Any number of things. After market items? Boot light staying on? (Test by leaving it open - light will turn off after 16-20 min). If those tests and checks are good - leave the car locked with the boot open for 20 min and then start to look for draining currents from the battery. If you have that problem pull fuses until it goes away - then check that circuit. Other possibilities are bad alt diode, battery not getting charged in the first place etc etc.