My new MBP has 5 hours of battery when doing light web browsing at medium brightness. Only 2-3 hours when playing low res youtube clips, and 6:45 hours with all apps and wifi turned off and screen brightness at minimum. When I first turned on the MBP I charged it to 100% and then discharged it down to 0. Is this battery life normal? What happened to 8-9 hours with wifi and web browsing? I can't even get 7 hours with everything off!