Parasitic draw is often a problem. Perform Closed Current Draw Test
Maximum closed current by vehicle

E31 50mA, E32 50mA, E34 40mA, E36 30mA, E38 50mA, E39 40mA, E46 40mA, E52 50mA, E53 40mA, Z3 30mA

Performing Closed Current Draw Test

Select proper measuring device (DISplus, MoDic or DVOM). Remember amperage draw in excess of 10A will damage DVOM. Use inductive amp probe of DISplus when amperage draw is high. When using DISplus inductive probe, clamp on negative battery cable with arrow pointing away from battery. Switch off all consumers. (It is not necessary to disconnect B- from body when using inductive probe)
Connect (-) test lead to negative battery terminal and (+) test lead to a known good ground.
Ensure all systems are OFF !
Be sure DVOM is on and set in proper mode.
Disconnect battery ground lead from body.
Observe meter reading, wait 16 minutes for vehicle to enter sleep mode.
Identify faulty circuit by disconnecting fuses, relays, control modules or connectors, observing meter readings.
Defective circuit is found when current is below maximum closed current for vehicle being tested.

use this link for more details http://www.e38.org/battery.pdf