hey, I have owned an early 08 mbp for a little over 1.5 years and the battery health is already at 67%. yesterday the battery health was at 84%. i just decided to calibrate the battery so i drained the battery until it went to sleep then left it in sleep overnight till it shut down. After that i let it charge till the light went green and to my surprise found that the battery health dropped quite a bit. so what does this mean? is this 'normal', or is my battery on its last limb?