DEAD BATTERY.

HI,i would like to post my problem to every one can help me and thanks for help, the problem is i bought a BMW car 318 i x ridge the problem started after 10 days i try to started the engine but the battery is dead, so i tryed to fix the problem with an electrition near my house, and we check the alternator and nothing rong with it, then we check the battery if it's damaged but it's nothing wrong with it, then we change the new dvd which Halford's technetion instaled it in the car, because i thought may be this caused problems due to bad installation, i change the dvd with anew one and the electrition fixed it this time then i bought a new baterrey, then i thought the problem was solved because 30 days nothing happened, but the problem started againe the new battery is dead, so i went to BMW company in ISELWORTH they checked all the car and they said that the alternator is falty, they change it but after 2 days the problem happened again, so please help me to find the wright solution for this problem and thanks.

best regards;
batteryman.