18650

The 18650 format dates to the commercialization of the lithium-ion battery itself. Sony brought the first mass-produced lithium-ion cells to market in 1991, initially for its own camcorders, and the cylindrical 18650 quickly became a workhorse size as laptop makers through the 1990s and 2000s -- including IBM and Dell -- adopted it for removable notebook battery packs, typically wiring several cells together in series and parallel.

The format's name follows a simple IEC convention still used across lithium-ion cylindrical cells: the first two digits are diameter in millimeters, the next two are length in millimeters, and the trailing zero denotes a cylindrical (as opposed to prismatic) shape -- so 18650 reads as roughly 18mm by 65mm.

The 18650's combination of established manufacturing scale, good energy density, and predictable form factor carried it well past laptops. It became the cell of choice for early high-output flashlights, cordless power tools, and -- most visibly -- Tesla's first electric vehicles, which packed thousands of 18650 cells into a single battery pack rather than using fewer, larger cells. That automotive demand helped drive further refinement of the format even as newer, larger formats like the 21700 have since taken over in some applications requiring higher per-cell capacity.