26650 batteries are a type of rechargeable lithium-ion battery known for their high capacity and long lifespan. The numbers "26650" refer to the battery's dimensions: it is 26mm in diameter and 65mm in length, making it larger than other commonly used lithium-ion batteries such as the 18650 and 21700.

26650 batteries are commonly used in high-drain applications such as electric vehicles, power tools, and high-powered flashlights, as they can deliver a high amount of current without becoming damaged. They offer a higher capacity than smaller lithium-ion batteries, making them ideal for devices that require long-lasting power.

As with all lithium-ion batteries, it's important to use a dedicated charger specifically designed for 26650 batteries in order to safely and effectively charge them. It's also important to note that not all devices that use lithium-ion batteries are compatible with 26650 batteries, so it's always best to check the device's specifications before using a 26650 battery as a replacement.