An 18650 battery or 1865 cell is a cylindrical lithium-ion battery common in electronic devices. The batteries measure 18 mm (0.71 in) in diameter by 65 mm (2.56 in) in length, giving them the name 18650. The battery is 3.7 volts.
A generic BMS for 4 series lithium-ion cells ('4s BMS')
A lithium-ion or Li-ion battery is a type of rechargeable battery that uses the reversible intercalation of Li+ ions into electronically conducting solids to store energy. In comparison with other commercial rechargeable batteries, Li-ion batteries are characterized by higher specific energy, higher energy density, higher energy efficiency, a longer cycle life, and a longer calendar life. Also noteworthy is a dramatic improvement in lithium-ion battery properties after their market introduction in 1991: within the next 30 years, their volumetric energy density increased threefold while their cost dropped tenfold.
A 3.6 V Li-ion battery from a Nokia 3310 mobile phone
Cylindrical Panasonic 18650 lithium-ion cell before closing.
Lithium-ion battery monitoring electronics (over-charge and deep-discharge protection)
Left: AA alkaline battery. Right: 18650 lithium ion battery