Using battery and not AC power???

I'm a new macbook pro user. Just got the new 15" retina display. The charger is so large and a pain to bring with you so is it that bad to the overall battery life if you're not plugged in??

I'd say its only about 2-5 hrs of battery use a week. I have it plugged in all day and most nights when I'm using it.

I bought applecare, does that replace the battery for free?