18650 battery reference

The 18650 is an 18 mm × 65 mm lithium-ion rechargeable cell. It is NOT a drop-in replacement for AA batteries — the voltage (3.6–3.7 V) is more than double that of an AA (1.5 V).

Specifications

Property	Value
Diameter	18.6 mm
Length	65.2 mm (unprotected); up to 69 mm (protected)
Nominal voltage	3.6–3.7 V
Fully charged voltage	4.2 V
Minimum discharge voltage	2.5–3.0 V (varies by cell)
Typical capacity	1800–3500 mAh
Chemistry	Lithium-ion (Li-NMC, Li-NCA, LiFePO₄ variants)
Charge rate	Typically 0.5–1 C; check manufacturer spec
Cycle life	300–500+ cycles to 80% capacity

Protected vs unprotected

Type	Has PCB circuit?	Length	When to use
Unprotected	No	~65 mm	Devices with built-in protection (most modern flashlights, laptops, EV packs)
Protected	Yes — prevents overcharge, over-discharge, short circuit	~68–69 mm	Devices without board-level protection, single-cell devices

Important: always check that a protected cell physically fits your device. The added PCB adds ~3–4 mm to length.

Flat-top vs button-top

Type	Positive terminal	Use
Flat-top	Flush positive pole	Devices with spring contacts or direct cell holders
Button-top	Small raised nub (~0.5–1 mm)	Devices designed for AA-style cells or requiring positive contact clearance

Common devices using 18650

High-performance flashlights (most Fenix, Olight, Nitecore models)
Laptop battery packs (multi-cell)
Power banks (multi-cell)
Vaping/e-cigarette mods
Electric vehicle battery packs (Tesla Model S/3/X/Y, many others)
Cordless power tools (some models)

Safety notes

Only charge with a charger rated for Li-ion cells at the correct voltage
Never charge a damaged, dented, or leaking cell
Do not short circuit — Li-ion cells can deliver extremely high current and will overheat
Store at 40–60% charge if storing long-term
Do not dispose in household trash — recycle at a battery drop-off point