I've had this happen too and they replaced the battery under warranty. I've read that lockinng the doors (even in your garage at nnight) helps the sensors/modules go to sleep sooner. I'm not sure if this is true or not, but the new battery is still holding up.

It ticks me off that I have to put a battery charger on it over night to protect the battery. That shouldn't happen on any car, much less one that costs as much as these do!