Standard battery shapes

[akis]

Why are common batteries cylindrical?

When you try to build, say, a battery pack for a laptop, or some other application, you lose space due to the awkward cylindrical shape of the most common and prolific batteries: AAA, AA, C, D, 18650, 26650...

Why can't they be made square ?

Just wondering

[IanB]

Well, they can be made square, for example 9 V batteries, or LiPo cells. A square or rectangular cell is called a prismatic cell.

But for other cells (a cell is a single unit of a battery), a cylindrical shape is easier to manufacture. Many types, such as lithium or NiMH are made by creating a roll of thin sheets. If you roll up a sandwich of thin sheets you naturally get a cylinder.

If you take other types of cell such as zinc chloride or alkaline, you want an equal distance from the center to the outside all the way round, and you want a container which is strong and easy to manufacture. Once more, cylinders come out as the most practical shape.

[David Hess]

A cylinder is also a good shape for holding pressure which would distort a prismatic container.

[james_s]

The seals are a lot easier and more reliable with a cylinder too, round seal rather than corners that have to fit precisely.

[Kilrah]

Prismatic Li-Po cells are ubiquitous, and will be used when filling the volume is considered important.
Apple are even known for manufacturing batteries with complex shapes for their thinnest machines - of course that has a cost, so for your bog standard machine most will just throw cheap 18650s and call it a day.

[Robomeds]

Li-ion cells also come in prismatic shapes (though Lipos have largely taken over that market).
Here is a Sanyo prismatic cell
http://www.megabatteries.com/item_details2.asp?id=17000&cat1=&uid=1753&vfpr=&vfbr=Sanyo&vfcat=Electronics+Electronics+Accessories+Power+Batteries&vfsku=3325495&vfbi=no_bid&sid=gpla&vfsku=17000&gpla=pla&gclid=CNy8g4elm9MCFQ6oaQodR08FAQ
I suspect it was just easier at the time to make higher capacity cells in cylindrical form.   

[PointyOintment]

Li-ion cells also come in prismatic shapes (though Lipos have largely taken over that market).

They are the same thing.

[james_s]

They're similar. There are a multitude of different Li-based cell chemistries and physical construction. Most of the time when someone refers to "LiPo" they mean the high discharge current rated packs as used for electric RC vehicles.

[PointyOintment]

I refer you to this guide by Davide Andrea (founder of a battery company, battery managment book author—you can look him up): https://www.reddit.com/r/electronics/comments/559b1b/liion_cell_terminology_oc/

[Robomeds]

Li-ion cells also come in prismatic shapes (though Lipos have largely taken over that market).

They are the same thing.

When I was in the battery business it was understood they were chemically the same but not mechanically the same.  The mechanical differences can matter.