2001 Jaguar S-Type Battery Keeps Going Down

First, I am female so please answer in laywoman terms. The battery keeps going down. I have replaced it three times. I constantly have to drive with an electric battery charger in the car to crank it. I have it at the Jaguar dealership now and they cannot find anything wrong with it. Can you offer any suggestions as to what may be pulling the battery down. Where do I go to get it fixed if the Jaguar dealership cannot help me. Thanks you so much for this website for people like me who have no clue what to do.

Dale