Battery Cable Diagnosis

Battery Cable Diagnosis





For the E60, the battery cable is routed from the luggage compartment outside on the underbody of the vehicle into the engine compartment.
If the cable is damaged in an accident or when driving over an obstacle (e.g. crash barrier), the battery cable is disconnected from the battery and the alternator is switched off. This prevents a short circuit and the risk of sparks.
The battery cable is fitted with a low impedance metal mesh, which is insulated against the body and against the battery cable. This metal mesh is referred to as the monitoring shield.
The battery cable is diagnosed by a special circuit between the SBSL and SBSR satellites.





The following circuit shows the functional principle of the battery cable diagnosis.
Battery cable diagnosis takes place by means of the low impedance metal braiding of the battery cable (= monitoring shield). A connection cable exits from each end of the monitoring shield (at the safety battery terminal in the luggage compartment and at the battery earth point in the engine compartment).
The connection at the safety battery terminal in the luggage compartment is connected to the right B pillar satellite. The second connection cable in the engine compartment is connected to the left B-pillar satellite.
The satellites contain analog/digital converters that are connected to the microprocessor of the satellite. The connection cables of the battery cable diagnosis are connected to the analog/digital converters. The right B-pillar satellite contains a pull up resistor. The left B-pillar satellite contains a pulldown resistor of the same size.
The voltage supply of the satellite (approximately 10 V) is applied at the pull up resistor. Ground is applied at the pulldown resistor. The very low impedance cable and the resistors of the same size mean that around half the voltage (approximately 5 V) is applied at the A/D converters.





In the event of a fault, significantly different measured values result as follows:
Every 250 microseconds, the values are measured, triggered by the synchronization pulse. If the battery cable is OK, the values are transferred every 20 ms to the SIM. If a significant deviation of the values occurs, the new values are transferred immediately.
In the following cases, the battery cable is cut off by the safety battery terminal from the battery:
^ Short circuit to earth (body)
^ Short circuit to battery positive
The alternator and the electric fuel pump are also deenergized at the same time.