The maker space I am a member of, got a lot of 18650 cells donated. We have a full soldering station (multiple ones actually) and a guy managed to build an electric bike from these cells - using a spot welder to weld on the thin metal strips for the battery terminals, then soldering wires to them.
So I brute-forced the case on the completely dead battery, and spent at least 30 minutes documenting the wiring. Inside the battery are 2 small circuit boards, one of which is attached to the connector, and wires that go to certain locations. There are 8 cells total, and they must be connected in a certain way.
After much spot-welding, soldering, and testing, and lots of electrical tape being used to keep from making unwanted connections, I re-assembled everything and used some duct tape to keep it together. If I was doing it again I would figure out a better way to seal the battery back together, but the duct tape worked.
Now I have a 3-hour battery or so, on the TiBook! Haven't really tested longevity yet but it does last...