From Apple:
Apple has determined that a very small percentage of iPhone 5 devices may suddenly experience shorter battery life or need to be charged more frequently. The affected iPhone 5 devices were sold between September 2012 and January 2013 and fall within a limited serial number range.
If your iPhone 5 is experiencing these symptoms and meets the eligibility requirements noted below, Apple will replace your iPhone 5 battery, free of charge.
If you have an iPhone 5 that falls within that date range, it's not a bad idea to follow the title link above to check your serial number. Both mine and my wife's iPhones were fine and within the range. Your mileage may vary.