Back in the Good Old Days, 9 V batteries had a stack of half a dozen pancake cells inside that completely filled the outer case. These days, it seems they use cylindrical cells similar to AAAA cells, with more wasted space around the edges.
I retrieved these batteries from our smoke detectors. I tend to poke the self-test button occasionally and wait until the low-battery alarm starts chirping, rather than throwing half-used batteries out. Being that sort of bear, I date the batteries when I put them in; they usually last two years.
This one is from batteries.com and lasted 18 months. The obvious corrosion inside the shrink-wrap plastic sleeve says that the cell sealing isn’t nearly as good as you’d wish. The two most heavily corroded cells are completely dead, but the rest have about 500 mAh of life left in them (at a rather low 50 mA discharge rate). With a bit better QC, it’d be a winner.
Notice that the case contacts have sharp points that ensure a decent connection, perhaps despite the crud. Only one or two of the points actually make contact, which probably contributes to a faster assembly time: just get the ribbons in the right neighborhood and crimp the case closed. On the other hand, all the current must flow through one or two points, so don’t use them as a high-current source.
On the other end of the scale, this Eveready Gold battery has six loose cells and lasted 2.5 years. All the cells have roughly the same level of charge remaining: they’re thoroughly dead.
The build quality seems better, with individually shrink-wrapped cells and a compliant closed-cell foam layer on the bottom of the case to maintain pressure against the contacts. No sharp points, so they need more pressure.
The cell polarity is exactly reversed from what you’d expect: the button end is negative. So, even though this looks like a cheap source of AAAA cells, that’s a cruel deception…
Lots more info at http://data.energizer.com/
If you’re heavily into battery testing, you need something like a West Mountain Radio Computerized Battery Analyzer.