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My iPhone 5 runs out of power very quickly. First it was 50%. Then it was 60%, then 70%, and now 80%. At 80%, the phone will shut down like it would when the power gets to 0%. When it powers back on, it says 80%. It's almost like the limit of when to shut the phone off was, and still is getting to a higher percentage.
However, the phone is unusable. Not only does it run out of power quickly, it also drains power at a ridiculous rate. In half an hour, with wifi enabled and the screen off, it takes about half an hour to turn off. When I'm using it, it takes 10-15 minutes after a full charge to die. What's the cause of this issue, and how can I fix it?