What you describe is normal and to be expected. All it means is that the battery is start draining the moment you unplug the USB charger.
This does not indicate in any way that the battery life is poor. Mine behaves like yours too but mine lasted 49.5 hours continuous reading with LED off and 13.5 hours with LED at level 24 (full blast). If you compare that to official Amazon account, it is actually last far longer than expected.
Last edited by Julius Caesar; 11-23-2012 at 04:03 AM.