Cell basics
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.
What it changes in practice
A 3,000 mAh cell discharged at 3 A is running at 1 C; at 30 A it is running at 10 C. The same current is a very different duty for a 5,000 mAh cell. This is why C-rate, not amperes, is the figure to compare across formats. It is also why a cycle life claim is meaningless without the C-rate it was measured at: a cycle at 0.5 C and a cycle at 3 C age a cell at very different rates.
Why it matters when you specify a cell
Convert C-rate into amperes before comparing anything. Multiply the cell capacity by the C-rate to get amps, divide pack current by the parallel count to get amps per cell, then compare the two. Charge C-rate is the number that sizes the charger, and it often binds before the discharge figure does.
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 →Form factor
The mechanical envelope of a cell — cylindrical, prismatic or pouch — which sets the packaging efficiency and the interconnect method.
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 →