Last night when I went to bed, my iPhone 5S battery was at about 50%. I didn't put it on the charger. When I woke up, the battery was dead, and when I put it on the charger, it charged like it was fully dead, starting at 0%. I've never had this happen with this iPhone before, and I've had it a few weeks shy of two years.. Something similar happened on my old iPhone 4. However, does this suggest that there is a battery issue with my iPhone or that the battery is deteriorating? Why would this happen? Thanks.