E46 M3 convertible 2002 draining battery

I'm really hoping someone can help me. I have an E46 M3 convertible 2002 which keeps draining it's battery. We've replaced the battery twice in 2 months of owning her. I always open and close roof with engine running, turn off all electrical a (radio/ sat nav system) but if I don't use her for a week when I go to start her there's nothing happening, nothing at all - not even a slow turn over of the engine. Any ideas gratefully received. Thanks in advance!! :-)





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