I recommend a CTEK 4.3 trickle charger. They can be had for $55-65 and will greatly extend battery life for people like yourself that do not drive them every day. You can also use the same charger on your cars.
I use a CTEK on my Ferrari, Porsche, and S1000RR. The CTEK also "might" be able to save your battery, but I would start over fresh.
You can find the CTEK on Amazon and a myriad of other retailers.
This BMW is like an insane asylum and the inmates have taken over...