iPhone Dies with 80% Battery Remaining 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?
While the iPhone 6s has a service plan - reduced cost replacement1 for having an issue where the battery reaches an end-of-life state after around a year of use (causing the battery to die at ~30%), older iPhones normally reach this state when their batteries grow old and tired (lithium-ion batteries can only be recharged so many times before they become unable to hold most of the original charge). Your iPhone 5 has a 3-3.5-year-old battery2, and thus it's the battery's fault that the phone is dying so quickly.

The only fix is to replace your iPhone's battery, something you can safely attempt yourself following this video or bring your phone to the Apple Store to have them do it (cost is usually 80-100 dollars without AppleCare).

1 The battery malfunctions are blamed on a manufacturer leaving the batteries exposed to ambient air too long; note that it is not a safety issue, merely a convenience issue. Recall information here.

2 Based on a release date of September 2014.

tl;dr: It's the old battery's fault, bring it to an Apple Store and get them to replace the battery to fix the issue.