I see it looks like they are soldered to the batteries. Great work…good project

Looks good.

For anyone else out there thinking about doing this…the most dangerous part of this job was soldering those wires onto the batteries. That is about the only dumb thing I see here.

The required heat to get the solder to flow means the batteries get hot. Not typically a good idea to be standing over something you are heating past it’s max.

This requires something called a spot welder. Op chose risk over $. It’ll be better just just buy a ups if you don’t have a spot welder.