I know little/nothing about the physical construction / dimensions of 18650 cells.
What I do know is that many protected 18650 cells have a bus bar / flat conductor that runs up the side under the wrapper which provides an external electrical connection between the physical ends of the cell, and that likely means increased diameter which is not constant (is out-of-round).
Photos in this article illustrate this:
I also know there are some protected cells which apparently don't use this design, and here's an example of one which is advertised as not having such a 'bus bar', which would result is a slightly smaller max diameter which is more 'round'. I have a couple of these and they're good cells (
EDIT: with very low DCIR). My calipers are inaccessible at this time, so I cannot measure anything.
EagTac~EagleTac 18650 3600mAh 15.1Wh "Dark BLUE top" Li-Ion Rechargeable . Currently the highest capacity 18650 on the market. EagTac takes the best li-ion batteries in the world, and adds a TRUE button top and a PCB PROTECTION circuit for a complete package. IC PROTECTED Rechargeable Cell...
illuminationgear.com
The description includes this text:
"Brass button top contact with PTC/Protection circuitry now at "top" of battery (with steel plate used at bottom for added durability).
By moving PTC to top ~ reduced stresses are placed on the IC components for increased reliability. This design also creates shorter battery length, and no metal strip running along side of body for safer use ~ narrower width. All internal connection is made using pure nickel for better conductivity."
As with most things, there are tradeoffs. You may need to prioritize your requirements / criteria, and if diameter is critical to your application, you may find that means you may limit your possible cells to choose from. It is often impossible to optimize for all your requirements in the same cell. There are usually choices to be made.
Such is life in general;-)
