18650 3.7v Rechargeable 7800MAH.

The 18650 is a type of rechargeable lithium-ion battery. Lithium-ion batteries have revolutionized portable devices. They're in everything, from our smartphones and cameras to baby monitors, fitness gadgets, and flashlights.

As battery technology has matured, cells, like the 18650, that were once reserved for device manufacturers, have found their way into consumer hands. However, these new lithium cells have not been standardized quite like the rechargeable AAs you'll find in the supermarket.
 

An 18650 battery is a cell that's 18mm x 65mm in size. The name, 18650, refers exclusively to the size of the lithium-ion battery cell, but there can be minor variations even here.