battery drains down overnight battery is new alternator was just replaced 3 weeks ago because it stopped charging. Before that the battery was not draining down.

 

If the alternator has a shorted diode stack it will drain the battery after shut down new or old.
Otherwise some other circuit may be doing it.
Remove the heavy lead at the alternator, charge the battery then touch the heavy lead onto the alternator terminal.
If it sparks the alternator is faulty.
It should be an open circuit when not running.
Another way is to feel the alternator case well after to motor has cooled.
If warm to the touch after the motor has cooled, the alternator is heating from the drain current.
Good luck.