AA cell left, 18650 cell right
Credit: Kaayee Electronics
Last year, I reported that the Tesla Gigafactory would be producing 20700 cells (20mm diameter 70mm tall), rather than the 18650 (18mm x 65mm) size used in all Teslas to date.
I was wrong! Size was wrong & the naming system too!
Turns out they’re going for 21700 size. I.e 21mm x 70mm.
Plus they are changing the naming system from the industry standard. Tesla is calling their battery size the 21-70.
In other applications, the last 3 digits specify the cell length to one tenth of a millimetre because it is a more critical dimension for making a good electrical contact. But other applications don’t include 7,104 cells1. Maybe with that many cells, alignment problems mean that getting the diameter correct is just as critical to getting a good electrical contact.
Names aside, why is 21-70 (or 21700) better than 18650? (Or is that 18-65 now?) In the Electrek article linked below, Elon Musk is quoted as saying: “It really comes from the first principles of physics & economics.”
Economics
A slightly larger cell of 21-70 size (i.e. 3 mm wider, 5 mm longer than 18650) will be 47% larger in volume2, with a similar effect on storage capacity, for negligible increase in cell cost.
For a given pack capacity, using 21-70 cells will yield a battery pack containing 47% fewer cells, & a similar reduction in wiring & connections, further reducing cost.
Physics
A larger object has less surface area for each unit of volume. This impacts cooling. There is a limit to how far the individual cells can be increased in size, without compromising cooling, &/or increasing complexity & cost of the cooling system.
https://electrek.co/2016/07/28/tesla-gigafactory-tour-roundup-and-tidbits-this-is-the-coolest-factory-ever/
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1 7,104 cells in 85 kWh battery – “Tear down of 85 kWh Tesla battery pack shows it could actually only be a 81 kWh pack”, https://electrek.co/2016/02/03/tesla-battery-tear-down-85-kwh/
2 18650 versus 21-70 cell volume
18 x 65.0 mm cylinder = pi x (18/2)2 x 65 = 16,540 mm3
21 x 70.0 mm cylinder = pi x (21/2)2 x 70 = 24,245 mm3
Relative volume = 24,245/16,540 = 1.47 = 147%
