An 18650 Battery is a lithium-ion rechargeable battery. The first 4 digits of the designation “18650” indicate the physical dimensions while the 5 th digit indicates it is a cylinder cell. The standard 18650 battery is 18mm around by 65mm long. This type of battery is very common in applications such as laptop battery packs, flashlights, electric vehicles, cordless tools and various other devices that require portable power.
Some types of 18650 have been modified adding either a button top and/or internal protection circuit. This can increase the physical length of an “18650” battery from 65mm to 70mm or in certain cases even longer. If you're using an 18650 battery for a consumer product, you should always verify with that product's manufacturer for the exact specifications required to power the device.