I lived this exact scenario with my E39. It turned out to be an electrical drain on the battery caused by a malfunctioning electric fan. While this may not be the exact cause of your problem, I would suspect a short or unexpected drain on your battery. Troubleshooting may find the problem - check the draw on your battery and start eliminating circuits one by one until the circuit that is draining your battery is identified.
2013 535xi, Carbon Black, beige interior, M Sport package, HUD, DDC & ARS
2010 Z4 35i, Crimson Red, beige interior, 6sp manual, Adapative Suspension
2000 528i, Cosmo Black, beige interior, Sport package (retired)