In the Mercedes-Benz W221 dual battery system, for example, the task of the power control system is to provide power to all electrical equipment and components of the vehicle as required. In order to maintain reliability, the power control system provides two power lines, one for the starter battery line and the other for the main battery line. The vehicle power supply control unit (N82/1) regulates the two battery lines to different parameters and characteristics. To ensure proper vehicle operation, the two battery lines can be connected together if required.
(1) Starter battery line The starter and starter battery belong to the starter battery line section. This power supply can be used by other electrical equipment only under special circumstances. After the engine has been started, the starter battery is charged for at least 1h via the main battery line, controlled by the changeover circuit in the vehicle power supply control unit.
(2) Main battery line All other electrical equipment belongs to the main battery line. Power is supplied by the battery (GI) and the generator (m).
(3) Central electrical equipment cut-off To prevent overdischarge of the battery under all operating conditions, the state of charge of the power supply control system is continuously monitored. The vehicle power supply control unit sends a signal to the electrical equipment, which will occupy the main battery line in a specific order and according to parameterized time and voltage values. By means of this signal, the individual control unit cuts off the electrical devices or reduces the power demand of these electrical devices.
(4) Standstill current cut-off In order to achieve a longer service life of the battery when the vehicle is stationary (engine switched off), the standstill current cut-off relay, minimizes the standstill current. For this purpose, the electrical equipment is disconnected from the power supply after a predetermined period of time. When the quiescent current cut-off relay is closed, the charge converter is not in operation. (5) Emergency operation mode If the main battery is overdischarged, the battery connection relay (Bzk1) mounted in the front additional fuse box will close. When the engine is running (terminal 6 is switched on), all unimportant electrical equipment is disconnected. Only the generator produces enough power for both batteries to be charged through the generator. If there is a fault in the main battery line (e.g. generator fault, disconnection in the on-board grid line), the starter battery line is reconnected. Only the ignition switch control unit (NT3) is still able to get power from the starter battery via the on-board power supply control unit. This is important for the emergency stop (P) function, as it ensures the safety of the vehicle via the transmission when the key is removed (emergency stop function).
(6) Emergency P function The EIS control unit switches the voltage to the emergency circuit mode. The spring accumulator in the intelligent servo module with direct selection operates and the transmission switches to the parking position. The electronic and mechanical systems in the intelligent servo module with direct selection are automatically synchronized after the function is resumed.
(7) Combination instrument display The vehicle power supply control unit sends status and malfunction messages to the central gateway control unit (N93) via the chassis CAN bus and from the central gateway control unit to the combination instrument via the central CAN, bus. The red battery symbol is displayed when 1) the engine is running and no signal is received from the generator. 2) there is a fault in the vehicle power supply control unit, e.g. charging converter failure.