UPS batteries are really dangerous because you cannot turn them off. They are also high enough voltage and current to harm you through arc flash and burns.

The other thing with small UPS batteries is that there are usually multiple batteries. There are fuses between the battery string and the circuit board but not between each battery in the string.

OP may have connected batteries wrong or he put a new battery in a string with a shorted/dead battery.

Whenever you are going to string multiple batteries together always disconnect batteries and check voltage on each separate battery to make sure that they are the same or very close in voltage. If different charge to the same level. Any difference in voltage between the cells will cause current flow that can cause a fire hazard.

Hm I have not heard about such an issue so far but I also don't have as many UPS as you.

I see you holding a red cable which could be +. When I switch UPS batteries, I do it the same way as it's recommended for car batteries to avoid sparks/arcs. Remove - (black) first, as it won't spark/arc. Then remove + (red) as it can't get a circuit closed any more, so also no spark/arc.

When plugging a car battery in, it's the other way around. + (red) goes in first and only then you connect - (black) to avoid spark/arc for both connectors again that way.

Replaced the batteries?

I'm in the process of topping up the water in mine intending to refresh them... but you've made me think... I need to check them for deadshorts / opencircuit

And I thought you'd burnt a horrible groove in your finger!

At first I thought you accidentally welded it to your hand lol

Who needs fuses anyway.