This is me charging my 2006 MINI S.
The battery was dead and I just assumed (HA!) that it was drained from being unused for months. Nope! The electric fan behind the radiator was not turning off. I charged the battery, went for a ride and when I left the car parked the battery was drained again.
You can read more about it here. I tried the “tap the black box” solution and it didn’t work for me. For now I have disconnected the fan and will have to take the car to the dealer.