BMW Moves to Big Cylindrical Battery Cells

By Ian Page – 2022.12.07

Tesla, uniquely amongst serious car companies, is moving from small diameter cylindrical battery cells to large cylinders called 4680.

You may wonder why packing cylinders together in a battery pack is better than packing prismatic (rectangular) cells which fit together much better.

The answer is two-fold

1. Connectors cooling and rigidity- cells need external stuff to make them work safely and this takes space. With cylindrical you fit it in the spaces between. With rectangles, you must separate the rectangles to make space. Overall, there's little difference, cylindrical makes have the benefit with lithium nickel as cooling is more important, and rectangular may be marginally better with lithium phosphate or sodium. Cylindrical cells also pack into a honeycomb shape which is very strong and rigid with no easy fracture directions.

2. Cell production cost. Cylindrical cells can be made roll to roll which is the cheapest way of making anything! Prismatic must involve some very careful positioning of multiple very thin flexible layers of materials for each individual cell. As a result, the best design of 4680 should be able to produce many times more megawatt hours of cells per factory production line at a much lower cost than rectangular. There are some advantages in the BYD very long cylindrical cells (the width of the car) in terms of assembly of the cells compared with positioning lots of smaller cylinders, but this does not make up for the raw production speed.

BMW/Volkswagen has been using prismatic cells in gen 5 and has very heavy cars for the energy capacity. They have announced their gen 6 battery technology which copies Tesla’s approach i.e., cylindrical 4680-sized cells and structural battery packs.

Given that BMW has some very good engineers they have no doubt gone through a much more thorough analysis than mine to come to this conclusion.

My conclusion at present is that the 4680 /structural pack is the winning design for high-performance cars based on lithium nickel. It's neutral or negative for lithium iron phosphate and sodium ion in terms of packing density but wins on the cost of production (since for sodium iron cyanide batteries the cost of materials is a much lower proportion of the overall cost, thus the cost of manufacturing processes and capital becomes more important.)

It would appear that Musk's focus on making the smallest factory design that produces the highest volume and dealing with the issues of packing cylinders into a rectangular space may also have been the right decision for the long term with very cheap battery materials.

I have no idea where BMW is getting its 4680 cells from. Panasonic is building a line in Nevada for Tesla using a slightly different manufacturing roll-to-roll technique with fewer innovations. In addition, there have been announcements from CATL, Samsung, and LG all for supply to Tesla. I believe Tesla has made the key patents free. It would appear that Tesla is going to get the new production capacity first until there is excess capacity since Tesla is working with these battery suppliers to get them up to scale.

As a random thought, cylinders fit just as well into non-rectangular battery compartments should that ever be important, while prisms don’t fit very well into nonrectangular compartments.