Yeah no, line voltage will kill you if you don’t know what you are doing, not to mention the fire hazard as was mentioned. The 12/24 volt used in computer systems much easier to mess around and not found out other than maybe a fried component. Unless you are an electrician/ electrical engineer with proper training don’t open/mess with Power Supply Unit (PSU) or UPS.

One mod anyone could do is swap their lead acid for a LiFePO4. You just need to make sure the same voltage,battery quantity (larger backups often have 2 batteries in a series) and the battery dimensions are the same. They should be drop in replacements and do last longer.

That being said, I myself, do have training and if you want to waste your time I probably would mod some UPS with a car battery for longer down time support. Watched a YouTube video of a person do it to find the pitfalls for me and the issue is heat as it’s not expected to run off battery + inverter for longer than the smaller battery normally allows it maybe 5 minutes compared to like 1 hour, so several fans and heat sinks on critical components would be needed adding minor complexity and planning.