I recall reading about police officers cooking-off pistol rounds by sticking spare bullets into a back trouser pocket and then dropping a handy-talkie into the same pocket. The charging studs on the bottom of the radio would be shorted by the bullets and the heat generated by the short-circuit would cause them to detonate. NiCd cells pack quite a wallop when shorted and could even potentially explode. AA NiCds can easily drive several amperes into a short circuit. Any batteries, primary or rechargeable, are potent energy sources and should be treated with respect.
This tale is no surprise to those of us who as budding young engineers learned that the easiest way to test 9V batteries -- at the time, the preferred power source for transistor radios -- was to touch one's tongue to the two terminals simultaneously. The 9V is made for this, as the terminals are side by side. (You can't do it with AAs, Cs, and Ds, where the terminals are on opposite ends of the battery.) So if you got a big sting, the 9V is good. The hit diminishes according to how run down the battery is; a spent battery has no wallop. When I say "hit," I should add there's nothing relaxing about it. You get a good shock. Supposedly this is a dangerous test to do, because if your skin resistance is low enough, you can die. Never heard of anyone dying from this, though.
As an adult, since transistor radios are no longer so popular, and the ones that are around use AAs, the 9V tongue test is applicable to smoke detectors. In testing them, I've come to the conclusion that the "change the battery once a year" thing, which has become common practice, is more a marketing ploy to sell more batteries than a necessity. When I am (forced to) change those batteries every year, they're still good. I'd keep 'em, but there's no use for them anywhere else.
It's most likely to happen with a 9-volt battery, Beth. The terminals are right next to each other. I'm not going discount the chances of it happening with a cylindrical battery, but I think you're ok with your regular junk transportation. Maybe skip the aluminum foil, but otherwise, ok.
Those little 9v batteries have always packed a real punch. Since the negative and positive terminals are right next to each other, you can put your tongue on the two terminals to see how much juice is left. The new ones are quite a shock.
Little batteries, even well-used, can pack a LOT of enegy! I've had a very similar experience with 9V batteries, which are more prone than most common types to accidental shorts like the one described because of the terminal orientation. AA cells could be even more dangerous if shorted, but the likelihood of something conductive contacting both terminals is much lower.
I use dry batteries to power some antique (tube) radios and have developed a very high respect for their power capability.
Beth, I think a spent battery is less of a problem than a fully charged one, but it certainly makes sense to remember that all batteries are actually power sources. Consider that model rockets are ignited with only a 6v lantern battery.
Now that's a lesson learned that everyone can benefit from. I can't tell you how often I pick up random things left lying around the house--nails, batteries, toy parts, change--and dump them in my pockets with the intention of disposing of them at a later date. I won't be so quick to load up my pockets with random, spent batteries any more after hearing this Sherlock Ohms tale.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.