Cell basics
21700 cell
A cylindrical lithium-ion cell roughly 21 mm in diameter and 70 mm long, offering about 50% more volume than an 18650.
What it changes in practice
The extra volume over an 18650 is spent either on capacity at similar current or on current at similar capacity. In practice that means fewer parallel strings for a given pack capacity, fewer welds and a simpler BMS. It is also the only cylindrical format in which 5,000 mAh and 60 A continuous discharge currently coexist in one part. The trade-off is thermal: the high-current grades assume the pack can hold an 80 C surface cut.
Why it matters when you specify a cell
The extra volume is not free thermally: surface area grows more slowly than volume, so each unit of stored energy has less area to lose heat through. Design the thermal path before choosing the 60 A grades, and measure cell surface temperature rather than inlet air. Where the pack is enclosed and unventilated, a mid-grade 21700 usually beats a headline part.
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.
18650 cell
A cylindrical lithium-ion cell roughly 18 mm in diameter and 65 mm long, the most widely tooled format in the industry.
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 →