...not to mentioned disconnecting the battery can damage the diodes in the alternator. If it wasn't bad before, it probably is now. I vaguely recall my E38 behaving as you described when the battery developed an internal short. With all that said, you would expect an engine to stay running even with a faulty battery (i.e. alternator is outputting). But, the electronics in these cars (i.e power management) can cause unexpected results/situations.