Our product manager Justin sent me a link to Kipkay's YouTube video on hacking apart a 9-volt battery into six AAA replacements. So, I had to try it for myself.
A 9-volt battery is actually made up of six 1.5-volt cells. Those cells are actually quadruple size, but you can make them work in AAA compartments.
. With some needle-nosed pliers, pry apart the casing of the 9-volt battery. As long as you're careful not to puncture the cells themselves, there's nothing harmful inside. Once you get it open, pull off any casing that's holding the cells together and you should get a sort of sausage string of cells.
Clip those apart, but leave some of the connector metal on each end of each cell. When you put them into a AAA compartment, bend the metal over to give you the extra room and conductivity to make them fit.
You may or may not be saving money here, since the cost of a 9-volt battery does run roughly 6 times as much as a single AAA. But in a pinch, the 9-volt battery could come in handy, just don't expect to use it as a 9-volt battery any more after you do this.
Also via Kipkay, The Energizer brand of A3 12-volt batteries can be pried apart to reveal eight button-cell batteries, useful for watches and other devices. Here you're saving money: the 12-volt is a couple bucks and each of the button cells can run about $5.
That's it for now. Be careful if you do this, by the way. Don't want you slicing up your hands or the batteries. Use proper protective measures.