This is what someone who knows stuff about batteries told me: Unless you plan on using your second battery almost as intensively as your primary, don't bother buying a second LiIon or LiPol battery; they age by simply lying there and are therefore almost always a waste of money. I should know, I have two spare batteries for my SE P910i that I barely even used and that now hold no charge.

If you do have an occasional need for longer battery life, one of those so-called "emergency" cellphone battery chargers can provide you, depending on how many batteries it holds, with 50 to 100% extra gogo-juice. The (now) added benefit is that there --still? -- are no rules about taking NiMh or alkalines in the airplane cabin.

And now, unless you are an under-the-counter supplier of novocain, go away. I want to die.