Does lead battery + trickle charger = UPS?
Not sure how well the trickle charger would deal with a varying load. Also, not sure that the trickle charger is protected from being back driven by the battery, or possibly draining it. These are things which you should investigate. Some well placed diodes here could really help.

For any sort of important application, I would recommend purchasing a dedicated UPS. There is quite a bit that goes into their design.