karlok:
retrolefty:
sooner or later the battery
voltage will not be able to reliable act as a
standby sourcesorry ,but I don't get what you mean
OK, a vandal steals the battery when you aren't looking. 
Or one of the cells starts to leak and corrodes the contact away. Or batteries have just self discharged faster then they were suppose to. Or a rash of frequent but short failures of the primary power source has caused the batteries to be discharged faster then you planned on.
The point is that UPS designs for critical safety related equipment always haave battery monitoring and testing as part of the design and operating procedures. We used dozens and dozens of large UPS systems at the refinery I worked at before retirement, and the battery subsystems were always the most unreliable part of the UPS and required lots of testing and preventive maintenance procedures to make sure they would work when called on to carry the critical loads.
None of this my apply to your use and project, I just desired to add as a part of a UPS that often is taken for granted or not planned for.