2002 Chevy Trailblazer dead battery aprox every 4 days

I have owned the car about one year. Within the last 2months it has had to be jump started about every 4 days.I have had "everything " checked electrical system , nothing seems to draw on the battery.I have not added anything to the car, like DVD players, security systems. Ect. Finally the mechanic tells me that despite the battery being new ( 1 month) it is a defective battery so 3 days ago they replaced the battery and this morning it was dead again. Several people tell me that it sounds like the alternator but this has been checked. How about the voltage regulator. Would this not be part of a diagnostic check up or should I request this as something extra?