Battery keeps dying in megane 53 plate?

So so pee'd off. Only had my car for 6 months, was running fine then it started cutting out and battery kept dying. Put new battery in it was running fine for a while then the same thing again, cutting out and then going dead. The AA tested battery and it was dead, only had battery for four months. So I changed battery again, haven't even had it for a week and it's dead again!! I'm at the end of my tether!! Don't know what to do, can anyone help :-(