Battery Safety Terminal
The E46 uses a trunk mounted battery which has a (B+) battery cable than runs the length of the vehicle. In the event of an accident, this cable could become compromised and there is a possibility of short circuit to the vehicle body. In order to reduce this possibility, the BST was added.
The BST is a pyrotechnic device which will disconnect the battery during an impact of sufficient severity. Although the main battery cable will be disconnected, the connection to the power distribution circuits will remain intact to allow other vehicle features to remain operational (power windows, locks etc).