I work on these all the time. Problem is normally cheap charger. Try and find a ups that puts load on the batteries everyonce and a while. Good luck. This is hard and never done. We got sick of UPS sitting there for sometimes years and then when needed - batteries collapse and the whole thing comes crashing down.

I have worked on UPS that are in charge of maintaining power to DP systems. When they fail you get things like oil rigs pulling off station costing millions. The company I work for just finished putting batter powered thrusters on an oil rig. First one ever.

Don't have a job right now. Rather sad.