Electrics
Page 9.11
Vehicle Electrical System Management
The main function of the vehicle electrical system control unit is to
protect the battery against excessive cyclical loads. Since functions that
are relevant to the driver are controlled by vehicle electrical system
management and technical safety must be assured at all times, a lot of
input data is required on the vehicle electrical system control unit. The
status of the vehicle electrical system is mainly determined by
comparing the determined voltage with the permitted lower voltage
limit.
The status of the vehicle electrical system is determined from the
battery voltage values and information relating to high current loads with
a short “on” time. The load switching states currently requested by the
driver are also stored in the vehicle electrical system control unit. The
vehicle electrical system control unit communicates with the DME
control unit in order to increase the idling speed if necessary for the
purpose of improving the vehicle electrical system status.
Comfort loads can be switched off if necessary, i.e. if the vehicle elec-
trical system status is too negative. The increased battery voltage is
then made available to other control units via the CAN bus. Before load
switch-off (e.g. plug sockets and cigarette lighter) is activated, the driver
is alerted by the warning “Battery low” in the instrument cluster. Once
the vehicle electrical system status is no longer critical, the loads are
switched back on again in reverse order. Diagnosis of the vehicle elec-
trical system control unit is performed using the PIWIS Tester.
Determining Battery Voltage
The battery voltage is determined in a number of different ways. The
current voltage is measured once and the measured value is filtered so
that short-term voltage drops, e.g. when servo motors are switched on,
do not render the value incorrect. A time based mean value is also
determined (unweighted, mean value over a fixed time period) – this has
very little effect on short-term loads. If a high-current load is switched on
(e.g. status on the CAN bus) and if the current voltage value is lower
than the determined value, the mean value is used to detect the vehicle
electrical system status. Otherwise the current measured voltage value
always has priority.
Notes
Short term loads are defined by an
expected “on” time “t
ON
” of less than 30
seconds at a current of more than 10 A.
To Next Page
Loading...
Need help?
General
