Why not leave the battery connected to a smart battery charger like the C-TEK while you are away? It will trickle charge and keep the battery at an optimum level when you return after some months. It even has acharge level memory and a re-start ability after a inadvertent electrical power cut off. BMW sells the same charger branded as its own.
http://smartercharger.com/battery-ch...ti%20US%203300