I have never travelled with a spare battery, however on every single laptop I have owned, I have purchased replacement batteries due to battery life reduction.

I have never travelled with a spare battery, however on every single laptop I have owned, I have purchased replacement batteries due to battery life reduction. My last Macbook and Dell both went from 1.5-2 hours per charge to 20-30 minutes per charge in a matter of weeks (after owning them for 18 months first of course).