Not if you work quickly and use 63/37 0.6mm rosin core tin-lead solder with a 30W iron. I never need to apply the iron for more than half a second. I've done

Not if you work quickly and use 63/37 0.6mm rosin core tin-lead solder with a 30W iron. I never need to apply the iron for more than half a second. I've done this more than a few times, and I realize that it goes against standard safety recommendations, but I'm going to keep doing it. And if you're recycling cells from other packs, they are likely to have remnants of old tabs still connected, so you can solder to the tabs instead of directly on to the cells, which gives a little more thermal wiggle room.