Can someone tell me how to know when a 520 battery is fully charged?  My usb plug in has an LED that is red and green.  Usually it is red when I first plug in the battery, then after a while it will change to green then back to red/back to green.  Is this indicating it is charged or that it is insecure and does not know how to make up it's mind?
	
		
			
		
		
	
			
			