Hi phortize, a battery can't be kept young, as you suggest. Many people think that if you don't use a battery, it remains new, but that is not true.

I'll make you an example for a better understanding:
buy two IDENTICAL batteries for your N900, and put one of them in your phone. Use your phone every single day of your life with that battery you put in, never change it and recharge it every day. After 3 years, remove that battery and compare it with the one you have never used: they'll be nearly identically worn-out (just few differences).

This happens because inside lithium batteries there are many security circuits that consume the battery. Additionally, even if you do not use a battery at all every year its capacity decreases.

Every recharging of the battery worns it a bit, but the self-worning out is much more relevant than that.

If you want to deepen:
http://en.wikipedia.org/wiki/Lithium-ion_battery