What is an 18650 battery? Size, voltage and capacity explained

Short answer. An 18650 is a rechargeable lithium-ion cell shaped like a cylinder, nominally 18 mm in diameter and 65 mm long. The name is the size: 18, 65, and a final 0 for a round cell. It is a component for battery packs and for devices built to take it, not a swap for AA batteries.

Size

The 18 × 65 mm figure is nominal. Data sheets give a maximum or a toleranced size: Samsung lists the INR18650-30Q at Ø18.40 × 65.00 mm maximum, and Molicel lists the INR-18650-P30B at Ø18.6 × 65.2 mm maximum. Protected and button-top cells run longer, so measure the space in your device.

Voltage

Most 18650 cells are 3.6 V or 3.7 V nominal. They charge to 4.2 V and are empty at the cut-off voltage on the data sheet, often 2.5 V. Lithium iron phosphate (LiFePO4) 18650 cells are different: 3.2 V nominal, with their own charger setting.

Capacity and current

Capacity and current trade off. Cells built for high current hold less, and cells built for capacity deliver less current. Choose for what your device or pack draws.

Common questions

Is an 18650 the same as a AA battery?

No. An 18650 is larger and has more than twice the voltage of an alkaline AA (3.6 V against 1.5 V). It does not fit a AA holder and must not be used in a AA device.

Are all 18650 cells the same?

No. Capacity, current rating, chemistry and protection differ by model. Compare cells on their data sheets, and read our 18650 buying guide.

What are 18650 batteries used for?

Battery packs for e-bikes, power tools and solar storage, and single-cell devices such as flashlights. Browse 18650 cells by maker and rating.

Sources

Ratings are from each maker's data sheet, linked on the product pages and in our data sheet library.