My MBP is brand new. Tonight was the first time that I used it on battery. I just did some light browsing (no Flash or video) and word processing for a couple of hours. And now it's telling me that it has 79% of the battery charge left, and only 3 hours and 2 mins. Is that normal? Wasn't the battery supposed to last 8 hrs or so? iStat Pro says the battery is at 99% health. Also, the estimated remaining time keeps changing, though not by much. I'm worried.