My battery calibration seems to be all messed up. If I charge it to 100% it takes a little over 3 hours (via USB with original charger), but the last 1% takes disproportionately long to complete. When discharging, it will discharge slowly from 100% to around 50%, then increasingly rapidly until it gets to 3%. That last 3% then seems to last for ages. Is there any way I can re-calibrate it?