Chevrolet Volt battery pack uses proprietary cells
Credit: GM/Green Car Reports
Electric vehicle (EV) manufacturers use lithium iron phosphate cells, connected together to form a battery pack for electricity storage. Almost all use proprietary cells.
Almost all — Tesla is the exception, using 186501 size laptop cells. Why 18650? Because they are commodity cells. I.e. multiple large volume manufacturers world-wide, for widespread use in multiple applications (including laptops), has reduced cost by competitive, volume production.
AA cell left, 18650 cell right
Credit: Kaayee Electronics
18650 cells are cheaper than the proprietary cells used by other EV manufacturers, both individually & per kWh of capacity. But it’s not all good news: each individual cell is smaller, so more of them are required to construct a battery pack of a given capacity. Thats more wiring, & more connections, which adds to cost.
Tesla’s Gigafactory, as its name implies, will not just add a few percent to annual world production. Planned 2020 Gigafactory production will exceed 2013 global production2.
In addition to volume of production, there is reason to believe that Tesla will also force costs down by changing the volume of the cells themselves3. A slightly larger cell of 20700 size (i.e. 2 mm wider, 5 mm longer than 18650) will be 33% larger in volume4, with a similar effect on storage capacity, for negligible increase in cell cost. There has also been speculation about 26650 cells5 (8 mm wider, same length as 18650).
For a given pack capacity, using 20700 cells will yield a battery pack containing 33% fewer cells & a similar reduction in wiring & connections, further reducing cost.
In summary, pack cost is expected to be reduced by:
1. volume cell production
2. reduction in cell count & cost
3. reduced pack assembly costs
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1 18650 — what do those numbers mean?
Rather than continue with the multiple, chaotic naming systems used for other battery chemistries, a naming system which specifies the physical dimensions of lithium ion cells has been adopted:
first 2 digits: diameter in mm — 18 mm diameter for 18650
last 3 digits: length in tenths of a mm — 65.0 mm length for 18650
AA size in a lithium cell is 14500 — 14 mm diameter x 50.0 mm length
AAA lithium cell is 10400 — 10 mm x 40.0 mm
Length is specified to one tenth of a millimetre because it is a more critical dimension for making a good electrical contact.
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2 Tesla Motors: "Planned 2020 Gigafactory Production Exceeds 2013 Global Production", https://www.teslamotors.com/sites/default/files/blog_attachments/gigafactory.pdf
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3 Advanced Automotive Batteries: "The Tesla Battery Report", p10 https://www.advancedautobat.com/industry-reports/2014-Tesla-report/Extract-from-the-Tesla-battery-report.pdf
CleanTechnica: "Lighter Batteries May Prove The Tipping Point For Electric Vehicles",
http://cleantechnica.com/2015/03/17/lighter-batteries-may-prove-tipping-point-electric-vehicles/
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4 18650 versus 20700 cell volume
18 x 65.0 mm cylinder = pi x (18/2)^2 x 65 = 16,540 mm3
20 x 70.0 mm cylinder = pi x (20/2)^2 x 70 = 21,991 mm3
Relative volume = 21,991/16,540 = 1.33 = 133%
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5 Manager Magazin: "Tesla baut seine Autos künftig ohne die legendären Laptop-Batterien" — Google Translate assures me this means, "Tesla builds its cars in the future without the legendary laptop batteries"
http://www.manager-magazin.de/unternehmen/autoindustrie/tesla-stellt-in-gigafactory-nicht-batteriezellen-vom-typ-18650-her-a-998973.html / Tesla Motors Elon Musk LiIon lithium ion lithium iron phosphate LiFeO4 charge recharge battery Model S Model 3 III range capacity kilo Watt hour Supercharger /
