Not luck or design...it just happened.
When I bought my Mini it had about 5 miles on it. I spec ordered the car. When I got home I put my battery charger on it (80 mile daytime ride home). Well, it took 40 minutes to top off the battery!
In a year of ownership, I find the battery needing topping off any time I feel like hooking it up. I don't think the car would ever "top off" the battery through any normal driving. I hook the charger up at least once every thirty days and top it off, which usually takes 15-20 minutes. It's a good practice, imo. Batteries develop "memory" and if left at a constant level of discharge, will eventually refuse to charge beyond that "memory" mark.
All you guys with Minis and a good charger...try that out tonight and time how long it takes to top off. I'm interested in hearing. Set your charger on the ten amp setting and see how long it takes.

 

Battery Tender makes a great product for keeping them at a fully charged status. I keep the batteries on a couple motorcycles in great shape with their chargers. Plug them in and forget about it til you're ready to ride.