I have to agree with Karel when it comes to keeping spare batteries around.. it's mostly a waste. I've given in to the temptation of getting a spare battery now and then, but they only end up in the fridge (at best). For example, I got a great offer for a spare battery when I bought a digital camera, with higher capacity than the original. In 1.5 years I've swapped batteries once. Later I bought a replacement camera, and now I have three batteries.. two are never used.

(As for airplanes - no worries, in-cabin isn't a problem. The restrictions are in checked-in luggage.)