The battery must be weak already and needs to be replaced. It might be a good idea to have the internal switching circuitry of the UPS checked as well.
The battery part is pretty easy (just buy a new battery).
But how does one check the "internal switching circuitry" of the UPS?
My sister brought their Giant UPS 500VA for me to check. I notice its battery is weak so I replaced the battery with new ones. But then the new battery also don't last long even at very low loads (testing my PC with my APC 500VA UPS, my PC lasts nearly 10 minutes, but with the Giant UPS 500VA that I replaced with new batteries, it lasts only 3-4 minutes..)
Perhaps your suggestion is right, this Giant UPS may have some problem -- how do I check it? (sayang, bumili na ako ng UPS battery for this Giant UPS...)