The battery in a smartphone and tablet really does get a workout. They're amazing things when you consider what they power and how they power it. But from time to time, they have issues which can arise for absolutely no reason at all. One of these problems is swelling.
This picture is of a swollen iPhone 5 battery (left), and a new iPhone battery (right). This example is relatively mild to some others I have come across.
Apart from the obvious problems such as the phone not powering on properly, or the battery not holding charge; there are other problems which can be caused by a swelling battery.
The insides of a smartphone, and in particular an iPhone is very compact. there's not really any wasted room. So when a battery swells and expands it place a lot of pressure on the back of the LCD assembly. This can lead to either the screen lifting off the frame, the touch functions being compromised, or the LCD display being completely damaged.
So if you notice any discolouration or bleeding in the LCD, or the glass lifting off the frame (in most cases this begins in the top right hand corner), it's highly likely it's time to get the battery replaced.