batteries - Could my Cell Phone Battery be degrading?

I've noticed that if I recharge my cell phone battery to 100%, then take it out, and put it back in and put the phone back on charging, it resumes charging the battery and continues to do this for upto 30 mins, despite having just received a 'battery full' message about 30 minutes ago.

Why does this happen? Is this normal or does this indicate that the battery is degrading?

It is a Li-Ion battery.

Answer

Your battery is degrading from the charging station. The person whom makes your charging station probably expects you will use it before placing it in. They are then pumping a top-off charge into for 30 minutes when, although it did self discharge a little in those couple seconds, it should not get one.

It is possible the charging station is pumping way too much charge, but I am hoping the system can recognize it is almost done charging and go to slow-charge. From battery university about lithiums:

No trickle charge is applied because lithium-ion is unable to absorb overcharge. A continuous trickle charge above 4.05V/cell would causes plating of metallic lithium that could lead to instabilities and compromise safety. Instead, a brief topping charge is provided to compensate for the small self-discharge the battery and its protective circuit consume. Depending on the battery, a topping charge may be repeated once every 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05V/cell and turns off at a high 4.20V/cell.

There are phases to charging a lithium battery, as show in this picture: http://www.batteryuniversity.com/images/partone-12.gif">

I have seen charging circuity before, when I have helped students build these, that will start with the heavy current phase and test the voltage, this means that if you unplug and plug in you can degrade the battery significantly.