I just got my MacBook Air last week, and I love it! (It's a refurbished 2013 13" 512GB, i7, 8GB RAM.) I notice that some days the battery shows over 12 hours at 100% and drains super slowly. Other days it shows 8-9 hours at 100% and drains faster. I use it to surf the web, play brief casual games, etc. I do turn the brightness down a bit to save battery. Any ideas why it varies so much? I do the same type things every day. Does this seem normal? Thanks!