The i3 has a better layout than I had thought. My main criticism is that they use nowhere close to the entire undersurface–it’s less than 2/3 of the width. As such, they also need frame stiffeners on the sides, which itself take up space and weight. The Tesla in contrast uses the entire pack as a load-bearing device. It stiffens the entire car and so they don’t need extra structural members.
The Leaf is less impressive IMHO. It’s also not really flat; the modules are stacked somewhat haphazardly to fill space under the seats. It also doesn’t fill a large portion of the underbody, nor does it act as a significant structural member. Even the internal packing doesn’t look so hot; there’s a big central channel for wiring that doesn’t seem very efficient.
The Tesla instead uses broad, flat plates as the module-level buses; each cell connects to the plate via a small wire which doubles as a fuse. There’s very little waste and most of the components serve more than one purpose.
They may be right but it remains to be seen. As best I can tell, current manufacturers do not use fully custom prismatic cells. They instead use one of a handful of possible form factors which do not necessarily lend themselves to compact packaging.
Tesla got lucky in that the height of an 18650 cell is just about what you want for a cell that covers the floor. They can then be tiled in a dense hexagonal arrangement and retain a lot of flexibility in dimensions.
The 18650 cell that Tesla uses is actually becoming increasingly distant from off the shelf cells. They use custom end caps which remove the venting mechanism that normal cells use. This saves money and volume, but without changing the production process significantly. They retain good safety though other mechanisms.
The prismatic cells currently in use don’t seem particularly efficient in and of themselves. Check out this cutaway picture. You can see that they’re probably not getting more than 80% packing density internally; there are plastic structural members; the separator bags are not infinitely thin; the housing has extra space for thermal expansion; etc. Tesla’s 18650 cells have much better internal packing efficiency.
No denying that. I meant “unique” only in that they’re the only auto manufacturer using them. Anyone else that was building a 200+ mile pack could use them (actually, they could get away with less since not all cars need the same performance).
The gigafactory will produce cells with a slightly larger form factor, though Tesla hasn’t said exactly what yet. They appear to like the cylindrical form factor, and for them the 18650 is (allegedly) close to but not exactly optimal. Whatever they produce, though, will seemingly be proprietary. Though maybe not exclusive, since Panasonic is a partner and they may well resell the cells.