Cell basics
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.
What it changes in practice
Volumetric density binds when the enclosure size is fixed; gravimetric density binds when the pack has to be carried or lifted. A cell can be strong on one and unremarkable on the other. Energy density also has to be read against the discharge rate: a figure quoted at 0.2 C does not describe what the cell delivers at 10 C, where voltage sag removes usable energy before the capacity is exhausted.
Why it matters when you specify a cell
Decide which density binds first: Wh/L if the enclosure is fixed, Wh/kg if the product is carried. Then check the rate the figure was measured at — a 0.2 C figure describes a storage duty, not a tool. Designing a high-rate pack from a low-rate density figure overstates both range and thermal margin.
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 →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 →