Two claims here - that the batteries have lower energy density, and that they're intended only for lower cost vehicles - go against the common narrative among Tesla people. Can you provide any background on this?
Weight is important; doubling the weight of that battery would affect the S significantly.
Obviously battery cost is important; doubling that would add a huge blow to the bottom line of the S let alone the 3.
So it's not surprising that Tesla would optimize for cost, weight & other engineering factors such as cooling and reliability over size.
(I'm almost certain his claim that the new cells are meant solely for the "lower cost vehicles" is wrong)
At least that's how I read owenversteeg's comment.
I'll be honest: I've got much more knowledge about batteries than I do about Tesla's strategic direction.
I do know that there are currently problems with Teslas sustaining their power output when on high performance mode. Since the power density of 21700 cells will almost certainly be less than 18650 cells (this is basic physics) [0] I think they will use 18650s for their performance vehicles. However, I could be wrong; they -could- simply increase the number of cells used and make the vehicle heavier. Strategically, I would use 18650s (at least with the performance cars) if I was Tesla. However, for PR reasons they may choose to use 21700s and compromise the performance of the performance cars.
My claim that they have a low energy density is on less strong ground, of course. As Musk said during the Q3 2014 conference call, "show me the cell, not the powerpoint." I only have powerpoints and no cells. However, the powerpoints I do have show that the energy density will be between 220 and 260 Wh/kg. They may be up to 280 Wh/kg. But I doubt that they'll beat the current maximum of 310 Wh/kg that you can get today.
Tesla does a lot of really cool stuff, but there's a bit of a Tesla reality distortion field going on. For example, this post here [1]. Some genius speculates that batteries grow at a 5.94% compound rate, and then extrapolates to assume that Tesla's cells will reach 350 Wh/kg. Here's the kicker: li-ion battery cell capacity has been practically stagnant for 11 years. In some of these 11 years, the cells have gotten cheaper, but not by much. If we truly had grown at that 5.94% compound rate for the past 11 years, we'd currently have a 528 Wh/kg cell today.
Li-ion battery energy density grows in spurts. Realistically, there have been a few major improvements, with very small incremental improvements in between. For example, in 2001 we were at 180 Wh/kg. In just a handful of months we jumped to 260 Wh/kg, then to 280 Wh/kg as manufacturing processes improved. In the last 11 years, however, improvements have been maybe a few Wh/kg per year, and there have been literally zero improvement whatsoever in the past four years, by anyone.
I think Tesla - may - get a bit north of the 310 Wh/kg we can get today, and I'm confident in their ability to reduce costs, but I don't expect anything revolutionary (energy density wise) like Musk implies in his presentations. I'd set the threshold of 'revolutionary' at 400 Wh/kg, a 29% improvement from today.
For future-me-check-if-I-was-right purposes: these estimates are for Tesla's cells released in early 2017.
[0] The larger a cell is, the lower its theoretical power density must be. Power density is a function of power dissipation, and larger cells are worse at dissipating power. These new cells are only a few millimeters wider, but that's enough to increase their volume by 50%, which means their power density will almost certainly suffer.
[1] https://teslamotorsclub.com/tmc/threads/21-70-cell-informati...
Also, are they currently limited by the heat dissipation? If not, increasing the volume per cell might not be as much of a problem. The linked article mentions that they doubled the number of cooling loops per module, which might already be anticipating an increase in heat to be removed.
For reference, Tesla's current cells are about 215 to 225 Wh/kg; they are not the most energy dense cells you can get. What I'm comparing are Tesla's new cells to the best cells out there. The reason why I do this is because that's what Tesla fans - and Elon Musk - repeatedly do. Also, I'm almost certain that Tesla's new cells will have a higher energy density than 215-225 Wh/kg; if they didn't, they'd be on par with batteries from a decade and a half ago, which would be pretty pathetic.
> Also, are they currently limited by the heat dissipation? If not, increasing the volume per cell might not be as much of a problem. The linked article mentions that they doubled the number of cooling loops per module, which might already be anticipating an increase in heat to be removed.
It's complicated. Technically, at the rate of discharge that Tesla uses right now, they shouldn't be. Teslas right now discharge their 65-100kWh pack in 3.5 to 5 hours. That's about 20 kW power continuous. There are roughly 6000 to 9000 cells per pack, so very roughly you discharge, while driving, between 2 and 3.5 watts per cell, so around 0.2C discharge. However, Teslas have their packs in an enormous, insulated, heavily managed sealed blob of batteries. This confines the heat to the pack, which requires their water cooling system.
Another complicating factor is that heat dissipation matters in a few different contexts. First, there's the important matter of getting the heat from the inside of the battery to the battery surface; this is a matter of battery design and solved by the people that make the batteries. Then, there's the matter of what you do with the heat once it's at the battery surface (Tesla water-cools their cells); this is a problem for whoever makes the battery pack. Then there's the problem of what to do with the heat in general; now that you've drawn the heat away from the batteries and into the heatsinks or water, how do you cool your hot water/hot heatsinks?
Switching to 21700s 'only' changes the first part of the equation - heat dissipation within the cell - but this is one of the trickiest parts to deal with. You can put a water cooling system on the outside of your battery, but you can't put a water cooler in individual cells. You can change the chemistry of the battery to better dissipate heat, but changing the chemistry of course also changes the battery's characteristics: energy/power density, cycle life, weight, cost, etc.
But how could the 2170 possibly be less energy dense? You have more active material per unit casing, so it almost certainly is more energy dense, based on geometry alone. Add the fact that you will have fewer connections, and the pack will be more energy dense as well.
In regards to "But how could the 21700 possibly be less energy dense?" the answer is "Cause batteries are magic." Batteries are weird; we know how they work, and how we should be guaranteed to improve energy density if we do x, y, and z. But when we do x, the batteries explode; if we do y, they only last a few cycles, and if we do z they heat up too fast.
If it was my head on the chopping block and I had to explain why the 21700s weren't as good, I'd say 'heat', but that's about as useful as saying 'magic', to be honest, and this is because pretty much every time there's a problem with batteries, it's due to heat. High power discharge? Heat. Cycle life? Heat. Thermal runaway? Well, duh, heat. Chemistry works in the lab but not in reality? Heat.
Eventually, you get tired of seeing heat as the source of all your problems and move to Siberia. But in all seriousness, as I said in the comment you're replying to, I think it's certainly possible Tesla's 21700s are better than the current max energy density, I just doubt that they'll get anywhere near Musk's claims of an energy density revolution.
Also worth noting that Panasonic's best 21700s are less energy dense than their best 18650s. (Panasonic is supplying the battery tech.) Another example of 'batteries are magic'.
Telsa seems to be selling the new battery factory as a prerequisite for the model 3. Both for the volume of batteries and the cost.
However the model 3 is smaller, less air drag, less rolling resistance, weighs substantially less, and (from what little telsa has said) will be much slower to accelerate.
The 21700s should be cheaper to build a battery pack out of, less temp sensors, less connections, less monitoring circuits, and less labor.
So maybe the 21700 based battery packs will be used across the different teslas and provide the cheapest way to hit a given range. Then the 18650 based packs (that can deliver a high peak power because of better cooling) will be more expensive, but provide better 0-60 times.