I have a 13"/256/4 macbook air. I'm losing 10% of my battery life per 30 minutes on average. Just using at 50% brightness, iTunes, basic flash-less web surfing and MSN Messenger for Mac in the background. Which equates to about 5 hours till fully drained. Have already calibrated it once, no difference. Is this normal? I'm beginning to think my 13" has a below average battery compared to what others are seeing here. (i.e. 10hour + for even the 11" which supposedly has a smaller battery capacity than my 13" etc.) Thanks in advance.