Briefly relating back to overall size... many lithium 18650's will differ ever so slightly in length (measured in millimeters). This is mainly due to an added protection circuit. The circuit helps to prevent the battery from over-discharging since Li-ion cells can easily become damaged if their power is drained below a particular voltage. The protection circuit also prevents the cell from being over-CHARGED as well. These cells are very sensitive. They must be handled carefully, gently, and as described above, must never run-down too far, or be charged over and above their threshold. They can actually explode if severely mistreated!