What is a Lithium Ion battery?
Lithium-ion batteries are rechargeable electric devices where lithium moves back and forth from the negative to the positive electrode during the discharge and charging process. A lithium ion battery comes in many sizes. The most common are the 18650 which measures 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. The 18650 has become the new gold standard for replaceable and rechargeable batteries.
They offer the performance of a lithium-ion cell, a capacity in the range of 1200mAh to around 3500mAh, and an nominal output of 3.6 volts. They are used in a huge range of devices from laptops to power tools, camera accessories like gimbals and sliders, RC toys, door bells, CCTV cameras and much more.
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.
The 18650 cell offers the best performance of any consumer-grade rechargeable battery. They're not susceptible to damage from charging before being fully discharged (as was the case with the old nickel cadmium cells), though they'll degrade at about the same rate as your smartphone battery.
You can't simply buy a 18650 battery off the shelf by looking at the capacity (measured in milliamp hours or mAh) alone. The right battery depends entirely on what you're using it for.