On Tuesday, Apple has acknowledged a battery manufacturing defect that was affecting a very small amount of iPhone 5s units that could lead to decreased battery life and longer charging times. Apple spokeswoman Teresa Brewer explains the situation below:
Originally Posted by Apple
Apple is reacting to the situation by reaching out to those that are affected by the issue and offering replacement units. It's unknown how Apple is actually planning to contact those that are affected, but we expect that Apple has a database of affected units sorted by serial number and can track down the owners to contact them accordingly.
The issue is not known to be plaguing the iPhone 5c.
Sources: New York Times