.
Then I sent my FSP SAGA-II 500 for RMA. Mean while I was using an Intex 450W PSU and even it ran fine. After I got my PSU from RMA, replaced with FSP APN 700, my UPS doesnt gaive any backup. It hardly gives backup for 7-8sec.
But my UPS ran fine on another system with Zebronics 450W PSU. So, its definitely not UPS problem.
To check the battery, take it to the EXIDE store where you boought the battery from.
To check the UPS, take it to the service center or any other local Stabilizer maker.