Basic construction procedure:
The battery gets hooked to the USB charging board. The board deals with charging and discharging the battery and steps up the voltage from 3.7 to 5 volts. It would be nice if the charge board didn’t step up the voltage because the welding helmet wants 3.5V but it does, so the voltage needs to be brought back down. To do this we use the step down regulator, it brings the voltage from 5V down to 3.3V a little lower than what is needed but it works. 3.3V is used instead of 3.5V because there are tons of things in the electronics world that need 3.3V so regulators are cheap and easy to find.