Power and Ground Distribution: Description and Operation
Vehicle System Voltage Supply
Voltage supply
A composite unit of hardware and software ensures the voltage supply for the vehicle systems. Two software functions are of fundamental importance for the voltage supply:
1. Energy management
2. Power management
The energy management ensures that sufficient starter motor current is always available. The energy management system monitors the vehicle even at engine standstill. The energy management system monitors all components in the vehicle which generate, store and consume energy. The data for the energy management is distributed across a number of control units.
The power management a subsystem of the energy management. The power management is run from the engine control unit (DME or DDE: Digital Engine Electronics or Digital Diesel Electronics). While driving, the power management system adjusts the power of the alternator and battery charging.
Brief component description
The following components for voltage supply are described:
- Battery
- Alternator
- Junction Box Electronics
- Power distribution box
- Intelligent battery sensor
- Car Access System
- Digital Motor Electronics or Digital Diesel Electronics
- Multiple restraint system
- Bit-serial data interface
- Battery cables
- Relay box
System wiring diagram
Battery
The capacity of the built-in battery depends on the engine used and the vehicle equipment. Selection criteria for the required capacity are:
- the cold-start behavior of the engine
- the standby current consumption of the vehicle
- the energy requirement of the auxiliary consumer units (auxiliary heater, telephone, etc.)
Alternator
With the engine running, the alternator generates a variable charging voltage for battery charge. The variable charging voltage is influenced by the power management depending on the temperature and current by the DME/DDE raising the engine speed.
JBE: Junction Box Electronics
The JBE is the central data interface in the vehicle (gateway for the data buses).
The JBE is a component of the junction box. The junction box is an assembly consisting of the junction box electronics and power distribution box. The power distribution box and JBE cannot be exchanged individually.
The power distribution box contains fuses and relays. Particularly important for the voltage supply are the following relays:
- Relay terminal 15
- Relay terminal 30 switched for consumer shutdown
- Closed-circuit current cutoff relay for switch-off in the event of a fault
Power distribution box
There are the following power distribution boxes:
- Power distribution box in the engine compartment: Electronics box
- Power distribution box in junction box
- Fuse block
The fuse block can only be completely renewed. Fuses cannot be renewed individually. The fuse block contains fuses for the following consumer units:
- Valvetronic
- Common rail (fuel injection on diesel engine)
- Electric auxiliary heater
- Power distribution box in the junction box electronics
IBS: Intelligent battery sensor
Only vehicles with high equipment, e.g. CCC (Car Communication Computer), are fitted with the IBS.
The IBS is a mechatronic, intelligent battery sensor with its own microcontroller. The microcontroller is an element of the electronics module. The electronics module records the voltage, the current flowing and the temperature of the battery. The following components are fitted in the electronics module:
- a precision resistor (resistor for current measurement)
- a temperature sensor
- evaluation electronics on a printed circuit board
The IBS continuously measures the following values on the battery:
- Terminal voltage
- Charge current
- Discharge current
- Battery temperature
For data transfer, the IBS is connected to the DME (Digital Engine Electronics) or DDE (Digital Diesel Electronics) via the BSD (bit-serial data interface).
These measurement data are queried while the vehicle is being driven and when the vehicle is at a standstill:
- Driving:
- Calculation of the battery status as basis for the battery charge state (SOC: "State of Charge") and battery condition (SOH: State of Health"). Balancing the charge and discharge current of the battery. Constant monitoring of the battery state of charge and transmission of the data.
- Calculation of the current characteristics on engine start to ascertain the battery condition.
While the vehicle is being driven, the IBS transfers the data across the bit-serial data interface (BSD) to the engine control unit (DME/DDE). The software in the IBS controls the communication with the higher-level engine control unit (DME/DDE).
- Vehicle standstill
When the vehicle is at a standstill, the measured values are queried in cycles to detect energy losses. The IBS is programmed in such a way that it wakes up every 40 seconds to update the measured values with a new measurement. The measurement takes approx. 50 milliseconds (ms). The measured values are entered in the IBS in the memory for recording standby current. After restarting the engine, the DME/DDE reads out the course of the standby current. If there is a deviation from the defined course of the standby current, an entry is made in the fault memory of the DME/DDE.
CAS: Car Access System
The Car Access System participates in the terminal control (terminal R, terminal 15, terminal 30g). The terminal control supplies important messages for the voltage supply.
The CAS is connected to the following components and control units:
- The Car Access System is connected by a direct cable with the START/STOP button and with the insertion slot for the ID transmitter. The START/STOP button and the insertion slot are located next to the steering column.
- The starter motor and the DME or DDE are connected to the CAS.
The CAS control unit is a bus user on the K-CAN.
DME or DDE: Digital Motor Electronics or Digital Diesel Electronics
The DME or DDE contribute to the voltage supply as follows: If the alternator voltage falls, DME/DDE increase the engine speed depending on the requirement. The software for this is called "power management".
The DME/DDE is bus user on the PT-CAN (Powertrain Controller Area Network).
If an intelligent battery sensor is fitted, the DME/DDE will evaluate the current battery condition. The DME/DDE therefore also influences the closed-circuit current cutoff relay.
MRS: Multiple restraint system
If the restraint systems trigger, the MRS control unit sends a message to other control units. Depending on the severity of the accident, the DME switches off, for example, the electric fuel pump.
Bit-serial data interface
The bit-serial data interface is the data line between the engine control unit (DME or DDE) and the alternator.
Battery cable
2 battery cables connect the battery with the engine compartment:
- One of the battery cables leads across the jump start terminal point to the starter motor and to the alternator.
- The other line is responsible for voltage supply and runs to the fuse block.
Relay box
The assembly depends on the engine and national-market version.
- Valvetronic relay in the case of petrol engines
- Secondary air pump relay (only US version with petrol engine)
- Relay for common rail (diesel engines only)
System functions
This section describes the following system functions related to voltage supply:
- Power management
Functions of "Basic Power Management" with standard equipment as well as "Advanced Power Management" with High equipment
- Emergency operation on failure of the bit-serial data interface
- Energy management
Voltage supply for the control units, consumer shutdown of auxiliary consumer units as well as closed-circuit current monitoring
- Data transfer for the voltage supply
Power management
Power management is software in the engine control unit (DME/DDE: Digital Engine Electronics or Digital Diesel Electronics). The power management system calculates the setpoint values for controlling the voltage supply. Depending on the vehicle equipment, there are 2 versions of the power management:
- Basic Power Management, without intelligent battery sensor, in the standard equipment
- Advanced Power Management on vehicles with high equipment, with intelligent battery sensor
Basic Power Management
With Basic Power Management, the alternator voltage is regulated depending on the calculated battery temperature. The battery temperature is calculated on the basis of the ambient temperature. According to the temperature of the battery, the setpoint values for the charging voltage are calculated. This information is sent to the controller in the alternator via the bit-serial data interface.
On vehicles with petrol engines, the idle speed is raised as soon as the consumer units subject the alternator to maximum load.
On diesel engines, no idle speed increase is required. Reason: between the alternator and combustion engine, the final gear ratio is higher than on a petrol engine. This means the alternator has a high speed even at idle speed. The performance of the alternator is also high at idle speed. Raising the speed is not necessary.
Advanced Power Management
Advanced Power Management is only available on vehicles with high equipment, e.g. if the Car Communication Computer (CCC) is installed.
The intelligent battery sensor (IBS) is crucial for the larger range of function of the Advanced Power Management. The IBS provides the power management system with information on the battery condition. It is no longer necessary to calculate the battery temperature with the help of the ambient temperature. The battery temperature is measured directly by the IBS.
In Basic Power Management, only the idle speed and charging voltage can be adapted. Advanced Power Management can run the following additional functions:
- Optimized charging voltage
- Battery test
- Energy diagnosis
- Switch-off of individual consumers or power consumption reduction
Only for vehicles with high equipment and IBS
On vehicles with IBS, consumers may be reduced or even switched off altogether, even if the engine is running. While driving, only consumers that are not safety-related and not readily perceptible are reduced or switched off altogether, e.g.: pulsing of the heated rear window or pulsing of the seat heating, reduction of the blower electric motor by a fraction etc. In the case of vehicles with diesel engines, the power consumption of the electric auxiliary heater is adjusted.
The shutdown of individual consumer units or reduction of the power consumption lowers the power consumption in critical situations. This prevents the battery from discharging.
Consumer shutdown is only activated under two preconditions:
- Battery charge state in the critical range
- Alternator subjected to full load
Control of the electric auxiliary heater
On vehicles with diesel engines, the heat exchanger for the heating system is additionally heated with an electric auxiliary heater. Due to its high power requirement, the electrical auxiliary heater is regulated by the power management as follows:
- The IHKA control unit requests the electrical auxiliary heater.
- The DDE regulates the maximum electrical power output of the electrical auxiliary heater (with a pulse-width modulated signal).
The maximum power output of the electrical auxiliary heater depends on the utilization of the alternator as follows:
- Alternator used up to 70 %: The electric auxiliary heater is assigned full electric power.
- Alternator load between 70 % and 80 %: the electrical auxiliary heater may retain the power output but not increase it.
- Alternator used up to 80 %: the electrical auxiliary heater must continuously reduce the power output to 0 %.
- The electric auxiliary heater regulates the heater output of the heater cells, depending on the input from the DDE.
Energy management
The energy management system monitors and controls the vehicle's energy balance. The monitoring and control are the result of merging various components on one circuit. The energy management links functions or signals and characteristic curves for the generation and output of control signals.
The following functions are described:
- Terminal control
- Data transfer in the energy management system
- Voltage supply when the vehicle is at a standstill
- Closed-circuit current monitoring
Terminal control
There are several consumers connected to the voltage supply by terminal 30 switched or by closed-circuit current cutoff relay.
Closed-circuit current cutoff relay is only featured in vehicles with high equipment and intelligent battery sensor (IBS). With this vehicle equipment there is Advanced Power Management.
Certain consumers, however, are still supplied directly by terminal 30. For example, the anti-theft alarm must also be active when the ignition is switched off.
Data transfer in the energy management system
When the engine is at a standstill, certain consumers are switched off via terminal 30 switched in the junction box as follows: The CAS (Car Access System) switches off the relay terminal 30 switched according to a timer.
Voltage supply when the vehicle is at a standstill
The following terminals are previously known for the voltage supply of the consumers:
- Terminal 30: Terminal 30
Various consumers are still connected directly to terminal 30.
- Terminal R
The CAS switches terminal R.
- Terminal 15
The CAS switches terminal 15.
- Terminal 30 switched: activated terminal 30
Time-dependent switch-off: the relay terminal 30 switched is always present. The relay terminal 30 switched switches off the connected consumers approx. 30 minutes after terminal R OFF. If a telephone is installed in the vehicle, the after-running period is extended to 60 minutes. The relay terminal 30 switched is activated by the Car Access System (CAS).
- Standby current cutoff relay: Deactivation in the event of a fault:
The relay closed-circuit current cutoff relay is only fitted with High equipment, e.g. if the Car Communication Computer (CCC) was ordered. In the event of a fault, the closed-circuit current cutoff relay switches off the connected consumers. The relay terminal 30g-f is controlled by the junction box electronics.
The junction box electronics (JBE) monitor the standby current when the vehicle is at a standstill. The following faults are detected:
- Unauthorized waking operations on the bus systems
- Control units that keep the bus systems continuously active (prevent them from "going to sleep")
The following switch-off and switch-on conditions are required for the closed-circuit current cutoff relay:
- Switch-off conditions:
- Reception of the message "Signal off". After 5 minutes, the terminal 30g-f relay is shut down.
- The battery status is continuously read out and evaluated in the engine control unit. If the starting capability of the vehicle battery is insufficient, the relay is also switched off.
- Data transfer on the data buses for 10 minutes after terminal 30 switched is switched off without any switch-on conditions for activation.
- The vehicle is "woken up" 20 times after terminal 30 switched has been switched off without any switch-on conditions being present. The closed-circuit current cutoff relay is a bi-stable relay. Each switching mode is retained even without current. Under normal preconditions, the relay is always switched on. In the event of a fault, the relay switches the connected consumer units off.
- Switch-on conditions:
When closed-circuit current cutoff relay is switched off, it can only be switched back on by defined switch-on conditions.
Conditions required for switch-on for relay terminal 30g-f:
- Unlocking vehicle
- Opening the luggage compartment lid / flap or door
- Switching on terminal R
Closed-circuit current monitoring
Closed-circuit current monitoring is necessary for a variety of reasons.
- Achieve starting capability of the battery:
The energy management system sends a request to switch off the auxiliary consumer if the starting capability of the battery is insufficient. The auxiliary consumers must deactivate their functions irrespective of the terminal status and must have reached their standby current after 5 minutes.
- Consumer shutdown
Certain consumers may be active even when the closed-circuit current monitoring of the energy management system is already running.
The consumers are switched off according to different criteria and are divided into the following categories.
- Comfort-related consumer units
- Heated rear window
- Seat heating
The comfort-related consumer units switch off automatically after engine OFF. The switched off comfort-related consumer units can only be reactivated after a new start of the engine.
- Legally prescribed auxiliary consumer units
- Parking lamps
- Hazard-warning flashers
The legally prescribed auxiliary consumer units must remain operational after engine OFF as long as this is possible. These auxiliary consumer units are not even switched off when the limit of starting capability of the battery is reached.
- Auxiliary consumers
- Auxiliary heater function
- Independent ventilation
- Communication components (displays, terminal 30 switched, telematics services)
The auxiliary consumer units listed can be switched on after engine OFF. The auxiliary consumers switch themselves off when the limit of starting capability of the battery is reached. The switch-off is requested by the DME/DDE by means of a CAN message.
- Units with after-run due to factors inherent in the system
- Electrical radiator fan
The units with after-run due to factors inherent in the system can retain their function for a certain time after engine OFF.
Data transfer for the voltage supply
The CAS (Car Access System) forwards the data of the terminal control as follows:
- Terminal R ON or OFF
- Terminal 15 ON or OFF
- etc.
The CAS (Car Access System) switches the corresponding relays for the following terminals:
- Terminal 15
- Terminal 30g
The JBE (junction box electronics) switch the corresponding relay for the following terminal:
- Terminal 30g-f
The control units at these terminals are supplied with voltage and "woken up". The corresponding vehicle systems are activated.
The consumer units are mainly supplied via terminal 30g and closed-circuit current cutoff relay (only on vehicles with high equipment). Certain consumers, however, are still supplied directly by terminal 30. For example, the antitheft alarm must also be active when the ignition is switched off. If the battery voltage exceeds a value of 80 milliampere (mA) when the vehicle is in rest state (68 minutes after terminal R OFF) (can be set at the factory), a fault code entry is stored in the DME/DDE.
Emergency operation on failure of the bit-serial interface
If the bit-serial data interface between the engine control unit and alternator is interrupted, the alternator voltage is regulated to a constant 14.3 Volts.
Notes for Service department
General notes
The following general information is provided:
- Trickle charging for the battery
- Protection of the intelligent battery sensor
- Battery replacement
- Alternator
Trickle charging for the battery
NOTICE: Do not connect continuous trickle charger to the cigarette lighter.
The cigarette lighter is supplied with voltage by the power distribution box in the junction box via a relay. This relay drops out after terminal 15 OFF. This means that a trickle charger connected at the lighter would be disconnected from the battery. Only charge the battery via the jump start terminal point. Only then can the energy supply be registered by the vehicle.
Protection of the intelligent battery sensor
CAUTION: Danger of destruction in event of mechanical strain.
- Do not introduce any additional connections at the battery negative terminal.
- Do not modify the ground cable. The ground cable also serves heat dissipation.
- Do not establish a connection between the IBS (intelligent battery sensor) and the sensor screw.
- Do not use force when disconnecting the pole shoe from the battery terminal:
- Do not pull on the ground cable.
- Do not place any tools under the IBS to lever off the pole shoe.
- Do not use IBS connections as levers.
- Use a torque wrench and set tightening torque in accordance with repair instructions.
- Do not release or tighten down sensor screw (Torx screw).
- Avoid contact between IBS and ground.
Battery replacement
CAUTION: Danger of destruction to IBS and wiring upon battery replacement.
The IBS (intelligent battery sensor) and wiring can be destroyed by mechanical strain upon battery replacement. Observe the following when replacing the battery:
- Always proceed in accordance with the repair instructions.
- Avoid mechanical strain of the integrated battery sensor.
NOTICE: When replacing the battery, perform "Register battery replacement" service function.
Use the battery size (capacity) installed as standard upon battery replacement. The size of the battery required for the vehicle is encoded in the Car Access System (CAS) and in the engine electronics (DME/DDE).
- When installing a battery with a different capacity, re-encode the CAS. Perform "battery" retrofitting with Progman.
- Delete fault code entries relating to battery replacement in the engine control unit.
Alternator
The installation of the alternator type depends on the engine used and on the vehicle equipment.
Diagnosis instruction
Energy diagnosis
A breakdown due to a flat battery or problems in the vehicle electrical system can have wide variety of causes. In most cases, the cause does not lie with the battery itself. For this reason, battery replacement will only rarely provide a sustained solution to the problem. Instead, systematic diagnosis of the source of fault is needed. Faults are often no longer present when the vehicle comes to the workshop. This is why data stored in the vehicle is the basis for fault diagnosis. Information on the battery condition as well as functional processes in the various bus systems are stored in the corresponding control units.
This information can be called up and evaluated by the BMW diagnosis system. The BMW diagnosis system has a testing procedure for this. The procedure for energy diagnosis reads all the relevant data from the corresponding control units.
The following information is displayed:
- Noticeable information An entry is only made when a fault is supposed.
- Standard information This information can be displayed at all times.
The energy diagnosis identifies the following faults:
- Operating errors
- Faults on the vehicle
Operating errors
- Side lights, parking light or hazard warning flashers were switched on for too long when the vehicle was stopped.
- Terminal R or Terminal 15 switched on for too long when the motor was off.
- Vehicle immobilization period too long.
- Frequent short-distance drives with several power consumers switched on
Faults on the vehicle
- Battery defective
- Alternator faulty
- Too high standby current, occasionally greater than 80 milliampere (mA) when bus systems inactive
- Vehicle does not "go to sleep": The vehicle does not enter rest state; the bus systems remain active.
- Vehicle keeps being woken up
Notes on encoding/programming
The data of the battery is coded in the Car Access System (CAS). The data can be read out using the BMW diagnosis system.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.