my macbook pro is about 18 months old and the battery life just got really bad. it was slowly getting worse at an expected rate until about last week i could get about 6 hours using the internet, watching movies and stuff like that. a few days ago just sitting on the desktop, doing nothing. bluetooth, wifi off, screen brightness on almost nothing. the time next to the icon was 2 hours, with 98% charge. is it normal for the sudden drop in battery life or is something wrong with it? what should i do?