An 21700 Battery is a lithium-ion rechargeable battery. The first 4 digits of the designation “21700” indicate the physical dimensions while the 5th digit indicates it is a cylinder cell. The standard 21700 battery is approximately 21mm diameter by 70mm in length. This type of battery is very common in applications such as electric bikes (e-Bikes), electric scooters, cordless power tools, laptop battery packs, torches / flashlights, electric vehicles, handheld vacuum and various other devices that require portable power.
Some types of 21700 have been modified by adding either a button top and/or internal protection circuit. This can increase the physical length of an “21700” battery from 70mm to 75mm or in certain cases even longer. If you're using an 21700 battery for a consumer product, you should always verify with the product's manufacturer for the exact specifications required to power the device.