Cell basics
Form factor
The mechanical envelope of a cell — cylindrical, prismatic or pouch — which sets the packaging efficiency and the interconnect method.
What it changes in practice
Cylindrical cells are steel cans with standardised dimensions and well-established welding methods. Prismatic cells are rectangular aluminium or steel cans that pack more efficiently into a box. Pouch cells are laminated film and give the highest volumetric efficiency but need external compression in the pack. The choice is usually driven by enclosure geometry and existing tooling rather than by electrical performance alone.
Why it matters when you specify a cell
The envelope decides more than the data sheet does. Cylindrical cells need holders and welded strip; prismatic cells need compression and tab support; pouch cells need external structure or they swell. Choose the form factor from the enclosure and the assembly process you already have, then pick the cell inside it.
Related terms
Read further
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 →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 →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 →