18650 Cylindrical Cells

The active layers of the cell are wound in a spiral. Normally these cells have the lower case as the negative terminal and the top centre as the positive terminal. However, a number of larger cylindrical cells have both +ve and -ve terminals on the top surface.

For this article we will concentrate on the 18650, but this has migrated to the 21700 and the 46xx

18650 and 21700 cylindrical cell drawings

Perhaps the most famous of the cylindrical formats is the 18650:

18650 => ~18mm in diameter and ~65.0mm long

These dimensions vary between manufacturers.

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In the specification sheets you can see that what is declared as an 18650 cell is sometimes quite different.

The cells at the extremes are ~7% greater volume than a nominal 18650.

With a drive to increase capacity in a given format you can see why over time some companies have pushed what are acceptable dimensions.