Rafael's phone dies while he still has battery life remaining. Leo says that the phone guesses how much battery life remains and often it's an inaccurate guess. So it should be taken with a grain of salt. It may have just crashed, though. Leo says to charge the phone all the way up and then run it down until it shuts down. Do this several times and he will essentially "teach" his phone how long its battery life is.
It may also be a defective phone. Leo advises documenting his experience with it, and taking it to the Apple Store.