You know everybody, there is physics that applies to the lead acid batteries we use to start our vehicles.
BMW batteries DO NOT last only 2 years. I have friends who bought 2006 330i's and they are still on their original batteries. You hit the nail on the head, you don't drive your car daily. Did you load test your battery after fully charging it? What is this "trickle charger" that you mention, what make, model.
I cannot understand why people dance around testing their batteries. It's like saying doc, I can't breathe and I feel as if my heart will stop beating. I seem to have gained 50 lbs. since I last saw you 3 years ago. Meanwhile, your doctor doesn't want to perform any tests and simply asks more questions and tries to guess what the problem is.