I have a new mbp 13 i5, and i think the battery lasts too little. With ONLY wifi and safari, browsing without flash, and with 3 boxes of display lightning and keyboard lightning off, the battery lasts around 5 hours max. I had finished charging it and it went from 100% to 50% in around 2.30 hours of use and now it says there is 3 left. Is that normal? Wasn't it advertised as 7?