The BMW Battery Safety Terminal (BST) is used to reduce the risk of a short circuit to the B+ terminal of the battery in the event of an accident, as a result of a short circuit on the cable running from the B+ terminal of the battery to the Starter & Alternator (an unfused/unprotected circuit).
Depending on the severity of an accident incurred, the MRS control module will provide a signal to the BST which will disconnect the B+ cable going to the Starter & Alternator. In the event that the BST is activated, power will continue to be supplied to the rear power distribution box (a fused/protected power distributor) to allow activation of all other vehicle systems (such as hazard lights, power-locks, power-windows, Engine Electronics etc.).
Because the battery cables are divided up in the rear distribution box, the rest of the vehicle electrical system remains operational when the safety battery terminal is triggered as long as none of the main fuses disconnect the circuit as the result of a short circuit. This ensures that all the important functions, such as e.g. hazard warning flashers, telephone, remain operational.
E36, E39, E46, E60, E61, E63, E64, E65, E66, E67, E90, E91, E92, E93, F01, F10, F12