My battery keeps dying?

My old battery on my LT150 was not starting the mower. I brought it to the dealer. HE checked it and said there was 0 charge. I bought a new battery. It worked for a week then I had to use a charger to charge it. After another week, that would not work either. Obviously something is draing my bateery but I don't know what. Didn't keep the liughts nor engine key on.