Cell basics
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.
What it changes in practice
Impedance decides voltage sag under load and how much of the capacity is usable before the pack hits its cut-off voltage. It is also the parameter that moves most with age: a cell that comfortably meets a current specification when new can fall out of specification partway through its life. AC and DC figures are not interchangeable, and a DC value is meaningless without the pulse duration and state of charge it was taken at.
Why it matters when you specify a cell
Impedance sets voltage sag and heat: I²R per cell, multiplied across the string. Two cells with the same capacity and different DCIR behave differently in the same pack, and the higher-impedance one reaches cut-off first. Specify a matching band at goods-in, and re-measure a sample of finished packs to catch weld problems.
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 →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 →