What's inside a 9V battery? More batteries! (…and why you never see a AAAA)
I came across a Gizmodo post with an eye-catching photo of a disemboweled 9V battery – apparently taken from the Axe Collector source post, "Why I Never Pay For "AAA" Batteries". Axe Collector has several photos of the innards of a 9V, which consist of six 1.5V cells that if you squint are about the same size as a AAA battery, and in a pinch could be used in its place.
Well. This is like getting permission to do something vaguely illicit – you mean you can tear apart a 9V battery and not be sprayed by flesh-eating acid, wait, alkaline? [Immediate disclaimer – I used a Duracell battery. No idea what's inside of the others, but hey, alkaline is alkaline.]
So I disemboweled a handy 9V battery to see what I could see. It came apart quite neatly, with the six 1.5V cells connected by bendy metal tabs, all in series, encased in a clear plastic wrapper.
How close are they to AAA batteries? As Axe Collector says, they're smaller, and he reckons they're actually AAAA size. I measured them at 0.303 in (diameter) by 1.592 in. (length). Trotting over to the virtual bookshelf and pulling down my copy of Wikipedia, it defines AAAA batteries as being 0.33 in (diameter) by 1.67 in (length)*. Not an exact match, but pretty close. Let's say that these are AAAA batteries.
I measured a stray AAA battery that was lying about (no doubt quivering when it saw what happened to the hapless 9V battery) and it came in at .400 in. (diameter) by 1.67 in. (length). As you can see from the photo, there's not much difference. But calculating out the volume, the AAA battery comes in at .21 cubic in vs. the AAAA's .11 cubic in: Almost double the volume — thus the power — for a fractional increase in length and diameter. It's easy to see why the AAAA battery was replaced by the AAA.
*Wikipedia gave the dimensions in mm: 8.3mm (diameter) by 42.5mm (length)