It seems like for most products, there is extra space around the batteries that could be filled up by more “battery” if they were square instead of round. But they’re all round, except nine-volts, which are still rounded. Any ideas?
WAG originaly the most common application for these cells was flashlights, where a round cross-section made ergonomic sense. Since that time tradition dictates that newer sizes (AA, AAA etc) would also be round.
I just wanted to point out that if you open up a 9-volt battery, it’s composed of several round cells.
There are rectangular cells. I hear they are less efficient for the given weight, though I’m not sure about the exact reason. Even laptop battery packs often use round cells - the Sony Vaio battery packs are cylindrical, and my ThinkPad battery looks like there are two rows of circular cells inside.
Could there be some edge-effect going on here? A round cell would have better surface to volume ratio than a square cell so that would make some sense. Perhaps some current leaks back along the surface creating a short circuit, reducing battery shelf life.
My guess is that you want the material to be equidistant from the anode/cathode. If the cell were square in x-section, the gunk in the corners probably wouldn’t get used up as well, and the energy potential might be wasted.
this page has a cool cross-sectional diagram:
Oh, and I direct your attention to the awesome photo of a dismembered 9-volt battery at howstuffworks.com (not square after all!):
And we all know that those freaky 6-volt lantern batteries are likewise composed of smaller round cells, right?
If you want cool battery info, check this out.