One problem related to battery cable has been reported for the 2006 BMW 530. The most recently reported issues are listed below.
Recall campaign no. 12v-126: battery cable insulated bulkhead connector the issue involves a battery cable bolt connection in the trunk. Specifically, this bolt connection is secured to the trunk floor panel by an insulated bulkhead connector. The insulated bulkhead connector may have been incorrectly attached to the trunk floor panel. Over time, this can lead to a loosening of the bolt connection and cause an overheating condition - exactly same thing happend to me.