system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Hexagons are the most efficient shape for stacking adjacent to each other while not wasting space. So why are lots of batteries cylindrical?

Also, wouldn’t electric cars and home battery storage, etc. increase its energy density if they could be packed with no gaps?
gpt
The contents of the cylinder are essentially a long flat sheet of thin materials, all rolled up into a round log shape.

To get the same contents into a hexagonal shape, you would need some sort of former in the center of the cell, which is wasted space.

So you either waste it inside the cell, or outside the cell. The cylinder is cheaper, and faster to make, and then you have plenty of room in the wasted space around the outside for things like cooling passages.