I was going through the light weight nuggets on the home page and found lots of useful information. Here is a perl I learned a while back. If you have a flashlight or device running on AAA batteries you can use a Energizer 9 volt battery as a source of spare AAA batteries. The Energizer Alkaline 9 volt battery contains 6 AAA cells inside, however the polarity is reversed. So the plus and minus are reversed. Its possible that other brands do the same thing, and its equally possible that Energizer has stopped making the 9 volt battery that way, so try it with a dead battery at home first. It takes a little practice taking the battery apart, but its not too hard.

Don't do this with lithium batteries. The lithium metal is extremely toxic and that wouldn't be good for your health.