I have no spare batteries for my N800, and there's a reason for that: I have spare batteries for other devices, including my main camera (where, due to replacements, I ended up with 3 batteries). Turns out I never use the spares, or at least extremely rarely. The only thing I get out of spare batteries is the knowledge that they are just now sitting in storage, slowly degrading as all lithium batteries do.

The battery I have in my N800 now is more than two years old and it's not as good as it was, so when I find a battery fresh from the factory (trying to avoid those with a long shelf-life behind them already), I'm not going to keep the old one as "spare": I'll give it to the shop instead (or another place where they accept stuff that can't be thrown in the garbage).