unlocking issue

my 03 s55amg's doors wont unlock! i had to manually open it with the valet key and once the door opened there were no interior lights on. My guess is that maybe the battery is dead, however the small alarm light still flashes implying that the car is armed. So since it still flashes even when the door is open and no power to the interior lights,is it a battery problem or something else?

any ideas?