What do you mean "can't change the battery"? You can change the battery yourself. It's just fixated in there under the cover because they last longer; also to look nicer or whatever the point of unibody is. It's also rather easy to do. Thought officially, the battery is not a user-serviceable part and may void your warranty. But I just hate that you say "you can NOT replace it."
it should be pretty obvious for people who know about this situation that I meant you will void your warranty if you do it yourself and it is not meant for mac owners to do it themselves.