Cell basics
Capacity
The charge a cell can deliver between its charge voltage limit and its discharge cut-off, stated in ampere-hours or milliampere-hours.
What it changes in practice
Data sheets normally give two figures: a typical capacity and a minimum capacity. Designing to the typical figure is designing to a value the manufacturer does not guarantee. Capacity is also conditional on rate and temperature: a cell rated at 3,000 mAh at 0.2 C will deliver measurably less at 10 A, and measurably less again at low temperature. The minimum figure at the rate you actually intend to run is the only safe design input.
Why it matters when you specify a cell
Design runtime against minimum capacity at your discharge rate and temperature, not the headline typical figure at 0.2 C. The gap between the two is the margin you do not have. For a worst-case calculation, take minimum capacity, apply the rate derating from the data sheet, and add the end-of-life retention allowance.
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 →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 →