Lots of devices are using battery packs that are spot welded together. Spot welding ensures great connectivity and vibration safe operation. In order to spot weld you need special equipment that can give high current output for very short time duration. This way you can get good weld joints without damaging the batteries with heat. Building a spot welder for small tasks is fairly easy.
KaeptnBalu has shared his instructable where he assembles his spot welder which is controlled by Arduino Nano. Spot welder require 7-12V power source capable to output high currents. So any lead acid battery works fine. Arduino is used to control welding pulse duration which can be adjusted from 1 to 20ms. With 0.15mm nickel strips he found that 9ms gives best result. First of all he prototyped the spot welder and then built his custom PCB where he put 8 MOSFETs in parallel that switch current on and off. He also built plywood foot pedal switch to keep his hands free while soldering.
We recommend EasyEDA for circuit design and PCB prototype