#UK1

Click the knob, press the tip, and you're welding. For house calls fixing shavers and electric toothbrushes I used to haul a big bag; now it's just a UK1 and a roll of nickel strip and I'm out the door. Still got a little thrill from the unboxing — the box is really nicely made. 0.15 nickel-plated is easy; 0.2 needs gear 70 or higher. The downside, to be fair: the battery is built in, so after a lot of welds you have to stop and let it recharge.

Capacity grading, matching, welding, fitting the BMS, heat-shrink wrapping — five steps. For the welding step I used the UK1, 0.15 nickel-plated at gear 60, just right for full penetration. The one thing I most want to say: matching matters ten times more than welding. If cells from the same batch differ by more than 5mΩ in internal resistance, no amount of good welding afterwards will save it. The most-asked question in the comments, "how much per pack?", is answered too — in step 4 of the project.