Here is Gil's video:
The main problem is with the manner of heat shrinking used to encapsulate the 3-cell pack. Gil inserts his battery pack into the shrink wrap lengthwise - the finished result of this leaves the terminals of the cells exposed.
|Problem: Cells being inserted lengthwise
|Result: this is the finished pack. Would you put this in your KX2?
Used this way, the shrink wrap only serves as a physical packaging to bind the cells together - it offers little in the way of electrical insulation.
In the video, he mentions that he wishes he had inserted a piece of cardboard insulating material onto the ends of the cells prior to shrink-wrapping them. That would have resulted in insufficient insulative qualities as well and is the wrong way to go about it. The cardboard material sold for 18650 pack fabrication is to protect the shrink-wrap, not insulate the cells.
The proper way to shrink-wrap a trio of 18650 cells is to use 85mm shrink wrap (Gil is using 75mm wrap) and insert the cells into the sleeve in such a way that results in the ends of all cells being completely encapsulated in the shrink wrap - not the sides of the cells.
|The exposed part of the battery should be its (non-conductive) sides as shown here - not the terminals.
The cardboard sleeving Gil mentions would be in addition to this - not instead of it. The cardboard's purpose is to prevent the (possibly) sharp solder connections from eventually poking a hole in the shrink wrap - not to provide insulative qualities.
At the end of the video, Gil mentions that he is not using protective circuitry for his pack. He is correct in mentioning that cells that contain such circuitry are slightly longer and may not fit into the KX2...
For my own homemade KX2 battery packs, I've used cells with built-in protection (they do still fit in a KX2) and cells without protection but with external protection added!
Please don't consider, even for a minute, using an unprotected battery pack in your KX2. Li-ion cells can produce incredible amounts of current. My preference is for cells that have the protection built into them already.
My own postings on homemade KX2 battery packs, along with sources, tests, etc can be found here.