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 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. You have to make sure you buy the right battery for the job, and you’ll need to know how to avoid fake batteries. What Is the 18650 Battery? An 18650 battery is a cell that’s 18mm x 65mm in size.