The pc isn't taking any load from the battery at all when the ups is plugged into the mains and the there has not been a power cut, to prove this point, if you look at the load indicator on the front of the UPS it should show nothing, then if you press the test button on your UPS this trips the relay inside so it runs off the battery and the load indicator will indicate what load the UPS currently has on it (this feature is on most UPS's), also the battery indicator will show remaining charge.
Inside the UPS is a circuit that detects if the UPS is getting power, when there is a power cut or a surge this circuit detects this and flips a relay inside the UPS, this relay switches the power being supplied to the pc from mains to battery, the battery power then goes through an inbuilt inverter which upps the power to 240 / 115 Volts (depending on what country you are in) which is then supplied to your pc.
Like has previously been mentioned, this is only an issue if you get a power cut in the first 24 hours as you will just get a lower than usual run time from the battery as it has not yet had the time to charge fully.
4The UPS should charge even when the computer is on. Have you consulted the manual if it says anything particular? Is there a light on the UPS that shows whether it's charging? – slhck – 2012-01-27T10:26:29.113
I see. The manual doesn't say anything specific to my question. There is only one light which indicates the UPS is on. It doesn't say if it's charging or not, it's just one steady light. This is APC brand btw. Anyway I think your answer is right. – IMB – 2012-01-27T10:35:07.437
1@IMB The UPS "working properly" is it powering your computer from a battery. You can use your computer during the first 24 hours, but if the power goes out, the battery won't be fully charged (although it should be within 24 hours). The wall is capable of delivering much more than enough power for both charging the battery and running your computer. – Breakthrough – 2012-01-27T12:33:42.707