A 21700 battery is a type of lithium-ion rechargeable battery. The numbers "21700" refer to the battery's dimensions: it is 21mm in diameter and 70mm in length. 21700 batteries are commonly used in high-performance applications such as electric vehicles, power tools, and high-powered flashlights. They offer a higher capacity than 18650 batteries and are known for their high energy density, long lifespan, and relatively low self-discharge rate. They are becoming more popular in recent years, and many electronic devices that used to use 18650 batteries are now transitioning to use 21700 batteries.