18650s are pretty much the only way to get decent energy densities and allow for high power output. What a lot of people don't know - which I find fascinating - is that discharge rates on batteries are set mostly by how much heat the battery can dissipate safely, and not as much by the cell chemistry itself. Looking at a random 18650, it may be able to (in theory) put out 200 watts, but the cell would be permanently damaged by the intense heat produced.
As a result, if you build an enormous single cell battery, even though it seems futuristic, it'll be horrible at distributing heat. A Tesla pack that was consolidated into one cell would basically be an enormous furnace at any reasonable discharge rate, and they'd have to discharge it at a very low rate in order to prevent it from permanent damage and/or fire.
If there's one thing to call "primitive", it would be the new low cost 21700 battery cells Tesla is producing; these have shockingly low power and energy densities, and disappointing cycle limits. They are cheap though, and that seems to be why Tesla's making them: for lower cost vehicles.
> If there's one thing to call "primitive", it would be the new low cost 21700 battery cells Tesla is producing; these have shockingly low power and energy densities, and disappointing cycle limits. They are cheap though, and that's why Tesla's making them: for lower cost vehicles.
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?
Cars aren't particularly size constrained, even if the Model S battery was twice the size it is now it wouldn't have a huge impact on the size or performance of the entire car.
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.
Thanks, and that's all obviously true, but I guess I asked for background when I should have asked for some citations. But owenversteeg indicated in another comment that his information is "not public yet," so I'll have to wait, I think.
(I'm almost certain his claim that the new cells are meant solely for the "lower cost vehicles" is wrong)
I didn't read that into owenversteeg's claim. Tesla is making and desiging its 21700 for the Model 3; they couldn't hit volume or cost targets without it. It's not being designed for the S; but that doesn't mean that they can't use it in the S later.
At least that's how I read owenversteeg's comment.
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.
Sounds like the 21700's have 50% more volume, likely slightly less than 50% more energy, but lower peak power output because of heat issues caused by a higher volume to surface ratio.
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.
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.
> 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.
Power density I can see. It would be harder to get the heat out of larger cylinders.
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.
The gains in energy density due to casing/connections are generally negligible. For reference, individual 18650s can range from 42 to 52 grams.
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'.
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?
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.
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'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.)
> discharge rates on batteries are set mostly
> by how much heat the battery can dissipate safely
Just a few comments up smart people are discussing the problems of heating the cabin. Why isn't cabin air being pumped around the batteries to heat the cabin and cool the batteries?
Musk has stated several times that this rather simplistic architecture works best for them. Tesla has evaluated other cell and pack designs, and still maintains that cylindrical cells are optimal. The Model 3 pack will use newer 2170 cells, same cylindrical format but larger, produced at the Gigafactory.
It only seems primitive until you try to figure out how to keep thousands of batteries all at the same charge level cycle after cycle. If even one manages to have more charge than the others in the pack, it could overcharge and catch fire.
Battery management has been the biggest problem among the DIY EV crowd and is responsible for a large number of fires.
18650 is mass manufactured: there are machines and robots to get consistent quality. You can get batteries in pretty much any size you want - but they will be made by hand and have the expected quality variations from hand made products.
