2003 Toyota Corolla Battery

Hi, I have a 2003 Toyota Corolla, mileage is 81,503. A week after I purchased the car the battery died. I purchased a new one from Auto Zone, 4 months later that battery died. I took the car to a Toyota Dealer service department they checked the charging system and the battery and they came up with that the battery was defaulted so they gave me a new Toyota Lifetime battery. Now its a little over 4 months later and guess what the battery is died. I brang the car back to the dealer and they told me its the battery again. How is this possible.