I was planning to buy a new iPhone 5 to replace my old iPhone 3Gs model until I spoke to my friend. He got his from AT&T. He says that the phone works OK until the phone shows a 60% battery power left and then it shuts-off. He must re-charge the phone. Is there a battery problem with this model or did he just get a defective phone? I have already read that some of the phones get hot after being used for a while but never that the phone died after 60% power is reached?