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. ... They offer the performance of a lithium-ion cell, a capacity in the range of 1800mAh to around 3500mAh, and an output of 3.7 volts.Mar 2, 2019