I just ordered a couple 18650ies to refresh older cordless tools. That worked very well so far. I purchased a 2900mAh Samsung type with soldering flags. This way I could solder them into the fixture (battery pack) without heating the actual cells. I have seen youtube videos where nickel strips were spot welded to the cells but that takes special tools. I have also seen wires soldered directly to the cells but felt this was unsafe. Seemed to work though.

I carefully avoided short circuiting the cells. I tested the old ones with a load resistor and amperemeter and though they were too bad to operate the tool properly, they still put out enough current to heat the resistor quickly. These cells are no toys.