Hi Guys,
I am also a firm believer of UPS's they have saved my arse on many occasions, especially in areas of fluctuation and power outages. It is very important for me that the systems that are connected to the UPS are also shut down properly by the UPS's management software to avoid corruption.
When i have replaced batteries in my UPS's here i have always replaced the whole tray or all the batteries in the unit, they are all getting used constantly so if one has failed the others wont be far behind, in personal experience.
Also note that when you get a battery from the official source replacement for your UPS they will always (as confined in laws) ship you a returns procedure. Pop the old battery in the box the new one came in and send it back to them for the correct disposal methods.
It is also a good idea to test your unit using a frequency of around once a month, it's really a simple process of just pressing the test button on your UPS.
James.