Batteries are cylindrical because it’s the most efficient shape to make them in. There is a central electrode with the electrolyte around it. By making it cylindrical it’s distributed evenly. Imagine having it square, then in the corners the layers would be thicker than on the sides.
So that explains why the cells and normal AA batteries are cylinders. So why not have a 9 volt cylinder? That’s because the chemistry used for alkaline batteries produces 1.5 volts. A single cell, regardless of size, only produces 1.5V. So how do you get 9V out of a 1.5V battery? By putting 6 of them in series. 6 x 1.5V = 9V.
An A battery is usually just called a 17500. They were used in laptop batteries and such but are now used in hobbies more. Mostly for flashlights, vapes, or Lightsaber replicas.
Usually they're used in thin devices where a bigger battery wouldn't fit.
Lots of computer styluses take AAAA batteries, including the Microsoft Surface Pen.
There are also some small flashlights and laser pointers that use them.
microsoft surface pen is my touchstone for this. the only way i could get new batteries was online and the pen lasted for so long if i bought a pack of 4 batteries i’d have lost the other two but the time they ran or so id need to but a whole new pack.
Which was the secret knowledge for those of us with pen lights or active stylii back in the day that required AAAA cells. And then you'd find a cheap brand of 9v that actually had a stack of nonstandard square cells inside it instead... Bastards.
I have a wacom-type pen for a tablet that uses one of those. It was a total pain in the ass the time I was traveling and accidentally discharged it by jamming the button in a tight-packaged bag. Turns out, they are pretty much only available online. No normal shop ever stocks them, not even electronics shops nor radio shacks. Barely anyone even heard of them. Tried disassembling a few 9V's, but all of them were the stacked kind. And with international shipping going 2-6 weeks and me changing locations more often than that, it was an extremely difficult to get hold of them.
When Dan's Data reviewed these, he said something like "even if you use this in space, you are still no further away from a shop that sells replacements than you would be at home"
Yep, I bought a pack of these, but then i didn’t need them because I didn’t use the stylus enough/stylus had a good battery life, BUT then when I did want to use the stylus I had no fucking idea where I stored those batteries.
The stylus for a surface tablet uses one AAAA battery, and it will last for hundreds of hours of use. I wish I still had mine, as my surface tablet was my art tablet and I miss doing art. Nobody pay attention to the fact I was bad at it.