According to Apple's iPhone User Guide, while the unit is being charged, the iPhone's battery status indicator (located on the upper-right portion of the screen) will change from a lightning bolt icon to a plug icon [see screenshot at left]. However, a surprisingly high number of users are reporting that the plug icon never appears, perhaps indicating that the battery never fully charges.
In our "Optimizing iPhone Battery Life" article, we noted that if your iPhone takes 10 or more hours to fully charge and never displays a plug icon, you should first try restoring the device. You may also want to try fully draining your iPhone battery and then recharging. UPDATE: It also appears that some iPhones will not fully charged unless they are turned off, then plugged into a wall outlet. Failing these options, you may want to considering contacting Apple for possible repair, maintenance or replacement.
In-house, at least one of our iPhones never displays the plug icon in its battery status indicator, even after restore and full power drain.
It is as of yet unclear whether or not this is a serious issue. Our in-house iPhone that ostensibly never fully charges delivers what we believe is normal battery life and does not exhibit any other significant problems.
If you have an iPhone that appears to never achieve a full charge, please let us know.