This has cropped up on this forum once or twice in the past. My AR51SU does the same thing. When I reported it to Sony they referred me to an engineer who stated that a discharge is perfectly normal. I disagree but decided to remove the battery when the laptop is switched off. When the battery is left connected it can lose up to 14% charge overnight when it is switched off. With the battery removed it discharges only 1 or 2%.
It has been mentioned that it may be caused by Sony's battery management program (ISBMGR.exe - or something similarly named).
