What do you use 18650 batteries (3.7 V) for?
These cylindrical cells are used to make the battery units for the majority of modern laptops (except the ultra-slims). Such a battery will usually contain 3, 6, or 9 such cells—some go as far as 12 or 15 cells. Inside one of these batteries, the cells will generally be pink in external color and interconnected by tabs that have been spot-welded together.
In some LED torches, there is a small cradle that holds three AAA cells. Once populated, this becomes the same size and voltage as the 18650 lithium cell. So you can purchase individual cells and chargers for these types of torches.
This same physical size has been used in computer batteries almost since the smaller laptops started to appear, although these were Nickel Cadmium (NiCd) or Nickel Metal-Hydride (NiMH). I suspect that this particular size is the original “A” that we never see, or maybe even the “B”.
Tesla vehicles and power walls use the Panasonic 18650 cells. They have the highest safety record, and it saves Tesla having to re-invent the wheel. The various number of cells required is simply shuffled into the right type of enclosure to fit the vehicle or wall.
Warning: Never attempt swapping NiCd or NiMH with Lithium cells. The voltages are obviously different (1.2 vs 3.7) and getting them mixed up could be a fire hazard.