Electrical issue

[Tiaan]

That is correct yes.

http://www.wikihow.com/Find-a-Parasitic-Battery-Drain (Ignore the ECU sleep mode thing, a Mini doesn't have an ECU)

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Also, I don't know if you have done it already, but to give the battery a better charge voltage, run a battery cable directly from the battery negative (connecting together with the original negative wire on the battery clamp) underneath the car up to the alternator mounting on the engine block.
This will give a direct charge wire for the battery so the charge doesn't have to travel through the car's body to reach the battery.