Drain it completely - from fully charged until it shuts itself off (you don't power it off). After it dies on it's own, let it sit dead for about 2 hours. Then charge it -uninterrupted- back to full capacity (and leave it plugged in for an additional 2 hours or so). It doesn't matter whether you use it during the recharging or not, it will still be calibrated as long as you leave it plugged in for about 2 hours after reaching 100% capacity.