Why my iPhone 4s dying

My iphone 4s is dying at 30%. This is concerning me because my last iphone did this too. So I had got my iphone in August of 2012 and it started dying at 35% about a week ago on Tuesday. Then by Friday it was dying at 60%. I thought it was the software update so I took it to the Apple store and the tested it and said that nothing was wrong with the battery, like normal. So they gave me a new phone. Well, it hasn't even been a week and this phone is starting to do the same thing. I try not to charge it until it has gotten down far and is about to die so I would not have this problem. Yes, I have tried restoring and no, it does not work. I have also reset it. Is there a difference between charging the phone on a iHome docking station and a plug into a wall? Is there anything else I can do to not have my phone die at 30%? I don't think it is normal because I also have an iPod and iPad and none of them do that. Also, the last iPhone didn't do that until last week. Could there be a reason why it dies unexpectedly?