2003 Lexus LS 430 - Battery keeps going dead

my battery keeps going dead its been checked at an electronic repair shop couldnt find any probleme 3 more garages and lexus who told me i need a new battery after a small discussion i told them this battery is only 7 or8 mnts old they put a new battery in 4days later same probleme toyota 1yr.told me i needed a new battery not so still have this same probleme 2yrs later

Have it checked for parasitic draw. No need to use a dealership shop, long out of warranty.