I leave my phone plugged into the charger overnight and sometimes I'll wake up to find out that the battery actually discharged. Also, plugged in and idle, the phone will occasionally discharge. How can I identify what's causing the problem? I did notice recently that the phone it was being charged via USB while it was plugged into a wall socket.
Below I have listed a few things to check with your device and battery. This might help determine where the charging problem is occurring.
The issues are happening all of a sudden since the upgrade to Android 2.2 / Froyo. They seem to be widespread - try Googling Android Ally charging problems.
Some chargers / cables work, some don't. I haven't found a pattern, other than that the stock VZW ones do seem to work. This does appear to be an Android issue, maybe applying to phones beyond the Ally.