Help please. My E53's battery's drained after parking for only a few days

[bulkwarrior]

Help please. My E53's battery's drained after parking for only a few days. This has never happened before. When I tried to charge it with a charger overnight, the battery is still not charged. I do notice every time when the charger's charging, the driver side headlight turns on and when it's not charging the light goes off. Maybe that's the reason the battery's drained and not charging properly. Does anyone know why is this happening since the head light is in the "auto" position and should not turn on by itself when the car's off. Also the passenger side headlight does not turn on.
Thanks.