1991 Buick Regal Battery not lasting more than a week.

Hi there,

Our Buick Regal GS 1991 is not able to hold the battery charge for more than a week or so, I try two new batteries and have the car checked by an electrical mechanic which found no drain and he kept the car for two days, I got a battery he recomend for the correct amount of crank power life time warranty and after 2 weeks the battery was dead again, I took the car to another mechanic who found no problem with it.I need some suggestions pretty bad as this is my Daughter's car.I will appreciate any advised. By the way I put a new alternator and belt on the car, new starter, plugs and battery (twice the battery).