No, this isn’t even partially true. Replacement packs only have to have a similar voltage spec. and the same connector, and be about the same size/shape (or smaller). Other than that, they can be anything.
I was thinking the exact same thing before I read Angler’s reply. Someone probably crunched the numbers and figured out that the 21700 is the optimum size to:
- Minimize empty space between cells. For instance, they could use a bunch of 10440’s but eventually it will cost you more money to manufacture all of these extra batteries, whereas it would become cheaper just to tow a trailer behind the car slap full of 26650’s
- Maximize density in the pack. As I said before, someone calculated the optimum ratio of empty space vs cost of manufacturing a TON of batteries
- Capacity vs weight. You could just a trailer loaded down with batteries hoping you would get a 10,000 mile range, but that extra weight hurts your range. Whereas a super light pack would reduce the amount of energy used, giving you a better weight to range ratio, but your range will be drastically shorter. Where is the perfect mix of weight to capacity? Hint: Ask a Tesla engineer.
In those 20700B discharge curves, did you notice the capacity stays dead-on the same at about 4200 mAh for 4A, 8A, 12A, or 15A? Nice! Wonder what the internal resistance will be like.
In comparing the 15A discharge curve to HKJ's for the EFEST IMR 4200 26650 2016, it looks like the EFEST still clearly beats it, maintaining a higher voltage. So, I'm not seeing any remarkable advantage in terms of low resistance or even capacity for it's size.
It's a little disappointing to me, I'd say. The 26650 will maintain it's lead, it looks like.
We could map the discharge curves to cells that perform similar I suppose. They perform much better than a SANYO GA or MJ1, and can handle a greater discharge, even if comparing a higher 8A discharge curve of the 20700B to a 7A discharge curve of the GA or MJ1. So the cells are no slouch and a good capacity.
There is also cooling requirement. Some space between cells is needed for cooling.
I don't know much about this, but I have read that the larger size of a 26650 cell (compared to 21-70), makes it harder to get the heat out. With thousands of cells in a battery pack, heat management is a significant design issue.
Although some posters have speculated above that PR or the desire to create a monopoly are motivating factors for Tesla, my speculation is that engineering concerns are the driving force behind the choice to use 21-70 cells. Battery packs are at the core of an EV. They are costly. Fiddling with them for any reason that does not improve efficiency will have a negative impact on ultimate vehicle cost.
Neither Tesla, nor any other EV manufacturer, can afford that.
I guess no one read the link i posted above :cowboy_hat_face:
I read the article and liked it.
It does make sense that the way the nearly 7000 batteries in Tesla S are placed in the number of battery packs is as important as the the individual being used.
I did not agree that the price of fossil fuel will necessarily be going up. As the public and governments better understand climate change and it relationship to fossil fuels, fossil fuel producers will sell at any price that brings a profit. The price of $145 a barrel in 2015 will most likely not happen again.
Glad to hear someone read it 
I’ve read articles that suggest supply constraints by 2025 in oil because exploration has been drastically cut as part of the forced adaptation to the current chronic low prices. Whether oil goes up again or not does not affect our climate change emergency, but does affect how quickly we will transition, for a start i think we should eliminate (instead of expand) fossil fuel subsidies.
If Tesla ends up with surplus cells that are sold cheaply enough, somebody in China will make a light to use them :money_mouth_face: That’s how we got to where we are now: Cells first, then lights.
Phil
Bort I read it too
And OK after the somewhat negative post of me before (sorry I see so many pitfalls for Tesla)
If you are going to make cells in a gigafactory it is totally understandable you make m how you want
26650 had trouble getting the heat out from the center because they are fat, 20/21 is thicket then 18 yet far off from 26.
And honestly lots of cells should be called xx700 as is and well just ~14% longer in unprotected form is a lot more content While not adding much of housing material so that is logical too.
I read it too, and I don’t see suppliers raising prices like they did in the past, that was short sighted. But we might be seeing a price increase due to a carbon tax. Yep, meet the phrase carbon tax.
I read it and found it superficial and content free, all rhetoric with no substance. Sadly. I wanted to learn something and came away disappointed.
okay
Did anyone notice the guy in the comments section that said the cells will overheat because of a 6C current draw? 
I read the article, too, Bort!
The chart was excellent. The prose was flip. I won’t say the article was short on facts, but it was a little long on opinions, and had too much “cutsie” going on.
yea I didn’t analyze it too much, but from the start I found it hard to keep reading. And it should have been interesting for me.
A tad off topic
BUT watch this from 18 minutes or so
It is interesting to see how BMW is doing
The press don’t give them a lot of attention but I remember vividly a comparison between Prius and BMW 1 series
For EU, the mile gallon compared to environmental pressure of a Prius made the 1 series win for the first 20 year of use and only after 20 years and no accu changing on the Prius it started to be better for our world.
I so hope our next car is a decent electrical one with a normal dinosaur burning engine on board.
One BMW available in the U.S. has a 600cc gasoline motor whose sole purpose is to charge the batteries.
Just about everone besides Tesla use prismatic cells.
That is on the way. But I guess you know that.
GM Europe is now owned by PSA. There is a Peugoet and Opel / Vauxhall BEV due for 2019. PSA have a Chinese partner for batteries.
JLR just introduced a car that is direct competition for the Model S. We will see how it does.
63 kWh Nissan LEAF is due in less than a year. I think Hyundai are going to be slighly ahead of Nissan in the rush to a ‘reasoably priced’ 200 mile.