High-voltage system, function

S54.10-A042-79

 Connector couplings of the high-voltage lines in the high-voltage on-board electrical system may only be disconnected by trained specialist personnel after disconnection of the power supply.

S54.10-A050-71

 High-voltage on-board electrical system with voltage class B (DC voltage 60 V < U  1500 V and AC voltage 30 V < U  1000 V):
Only specialist workshops with the appropriate specialists may carry out repairs and rectify faults on the high-voltage on-board electrical system.

 All plugs are contact-protected.
Seating of all connector couplings is monitored by an interlock circuit.
All high-voltage lines are orange.
All high voltage components are marked with yellow warning stickers.

Function requirements in general

 Circuit 30

High-voltage on-board electrical system, general

The high-voltage on-board electrical system described in detail here consists of the following components:

 Electric refrigerant compressor (A9/6)

 Electrical machine (A79/1)

 Electric machine control unit (A79/3)

 High-voltage batteries (G1/18, G1/27, G1/28)

 High voltage PTC heater control unit (N33/7)

 Battery heater control unit (N33/8)

 DC/DC converter control unit (N83/3)

 Alternating current charger for high-voltage battery (N83/11)

 High-voltage distributor module 1 (N116/5)

 High-voltage distributor module 2 (N116/10)

 230 V feed socket (X58/23)

 Battery management system control unit (N82/2)

The battery management system control unit undertakes complete monitoring of the high-voltage batteries with regard to temperature, voltage and currents. It defines safety limits for these values and transmits values and safety limits to the powertrain control unit (N127) via the hybrid CAN (CAN L).

The high-voltage batteries serve as energy storage units and are connected via high-voltage distributor module 2 (N116/10) and high-voltage distributor module 1 (N116/5).

Some high-voltage on-board electrical system consumers (e.g. electric refrigerant compressor (A9/6)) are protected by fuses in the engine compartment fuse and relay module (K40/10) and electrically connected to the high-voltage distributor modules with threaded connections or electrical connectors.

The high-voltage on-board electrical system is supplied with energy from the high-voltage batteries and from the electric machine during overrun mode (recuperation) via a power inverter. The high-voltage batteries are charged by recuperation in overrun mode or by connection to an external power source when stationary via the alternating current charger for high-voltage battery. If the high-voltage batteries are almost completely discharged, the drive system switches off. The high-voltage batteries are thus protected from deep discharging.

The powertrain control unit acts as an interface (gateway) between the following CAN bus systems: suspension CAN (CAN E), hybrid CAN (CAN L) and electric drive CAN (CAN ED).

Function sequence for high-voltage on-board electrical system

The following tasks of the high-voltage on-board electrical system are explained in more detail:

 Stationary charging

 Interlock circuit

 Switching on and disconnecting the high-voltage on-board electrical system

 High-voltage on-board electrical system cooling

 High-voltage batteries heating

Stationary charging

The electrical energy of the electric vehicle is provided by the high-voltage batteries which must be steady-state charged beforehand.

 Further information about this can be found in the separate function description, "Steady-state charging function".

Interlock circuit

The interlock circuit is used as contact protection to protect people against inadvertent contact with high-voltage components. The interlock is an anticipatory contact which opens a little before the high-voltage connection when opening a high-voltage connection. In this way timely discharging of the high-voltage intermediate circuit is achieved by the control units of the high-voltage components.

The interlock circuit detects open lines or access points in the high-voltage on-board electrical system. In order to do this, a 12 V/88 Hz interlock signal is looped through all assembly parts of the high voltage on-board electrical system that can be removed or opened. To do this, there is a jumper in each removable high-voltage plug connection which interrupts the interlock circuit when the high-voltage plug connection is detached. When detaching a control units plug connection the interlock circuit is interrupted via the contacts interlock input and output.

Under certain conditions, interruption of the interlock circuit causes the contactors in the high-voltage batteries to open and the entire high-voltage on-board electrical system to shut down. The battery management system control unit (N82/2) and the alternating current charger for high-voltage battery (N83/11) each have their own evaluation circuit for the interlock signal. The error statuses can be determined by evaluating the interlock signals in the active components (high-voltage sources) (open circuit, short circuit). Here, the respective error statuses are reported to the powertrain control unit via the hybrid CAN (CAN L). The central control and evaluation of the interlock circuit is located here.

Switching on and disconnecting the high-voltage on-board electrical system

The high-voltage on-board electrical system can or must be switched on or disconnected depending on the driving condition, e.g.  charging, service or accident.

 Further information about this can be found in the separate function description, "High-voltage on-board electrical system energy management function".

High-voltage on-board electrical system cooling

The operating temperature of the high-voltage batteries and the electric machine must fall in a specific range in order to ensure that the charging capacity, number of charging cycles and life expectancy of the high-voltage batteries or operation of high voltage components are optimized and secured. The high-voltage batteries and the electric machine are therefore part of the low temperature coolant circuit.

The low temperature circuit 1 protects high voltage components and the electric machine from damage caused by overheating. The temperature of the high-voltage batteries is controlled by low temperature circuit 2.

 Further information about this can be found in the separate function description, "High-voltage on-board electrical system cooling function".

S54.10-A059-76

High-voltage batteries heating

Because the efficiency of the high-voltage batteries is dependent on temperature, it decreases significantly as the temperature drops. In addition, the electrolytes of the high-voltage batteries can congeal at very low temperatures. For this reason, the battery pack is automatically heated up during operating status and, if necessary, during the charging process at battery temperatures of less than ­15 °C for an improved operating and charging temperature. This support is terminated automatically when approx. ­15 °C is reached.

Thermal management automatically controls temperatures in the vehicle interior and battery and electrics components after system start. A "Heat pump function" can be used for heating up water with the determined temperature values and the particular function of this system.

The high-voltage batteries are heated by the battery heater control unit (N33/8) integrated into the coolant circuit and this ensures that the interior temperature of the high-voltage batteries does not drop below ­15 °C.

The battery heater control unit is a high voltage PTC heater booster and is actuated by the powertrain control unit. The heating elements are heated by energizing them. The coolant flowing through the battery heater control unit absorbs the heat of the heating elements and is heated as a result.

The heated coolant is circulated via coolant pump low temperature circuit 2 (M75/17). The resulting heat is delivered to the high-voltage battery via coolant circuit low temperature circuit 2. Once the operating temperature of the high-voltage batteries has been reached (approx. 20 to 35 °C), the heating elements of the battery heater control unit are no longer energized.