Battery tender or CTEK is what you should buy and use. I have used them for over 10 years. My current M3/4 which I bought from the dealership has a battery I put in 2004. Using the tender I am still using this battery what almost 9 years later. My E93 3/2007 manufacturer same thing, just fine but I will most likely replace it anyway in a few months. Batteries are expensive and consider you also need to get it registered and also recoded if you use an AH battery that is different (if it is close in AH no problem) than what the OEM battery was.
I buy Battery Tender juniors and have 6-7 of them all in use.
You don't need a full Battey Tender. In fact I ordered one on Amazon tonight for a friend for 24.91 and also some Teflon Lube for our E93 seals free shipping.
You can leave the tender in the car by the jump posts in the engine compartment and just unplug it when you want to drive it. Takes maybe 30 secs.
That is what I have done to all my rides and no failures yet...