I had this issue a little while back. When I got my new battery there was a slip of paper in the box and I was amazed that as little as 0.5 volt down (offload) after a full charge then letting it settle for an hour or so before taking a reading means the battery is only at 50% capacity. I now use an Optimate and when I come home after two weeks I don't have any issues with starting. It does a battery no favours constantly letting it discharge to a low voltage and charging it back up again. One word of warning. Do not go for a constant charger, this will boil your battery dry and buckle the plates ruining your battery.