I wouldn't try soldering it back together. The remnants of that ring terminal look awful and you'll need to trim the cable that it came from down a bit since there's likely missing wires and probably corrosion. They do sell butt connectors (uninsulated) for heavy gauge wiring so what I would suggest is taking a section of wire to an auto parts store and size it up with an appropriate battery cable with ring terminal. Then you can cut the new battery cable to size, strip back some insulation and crimp it to the wire on the engine room harness and bend the ring terminal 90 degrees like the original. You will need special crimpers for battery lugs and heavy gauge wiring, which can be bought on Amazon (and maybe auto parts store?). You should also insulate the repair with shrink tubing or something.I meant it’s the harness connector that came off, nothing was wrong with the battery. I was thinking of soldering it back but not sure if that would be stable. Or can I buy a new connector somewhere and attach it to the wire again, cause they look like a whole, instead of separate parts?