Sounds like a weak/bad battery.
You did not post that you performed the proper test on the battery.

Find out the MAXIMUM current output available under UPS mode of operation.
Place a suitable load on the output, calculate that your computer is not
overloading the unit. Most UPS units will NOT handle both the computer and
the monitor for very long. Then pull the plug while monitoring BOTH the AC
output voltage AND the battery voltage at the SAME time.

FYI it is normal for the meter reading voltage to drop to below 90 volts
when the UPS detects a no load condition. 40 watt light bulb is simply not
large enough to test the output, since most computers alone have 300 watt
power supplies.

UPS sealed lead acid batteries last ALMOST 2 years before they should be
replaced. Occasionally one will last a bit longer, but for full back up
performance, replace them every two years.

David