I took out my battery to clean my MBP yesterday and I found this darker gray mark on the inside of the battery. It's about the size of half of a quarter (probably a little bigger) and is located on the edge of the battery. I wish I could take a picture of it, but I doubt it would help very much as it would be shot with my camera phone. Is this common? Is it a defect I should worry about? I got my MBP in late September and I've been taking care of it real well. Thanks, everyone!