The E38 with M73 TU features a two-battery system consisting of an electrical system battery (DME supply), a starter battery (supply for the starter and electrically heated catalytic converter) and the disconnect switch.
The voltage of the battery for the electrical system is checked on recognition of the key contact or terminal 15. If the voltage values of the system battery U_BB are < 9 V and of the starter battery U_SB > 9 V the electronic battery disconnecting switch for supplying all control modules of the vehicle electrical system is closed for 30 seconds even if terminal 15 is deactivated during this period of time. The electronic disconnect switch is opened again if no start attempt is made within this 30 second period. The switch can close again on renewed recognition of the key contact or of terminal 15.
After the engine has been started the “engine running” signal is detected and operation of the electrically heated catalytic converter (E-cat) is triggered by the E-cat control module. The electrical power required for this purpose is supplied by the starter battery. The maximum operating time of the electrically heated converter is 30 seconds. At normal voltage levels, the electronic disconnect switch remains open during this period of time. Once the E-cat heating period has elapsed, the disconnect switch is closed thus connecting the system battery and starter battery.
If a voltage value for the system battery U_BB of < 9 V is detected the electronic disconnect switch will remain closed until E-cat operation is detected.
In the event of a break in the “E-cat ON” line or a short to U_B the disconnect switch will always be closed 4 seconds after detecting the engine speed signal.
In the case of a short in the “E-cat ON” line to ground the electronic disconnect switch will always be closed 60 seconds after detecting the engine speed signal.
The electronic disconnect switch is closed during vehicle operation to ensure the starter battery has a high charge level at all times. The starter battery and the system battery are charged by the alternator.
If an excessively high alternator current is detected, the disconnect switch will be opened to prevent the alternator overloading. The alternator now only charges the system battery. The disconnect switch is not closed again before the voltage of the system battery U_BB is higher than the voltage of the starter battery U_SB.
However, if the system voltage U_BB drops below 9 V the disconnect switch will be closed in order to ensure vehicle operation can continue. If U_BB < 9 V is detected three times within 1 minute during vehicle operation, in order to maintain vehicle operation, the electronic disconnect switch is closed permanently and only opened again after “ignition OFF”.
The following safety functions serve the purpose of protecting the electronic disconnect switch from overloading:
The electronic disconnect switch opens if the preset limit current is exceeded. A check for this fault is carried out at 10 second intervals.
If the temperature in the electronic disconnect switch exceeds the permissible limit the switch is opened until the temperature drops again.
The module is permanently switched off under overvoltage conditions.
If the voltages of the system battery U_BB are < 1 V or starter battery < 1 V this is interpreted as a short-circuit. The electronic disconnect switch remains open.