Yesterday I turned off my iPad when its battery percentage was at 57%. Then I turned it on again, a few hours later, and the percentage had dropped to 29-- but suddenly it jumped up to 56. Since then it hasn't jumped up or down oddly. Previously, I've seen it go down or up by as much as ten points after being off for a while, but not anything like what I saw yesterday. And today, I noticed the clock bar go back one minute and then tick forward again. I've never seen this before; is it normal?