I purchased my 2004 R53 MCS new in June 2004. It is one of my summer cars and stored inside during winter for about 6 months. I use battery tender 12V 1.25A charger-maintainer to keep my battery charged and extend its life.
Today, the battery finally gave up and this is how I discovered it: rotten egg smell (hydrogen sulphide gas) inside my garage! My trickle charger is also flashing Red. I checked the voltage and only got 10.8V, started the car and never cranked, lights are dim and warning lights lit & flashed. 15 years of service from the OEM battery is extraordinary good. I was tempted to get a replacement from Mini if it gives me 15 years again but I heard the batteries are not made as durable as OEM. I decided to try AC Delco 47PG Gold Series, $176 CAD before tax. Size fits perfectly and has fume vent matching location of the OEM battery. The AC Delco has 2 vent ports on both ends and I sealed the (-) side with urethane caulk. It comes with 42 Months free replacement warranty.
AC Delco 47PG: CCA=650 : CA=810 : RC=115 : 60Ah
OEM Mini : CCA=480 : CA=480 : RC=90 : 55Ah

Just a warning to check your car and don't just leave the battery charger-maintainer at work. Hydrogen Sulphide gas is toxic and explosive.