Important note for fellow tinkerers, nearly lost my mind at 2:22: Do not ever solder directly on, or even adjacent to, lithium batteries. Only use spot welders and crimp connections. Once you cut a wire (NEVER cut two wires at once) that leads to a battery, place a Wago clamp on it to make sure it can not short accidentally.
What the guy did at 7:57 is even more inexcusable, one blob of solder misplaced and he'll short the pack. And the tiny main wire, that's NOT going to survive the 30 amps he's gonna pull at 100w out. JESUS. Take the time, install proper connectors, and solder them BEFORE the pack and the PCB are electrically hot.
And also, he left about zero airway for heat to escape. At full load, the inner cells will heat up!
Also, never just take random cells and place them together in a parallel connection. Always measure each individual cell's capacity, only use closely matching cells, always stay 5-10% below the cutoff voltage, and never work with charged cells, only ones with max. 10% charge - only charge cells once they are installed.
And ffs keep a bucket of fine sand nearby when you work with batteries. If a battery makes signs that it is about to go kaboom (it emits smoke, swells up, or catches fire), dump the bucket of sand onto the battery/device, leave the room, close the door and call 911(US)/112(EU).
Suggestion: (to video maker)
1. Leave the design open source, create a second design (more efficient/whatever you think defines the declaration of value).
2. Package second design parts together for sale and let the public purchase so they can run around and find disposable vapes and make their own.
3. Profit.
No need for steps 4 and 5. (Rinse & Repeat new models).
People still design new AA battery products because there's almost no other standards. It's crazy.
We need something like USB but for batteries and expansion modules.