Cell basics
18650 cell
A cylindrical lithium-ion cell roughly 18 mm in diameter and 65 mm long, the most widely tooled format in the industry.
What it changes in practice
The name is dimensional: 18 mm diameter, 65 mm length, with the trailing zero denoting a cylindrical form. Because holders, nickel strip, welding jigs and pack tooling are standardised around 18 mm, the 18650 remains the default where a design does not need the extra volume of a 21700. Current models span about 2,000 to 3,500 mAh, and continuous discharge from 8 A on energy grades to 36 A on power grades.
Why it matters when you specify a cell
Start from the current per cell, then choose the grade. A pack that draws 5 A per cell does not need a power cell, and paying for 30 A capability costs capacity in the same can. If the envelope allows, check whether a 21700 pack needs fewer cells for the same energy — the answer is often yes, and the saving is in welds and assembly rather than in cell price.
Related terms
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More in Cell basics
Other terms in this group
The vocabulary you need before you can read a data sheet.
21700 cell
A cylindrical lithium-ion cell roughly 21 mm in diameter and 70 mm long, offering about 50% more volume than an 18650.
Read the entry →Form factor
The mechanical envelope of a cell — cylindrical, prismatic or pouch — which sets the packaging efficiency and the interconnect method.
Read the entry →C-rate
Charge or discharge current expressed as a multiple of the cell rated capacity, so that cells of different sizes can be compared on the same scale.
Read the entry →Capacity
The charge a cell can deliver between its charge voltage limit and its discharge cut-off, stated in ampere-hours or milliampere-hours.
Read the entry →Energy density
Energy stored per unit volume (Wh/L) or per unit mass (Wh/kg), the figures that decide how big and how heavy a pack has to be.
Read the entry →Internal resistance
The opposition to current flow inside the cell, measured as AC impedance at a stated frequency and state of charge, or as DC resistance across a current pulse.
Read the entry →