This video about a house fire initiated by stored batteries showed up in my Facebook feed the other day: http://www.youtube.com/watch?v=OSJH21WmALc
Uh-oh, there’s a lot of batteries just thrown together in a cardboard tomato container in one of my kitchen drawers. So I looked into it further. As I understand it if anything metal/conductive makes a connection between positive and negative terminals there will be a spark. If there is anything flammable within range of the spark, it will catch fire.
It’s especially easy to do this with 9 volt batteries because the terminals are right next to each other. All it takes is something like a nail, some steel wool, or the scissors that used to lie among the batteries in my kitchen drawer to lie across both terminals at once and there’s a spark.
The batteries from things like cordless drills and dust busters can to this too:
http://experttooltips.com/Cordless_Drill_Batteries_and_Fire_Hazards.shtml They say to put a piece of electrical tape across the terminals to prevent sparking. This author says not to use adhesive tape because the residue may stay on the terminal and interfere with the function of the thing the battery powers. Another possibility is to store batteries in the original plastic container they came in.
It isn’t only batteries with both terminals at one end that can start a fire. In fact you can start your campfire with batteries on purpose if you want: http://www.ehow.com/how_4550284_start-fire-batteries.html
This right here is why I love the internet. I’ve been paying attention all my life and until now I just never knew about this, and I still wouldn’t if not for the internet.