"We recently discovered a manufacturing issue affecting a very limited number of iPhone 5S devices that could cause the battery to take longer to charge or result in reduced battery life," Apple spokeswoman Teresa Brewer told The New York Times. "We are reaching out to customers with affected phones and will provide them with a replacement phone."
It's not clear how many phones are housing a bad battery, nor did Apple say how it was able to identify which users own an affected model. If you happen to own an iPhone 5S that seems to drain unusually fast, you should contact Apple and see if yours might one of the "limited number" of models eligible for a replacement.
Apple unveiled the iPhone 5S and 5C last month, both of which are rated for up to 10 hours of talk time on 3G, up to 250 hours of standby time, up to 8/10/10 hours of Internet use on 3G/LTE/Wi-Fi, up to 10 hours of video playback, and up to 40 hours of audio playback. Some of those figures are big improvements over the iPhone 4S, such as 50 more hours of standby time.