How you know it's bad: the device "may suddenly experience shorter battery life or need to be charged more frequently," says Apple.
See if your iPhone is eligible by visiting the replacement program's website and entering your serial number, which is on the phone under Settings > General > About. But if the phone is damaged in a way that impedes battery replacement, you have to fix that first, PC Magazine reports.
The program will run until next March or two years after your purchase date, whichever gives you longer coverage, reports Ars Technica.
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