With all the technology available today with being able to boost voltage efficiently using SMPS, why do we still use 9V batteries? Is there some secret advantage with them that I am unaware of?
If you look at the size as well, the 9V is just big and bulky and I have designed projects where I can use 2xAA batteries and boost the voltage, which will give me longer battery life than a 9V. And it takes up the same amount of space.
A lot of circuits today also need regulating, and the easiest way to do that with a 9V is a linear regulator (usually to about 5V) and I am aware this is not the case for every design, but that right there is energy wasted, and yet again, boosting the voltage from 1 or 2 AA batteries will probably give your product a better shelf life.
I saw a comparison between a 9V battery and some AA batteries, where someone found the energy available, and ended up with this data: 
NOTE: These results were from Energizer Alkaline batteries. The page can be found HERE.
So with all this data, why are 9V batteries still used in designs? Are there some applications where it would be advantageous to use them? Or is it usually a better idea just to go for the AA or AAA solution?
There have been times where I have considered using a 9V battery for some of my projects but it always seems after doing my calculations, they just don't hold up as well as others, so am I missing something?
For reference, the datasheets for the compared batteries are here:
AA
9V
EDIT: I am not intending this to be an 'opinion-based' question, rather, I was intending to ask from a practical point of view, if there were advantages to choosing a 9V over any other solution (such as boosting AA batteries). Just wanted to make that clear!
Best Answer
First a AA is not a battery.. it's a cell. A battery is a "battery" of cells, i.e. more than one cell. A 9V battery contains six 1.5V cells. Rip one apart and see.
As for why we still use 9V batteries, it really is a matter of the design. The nice thing about 9V batteries is they give you a fairly wide operating voltage range during their life without being overly high a voltage. They also come in a really nice compact package that has a rather usable clip.
I would in no way recommend using one with a linear regulator to generate 5V though unless the current requirement on that 5V is very small. Better to design your circuitry using componentry that will work at the 9V directly.
It also really depends on the nature of your widget. If you have sensors or transducers that require larger voltages and it's battery operated, it's simpler and usually cheaper to go with a 9V battery.
One also has to consider the ramifications of boosting a lower voltage. By doing so you will introduce a lot of new problems not the least of which is the electromagnetic noise you will be adding and needing to deal with. Efficiency is also an issue.
But in the end, there is a lot of to and fro on the decision so this question really comes into the opinion ranks.