Best way to do that (IMO) is as follows:
- get the wire soaked with solder
- put a pea-size dab of solder on the nickel
- put the wire on the dab of solder and add heat from the top; keep heating it from the top until the wire goes all the way through the dab of solder; all the solder should be fluid at that point
That way you should get a solid joint. The wire will get really hot, probably too hot to hold in your hand. Best to hold it with pliers.
Since you’re going to do the soldering away from the battery pack, you can take your time, which is good.