Why does my UPS only give a few minutes of runtime?

Three causes, in order of likelihood. The battery has lost capacity, end of life is conventionally 80% of rated, and a bank at 60% delivers a fraction of its original runtime. The load has grown since the bank was specified, which is extremely common in a room that has had equipment added over a decade. A single weak battery in the string is dropping the whole string to its end voltage early, in a series string, capacity is set by the weakest member. The diagnostic order is: measure the actual load, then test the string, then test individual units.