What causes power loss when the car is not running? Both batteries replaced on 2001 BMW 750iL

When I open the door their is no power but when I put the key in the ignition to start it I get power and the car starts. But it eventually does the same thing. Tha car is just siting when it does this. I have already had both batteries replaced. One is used and the other one is new. I had a new stater put on also. What could be the issue with what it seems to be a battery issue or connection issue.

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1 answer
are the cables very clean and i would suspect poss the used battery is not holding a load.have it load tested to be sure