The active layers of the cell are wound in a spiral. Normally these cells have the lower case as the negative terminal and the top centre as the positive terminal. However, a number of larger cylindrical cells have both +ve and -ve terminals on the top surface.
For this article we will concentrate on the 18650, but this has migrated to the 21700 and the 46xx
Perhaps the most famous of the cylindrical formats is the 18650:
18650 => ~18mm in diameter and ~65.0mm long
These dimensions vary between manufacturers.
In the specification sheets you can see that what is declared as an 18650 cell is sometimes quite different.
The cells at the extremes are ~7% greater volume than a nominal 18650.
With a drive to increase capacity in a given format you can see why over time some companies have pushed what are acceptable dimensions.
IEC 61960-3 designation
“18650” is industry shorthand rather than a formal name. The performance standard IEC 61960-3 gives cylindrical and prismatic lithium secondary cells a more descriptive code made of three letters followed by the maximum dimensions [1]:
- First letter, negative electrode: I = carbon (conventional lithium-ion), L = lithium metal or lithium alloy, T = titanium, X = other.
- Second letter, positive electrode: C = cobalt, N = nickel, M = manganese, F = iron, Fp = iron phosphate, Mp = manganese phosphate, T = titanium, V = vanadium, X = other.
- Third letter, shape: R = cylindrical, P = prismatic.
- Numbers: for a cylindrical cell, the maximum diameter and then the maximum overall height, each in millimetres and rounded up to the next whole number.
Because the numbers are maximum dimensions rounded up, a nominal 18 × 65 mm cell becomes “19/66”. The standard’s own worked example is ICR19/66, a cylindrical lithium-ion cell with a cobalt-based positive electrode [1]. Following the same rules, a nickel-rich NMC or NCA 18650 would be INR19/66 and an LFP 18650 would be IFpR19/66. In practice many manufacturers blend the two systems in their part numbers, for example Molicel’s INR-18650-P28A [2].
Typical mass and capacity
Two current datasheets show how much the internal design can vary inside the same can:
| Cell | Design focus | Typical capacity | Energy | Mass | Gravimetric energy |
|---|---|---|---|---|---|
| LG INR18650 MJ1 | Energy | 3.5 Ah | 12.72 Wh | 46.8 g measured (49.0 g datasheet max) [3] | ~260 Wh/kg |
| Molicel INR-18650-P28A | Power (35 A max discharge) | 2.8 Ah | 10.3 Wh | 46 g typical [2] | 220 Wh/kg |
The masses are almost identical, so around 46 to 49 g is a sensible planning figure for a quality lithium-ion 18650. What changes is how that mass is spent: an energy cell packs in more active material, while a power cell gives up some capacity for thinner electrodes and lower resistance (see Effect of Electrode Thickness on Cell Energy Density). Treat unbranded cells with caution: an independent comparison of commercial 18650s found that some budget cells delivered less than 20% of their rated capacity [4].
Applications
The 18650 grew up in portable equipment, and the uses listed in the scope of IEC 61960-3 (notebook computers, power tools and power-assisted cycles) are still typical homes for it [1]. It also powered early high-volume EV packs: the cell Tesla developed with Panasonic before moving to the 2170 format was an 18650 [5]. Today it remains common in power tools, e-bikes, portable lighting and industrial packs, where the mature supply base and wide choice of energy and power grades matter more than the last few percent of pack cost.
Why the industry moved to 21700 and 46xx
For a cylinder, volume scales with diameter squared times height, so small increases in diameter pay off quickly:
A larger cell means fewer cells for the same pack energy, and therefore fewer cell-to-busbar joints, fewer cell holders and less handling per kWh, while the can and cap make up a smaller share of each cell’s mass. When Tesla and Panasonic started mass production of the 2170 in 2017, they described it as chosen to give the best performance at the lowest production cost for both vehicles and energy storage [5][6]. See 21700 Cylindrical Cells.
Going much wider brings its own problems. Current has to travel further along the electrode foils to reach the tabs, and heat generated near the core has further to go to reach the surface. The 46 mm diameter 46xx cells address this with tabless or multi-tab current collection, which shortens those electrical paths (see Tabbed vs Tabless Cylindrical Cells and The Thermal Circuit of a Cell).
References
- IEC 61960-3:2017, Secondary cells and batteries containing alkaline or other non-acid electrolytes – Secondary lithium cells and batteries for portable applications – Part 3: Prismatic and cylindrical lithium secondary cells, and batteries made from them, IEC, clauses 1 and 5.1
- Product Data Sheet, Model INR-18650-P28A, E-One Moli Energy (Molicel)
- T. M. M. Heenan et al., An Advanced Microstructural and Electrochemical Datasheet on 18650 Li-Ion Batteries with Nickel-Rich NMC811 Cathodes and Graphite-Silicon Anodes, J. Electrochem. Soc. 167 (2020), as summarised in LG INR 18650 MJ1
- V. Muenzel et al., A Comparative Testing Study of Commercial 18650-Format Lithium-Ion Battery Cells, J. Electrochem. Soc. 162 (8), A1592 (2015)
- F. Lambert, Tesla starts mass production of new “2170” battery cell at the Gigafactory, Electrek, 4 January 2017
- Tesla and Panasonic begin production of Li-ion cells at Gigafactory, Green Car Congress, 5 January 2017
See also: Cylindrical Cell Comparison 4680 vs 21700 vs 18650, Cylindrical Cells