I inadvertently ran my iPad Air down to 10% the other day, not something I usually do. However, it took over 12 hours to get back up to 100%. Having never run down this low before, this seems an awfully long time to recharge. Is this typical or is the battery flagging ? The pad is about18 months old and used daily. I timed it's charging rate over an hour period, against my wife's ' none Air' pad. Hers gained 17% over my 12%. Any ideas / thoughts on this ? Thnx.