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Inside the Tesla battery pack
- Thread starter Ingineer
- Start date
Todd Burch
14-Year Member
No. That changes the sensitivity of the stockholders and enthusiasts to exposure of IP.
I was just pointing out why some have concerns about this being posted, whereas the same concern is not shown for Apple.
Personally, I don't mind since others are going to tear it apart anyway...just pointing out the difference.
Nice photos. Looks pretty much like the scheme used in the Roadster battery pack. There were some diagrams a couple years back posted here from (dun dun DUUUUUN) China.
Roadster battery (ESS)
Roadster battery (ESS)
ToddRLockwood
Active Member
Interesting that they use such a small wire to carry the current from each cell. It looks to be no more than 26ga wire or similar? With over 7000 cells in the pack and a peak power draw of 310 kW, each cell is responsible for about 45W under peak load. With a lithium-cobalt cell having a nominal voltage around 3.7V, that's about 12A per cell over that tiny wire. Sure the wire is short, but I still have to wonder how much power is lost in each one under heavy load... I'm sure one of you EE guys will be able to calculate this quickly.
Don't forget that this small wire is intended to act as a fuse, to protect the cell from overload.
this. was going to point that out too. it's probably sized exactly for that purpose.
i like that in the pix and the parts of the charge cable you can see the trickle down tech from the space program in that they use silver plated connectors and the cable routing, etc. very reminiscent of many pix i've seen of rovers and satellites and iss components.
driving a spaceship.I'm curious, and I cannot tell from the photos, are these battery cells soldered in? Or are the connections simply being held together by force? I'm just wondering because I'm curious how hard/easy it would be to refurbish a Tesla battery pack in the future when it dies and is out of warranty. Being that it uses standard 18650 cells, I would think of all of the EVs on the road right now, Tesla packs would be the easiest to refurbish yourself. If no soldering is required, that makes it even that much easier. The cells in our Leaf and Volt are completely proprietary so we'll depend on Nissan and GM to provide replacement packs.
Oh, I realize that, I just wonder what compromises were made to accommodate that design choice.
vcor
Tech Specialist
Looks to me as if the battery wires are welded, which makes sense if you want the most reliable connections. If you open up almost any rechargable tool, you'll find the battery connections welded to the battery. As for the 18650, that really just a style/size and doesn't indicate the battery chemistry (there are a number of 18650 cells with different internal technical designs. Tesla has stated they use an Automotive grade version, so they may have something special or semi-custom.
I am curious why Tesla did not go with Li-Fe batteries that are much safer, highly stable, retains high capacity (>80%) after 3,000 cycles of charge/discharge and would be well ahead of competition (probably cost!). I don't know safety aspects of Li-Ion from crash testing and fire perspective. There has been reports of Li-Ion battery issues sporadically world wide (latest being Dreamliner). Any thoughts?
Thanks for the pics? I see no problems in sharing as this is not a violation of IP unless it was patented and copied. I am sure competitors have done it already.
Thanks for the pics? I see no problems in sharing as this is not a violation of IP unless it was patented and copied. I am sure competitors have done it already.
Because LiFePO4 energy density suck. You'd get a 35kWh battery instead of a 85kWh battery for the same amount of cells.
While the Panasonic cells are 3100mAh, 3,7V (~11,5Wh), a typical 18650 LiFePO4 cell is 1500mAh, 3,2V (4,8Wh).
There are many different chemistries in batteries that come under the Li-Ion umbrella. The chemistry used in the Model S is far different than the chemistry used in the Dreamliner. PenFed sent an email on this issue. (Post #14). I'd be more concerned with my ICE car catching fire.
JRP3
Hyperactive Member
What I'm struck by is the empty space and space used for cooling. It looks as if it cuts the energy density of the cells by quite a bit. A prismatic cell that didn't need as much cooling could potentially raise pack density even if the cell density was lower. A123 EXT cells maybe? Certainly room for improvement at some point.
stopcrazypp
Well-Known Member
Besides from the sucky energy density, Tesla makes up for the cycle life deficiencies of the cobalt based cells (LiCoO2 in Roadster, NCA in Model S) by using lots of them. Example: 73 miles*3000 cycles = 219000 miles of use, 265miles*500 cycles = 132500 miles of use (the numbers might be better for the larger pack than indicated, because the larger size also means the discharge rate is lower, which improves cycle life).
They make up for safety and thermal performance by having active cooling (which the Leaf notably skimped on) and multiple safety features. Electrically, they have a built-in thermal fuse, a built-in pressure fuse and two separate short circuit current fuses (one each for cathode and anode) for each cell. They also have a fuses at the module level. Mechanically, the cells have a metal casing and are small (so there is less damage per cell compared to the 5-10x larger prismatic cells other packs use, plus smaller cells ensure a much larger surface area for heat dissipation). Each module and the overall pack casing is also metal (as opposed to plastic, like in the Volt for example), which helps containment of the fire if one occurs (the Boeing pack was designed like this too, the fire was completely contained inside the box).
http://large.stanford.edu/publications/coal/references/docs/tesla.pdf
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NCR18650A is 245Wh/kg and 530Wh/L (prismatic).
https://industrial.panasonic.com/www-data/pdf2/ACI4000/ACI4000CE25.pdf
18650 has 0.02106 liter of prismatic volume (using 18mm*18mm*65mm rather than the cylindrical equivalent). NCR18650A is 11.16Wh and 45.5g.
A123 AMP20M1HD-A pouch cell is 131Wh/kg and 247Wh/L.
http://ecomodder.com/imgs/a123_systems_amp20_data_sheet.pdf
So Tesla can afford to use twice the space of the cells for cooling and still have the same module/pack volume density as the A123 prismatic cells. I think the main issue is the weight though (NCR18650A is twice as energy dense in weight also).
Plus, it's not like the A123 cells don't need cooling (their cycle life is drastically reduced at higher temperatures ~5-6x shorter at 65 degrees C compared to 25 degrees C). At minimum they need aluminum fins for liquid or air cooling (as shown in the modules that A123 sells). And they definitely can't be packed extremely tight in the first place (pouch cells need room for swelling).
http://www.a123systems.com/products-modules-energy.htm
JRP3
Hyperactive Member
The A123 pouch cells, and the Volt cells, are strapped together with thin cooling plates between them, I don't see any room for swelling, which should not be taking place as far as I know. Certainly we strap our LiFePO4 prismatic cells together with no room for swelling. A swelled cell means something went wrong. Since LiFePO4 cells, including A123, have lower effective internal resistance they don't heat up as much during use, and can put out much higher C rates. Yes the Fisker A123 cells have thin aluminum plates for cooling, but the newer A123 EXT cells are supposed to be even less affected by temperature. Since they don't heat up internally as much during use a less aggressive system might be used to keep the pack protected from external high temperatures, for example just circulating cool air around the pack perimeter. Also remember that in the Fisker the small pack is working harder, with a Model S sized pack I'd bet the A123 cells would be very lightly loaded compared to their potential.
Some of this may be their way of achieving passive safety. I remember a long time ago Tesla putting out some demo photos where they had deliberately put one cell into thermal runaway (ie. on fire), but it didn't spread into the rest of the pack. Smaller cells obviously helps with that (less energy in the initial failure scenario), plus a degree of spacing between them.
strider
Active Member
As for the IP arguments, I recall reading here that Tesla has EVs from their competitors torn down in their labs so they can't be hypocritical about this. Even if I hadn't read that here I would assume it would be true as every company in every industry does this. It's a part of doing business and any company that doesn't know what its competitors are doing is not going to be around long. Much of this is way over my head but I like reading about it and looking at pretty pictures.
JRP3
Hyperactive Member
That's because Tesla is riding the commodity battery curve. Consumer grade usage patterns will continue to push commodity batteries towards a combination of greater energy density, reasonable weight and lower cost at the expense of lower lifespan and lower reliability (unless carefully controlled) than desired in an aircraft, automobile or other "enterprise"-equivalent application.
Tesla is successfully using commodity cells in an enterprise application by taking on the job of making the battery pack behave reliably itself. As opposed to folks who use batteries like A123. They are basically delegating the problem of how to make the battery behave to A123.
Tesla's approach is harder but the advantage is that because you're designing both the battery pack and the car to work together, you can make the car accomodate the battery pack (and vica-versa) in ways that otherwise aren't possible or practical if two manufacturers are designing each part on its own.
By doing so, Tesla can ride the commodity battery curve so they'll always have the cheapest, densest, most cost-effective battery packs.
The Model S is basically their 2.0 version of a vertically integrated powertrain and battery-pack combination. They've got a big head start on anyone else who might want to try the same approach.
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