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TL;DR 21700s produce less power (not necessarily energy) for the same cycle length and weight, so Tesla will either have to reduce power consumption of their hi

TL;DR 21700s produce less power (not necessarily energy) for the same cycle length and weight, so Tesla will either have to reduce power consumption of their high end vehicles -or- add more batteries, which adds weight. Both of these things reduce performance. 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-information.74652/#post-1683564 https://teslamotorsclub.com/tmc/threads/21-70-cell-informati...

justin66 10y ago
Thank you!
aembleton 10y ago
They might build cars with 80% 21700s and 20% 18650s. That way it would be cheap but provide the ability to supply a large current for acceleration.
- yessql 10y ago
  The 2170 would be marginally less power dense. It would probably be easier just to use more 2170's than try to engineer some complicated mix.
cesarb 10y ago
Are you comparing the Tesla 21700s with all the commercially available 18650s, or with the specific 18650s Tesla uses? It's possible Tesla doesn't use the most energy dense 18650s available, so their 21700s could be an improvement for them. 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.
- owenversteeg 10y ago
  > Are you comparing the Tesla 21700s with all the commercially available 18650s, or with the specific 18650s Tesla uses? It's possible Tesla doesn't use the most energy dense 18650s available, so their 21700s could be an improvement for them. 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.

justin66 10y ago

Thank you!

aembleton 10y ago

They might build cars with 80% 21700s and 20% 18650s. That way it would be cheap but provide the ability to supply a large current for acceleration.

yessql 10y ago
The 2170 would be marginally less power dense. It would probably be easier just to use more 2170's than try to engineer some complicated mix.

cesarb 10y ago

Are you comparing the Tesla 21700s with all the commercially available 18650s, or with the specific 18650s Tesla uses? It's possible Tesla doesn't use the most energy dense 18650s available, so their 21700s could be an improvement for them. 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.

owenversteeg 10y ago
> Are you comparing the Tesla 21700s with all the commercially available 18650s, or with the specific 18650s Tesla uses? It's possible Tesla doesn't use the most energy dense 18650s available, so their 21700s could be an improvement for them. 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.