On-Board Electrical System - Mercedes-Benz E-Class Series 207 Service Manual

On-board electrical system

On-board electrical system management
(BNM)
The on-board electrical system management monitors
the loading of the on-board electrical system and
actively intervenes in the energy budget. The aim is to
guarantee that the engine can be started and that all
the electrical consumers receive a stable power
supply.
The following components are involved in energy
management:
• On-board electrical system battery for power
storage and supply when the engine is off
• Alternator for power generation
• Battery sensor
• CDI engine control unit (with diesel engine)
• ME-SFI [ME] engine control unit (with gasoline
engine)
• Rear SAM control unit with fuse and relay module
The load state of the on-board electrical system is
determined from the characteristic variables available
in the vehicle, and suitable measures are initiated as
and when necessary.
These include the following sub-functions:
• Alternator management
• No-load current management
• Consumer reduction
• Dynamic idle speed control
Alternator management
The state of the on-board electrical system battery is
determined via the battery sensor. This uses voltage,
current and temperature measurements to define
characteristic values which serve as the basis for the
on-board electrical system control. From these values
the rear SAM control unit calculates a nominal voltage
for the alternator.
The CDI or ME-SFI engine control unit reads in this
information and, if necessary, other values (e.g. if the
air conditioner compressor is on), computes an
optimum nominal voltage from these input variables
and forwards this to the alternator. In addition the
input values are checked for plausibility in order to
rule out the following faults:
• Battery overcharged
• Insufficient battery charge
70
Introduction of the New E-Class Coupé, Model Series 207
b
– This printout will not be recorded by the update service. Status: 01 / 2009 –
The relevant CDI or ME-SFI engine control unit
compares the values of the alternator request with
those of the alternator output, and is thus able to
recognize the energy state of the on-board electrical
system.
No-load current management
The no-load current management system guarantees
that the engine can always be started. The no-load
current management system is integrated in the rear
SAM control unit and lengthens the service life of the
on-board electrical system battery.
To do this, consumers are isolated from the power
supply as necessary by actuation of the no-load
current shutoff relay. The no-load current manage-
ment function tolerates an increased level of energy
consumption for a certain period of time
(e.g. t = 75 min). This is desirable, for example, when
the radio is being used while the car is being washed.
When a period of t = 6 h has elapsed, or as soon as the
on-board electrical system voltage drops below
certain limits, the extended powerdown monitoring
function initiates the following measures:
• The no-load current shutoff relay is opened
• The no-load current shutoff function is activated
via the data bus/CAN bus
Consumer reduction
As soon as the alternator is no longer able to provide
the requested electrical output, the consumer
reduction function is activated. The on-board
electrical load is reduced by cutting back convenience
functions.
This avoids a negative power balance and ensures that
starting ability is preserved.
Consumer reduction is activated when the on-board
electrical system voltage drops to below U = 12.2 V.
The power consumption of the first consumer is
reduced t = 20 s after engine start. If the shutoff
conditions remain unchanged, the power consump-
tion of another consumer is reduced every second.