An 18650 is a cylindrical lithium-ion battery cell, typically about 18 mm in diameter and 65 mm long. The format is common in portable electronics and other devices that need a rechargeable, relatively high-energy cell. The name describes its approximate dimensions, not its capacity or quality.
In cybersecurity and connected-device security, the battery matters because it must match the device’s charging circuit and safety design. An 18650 lithium-ion cell is not automatically interchangeable with a nickel-metal hydride (NiMH) battery: their voltage and charging requirements differ. Using the wrong chemistry can cause malfunction or overheating, particularly in compact devices with limited enclosure space. Check the manufacturer’s specified battery type and charger; never assume that a cell fits physically is electrically compatible.