My iPhone is shutting off with 20% battery life left?

It never did that before, it would stay on till drained.

This tends to happen if you are running high-power apps - they drain the phone quicker than it realizes what's going on.iOs will calculate that the current power in the phone, and the time it will last, represents 20% of battery life, though it doesn't factor in apps that take a lot of power. Maybe you have something running in the background (or perhaps a lot of apps) that are passively draining power - try close them all down when you arent using them