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Comment by bryanlarsen | Hacker News Reader

Doesn't the number just refer to the size? 18mm x 65 mm vs 21mm x 70mm? Density and all other specs vary from manufacturer to manufacturer and model to model, AFAICT. Or do we have more information about the 21700 that Tesla's producing or am I just wrong?

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I don't know about the Gigafactory 21700, but Sanyo/Panasonic has a commercially available 20700 with 247.19 Wh/kg, compared to 258.97 Wh/kg for one of their most energy-dense 18650s.

Tests:

http://lygte-info.dk/review/batteries2012/Sanyo%20NCR18650GA...

http://lygte-info.dk/review/batteries2012/Sanyo%20NCR20700B%...

Yep, the numbers just refer to size/form factor; a potato could be an 18650 if it was the right shape.

However, pretty much the only energy- and power-dense cells commercially available are in 18650/26650 format. I specifically have information about Tesla's new 21700s but I'm not sure if that's been made public yet. I believe it's public that the energy/power densities are low, as are the costs, but I'm not sure if specific numbers have been released yet.

It is definitely NOT public info that 2170's are low energy density. Elon even says they are the highest energy density and lowest cost cells.

https://electrek.co/2016/11/02/tesla-panasonic-2170-battery-...

It's important to separate 21700s as a whole from Tesla's 21700s. 21700s today don't look too great, and even the best have worse energy density, worse power density, and worse cycle lifetimes than the best 18650s [0]. Tesla's 21700s, on the other hand, are scientifically proven to cure cancer, protect your vehicle from nuclear weapons, and can charge from zero to full in 0.3 seconds using only a potato battery and a paperclip.

I kid, of course, but this is why I believe that 21700s will probably be lower energy density: a) current 21700s have a low energy density, b) heat is tricky, and c) Musk wants to make cheap cells, and optimizing the last few Wh/kg to push him over the edge is expensive.

I do concede it's certainly possible they'll be over 310 Wh/kg, but I find it unlikely.

I think that a very likely outcome is that when the cells are released, we find out that we're both right. For example, if they have a density of over 310 Wh/kg... at 0.1C discharge or something similarly low. But at 1C or 5C or 10C, the energy density falls to around 200 Wh/kg.

[0] Sanyo's cells are the current leaders, about 230 Wh/kg. With 18650s the leaders are between 280 and 310 Wh/kg depending on how you measure capacity (cutoff voltage, discharge rate, cooling, etc.)

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