Most E-Cigarettes use what is called a 18650 battery. This may seem like an arbitrary number, but it is actually describing the size. The number can be broken into two key parts: 18 and 65. The 18 describes the diameter of the vape battery in mm, and the 65 is the length of the battery, 65 mm. So an 18650 battery is 18 mm in diameter and 65 mm tall.