First off, I don't know if this is the correct forum because I didn't see a category for UPS. Here's my question. I have seen some YouTube videos about wiring a larger battery to UPS to get a longer run time. First off, I don't know how it would work with a larger battery because the charger in a UPS is only sized for the battery its meant for and secondly, why couldn't I just use an inverter instead of a UPS?
I would think that any alterations done to the UPS would violate the warranty.
An inverter runs on AC current to produce DC current so if the power goes out there is no AC current and that's what the UPS is for , to provide emergency power to the computer to give you time to save and close your work.
You are correct about charging circuit issue. The UPS is designed to only charge the size battery it was built with. It not that it would blow out the charging circuit since they are designed to only produce a fixed amount of current. The problem would be the charging circuit may be design to run for a couple of hours and now you are going to run it for a couple of days. It will cause it to eventually fail.
Can you make your own UPS with a inverter...maybe. You could say take a battery charger and a inverter an hook both to a battery. When you have AC power the batter charger would produce DC power which would then be converted back to AC by the inverter. When you lose power the battery charger would be gone but it would now draw power from the batter. This is how a true online UPS works.
The key problem is finding battery chargers ....ie rectifiers and inverters that are designed to run 24x7. Tend to be kinda expensive but you can get stuff like this a lot more cheaply that you used to because of the increase in solar energy that uses similar concepts.
I use a large pair of UPS batteries on several of my APC UPSs that would normally each take only one, I just wired them in series (they are also APC batteries, just for larger models). I've done it for years and they don't have any issues, once charged the Power Chute software shows the increased available time to run on battery.