High-voltage system energy management, function

S08.00-A021-79

High-voltage on-board electrical system energy management in general

High-voltage on-board electrical system energy management manages the provision (supply) and consumption (balance) of electrical energy. The goal is a low energy consumption and an optimal charging process.

The 12 V­on-board electrical system is supplied by the DC/DC converter control unit (N83/3), which works here like an alternator in a conventional combustion engine vehicle.

The energy management function is integrated in the powertrain control unit (N127).

The following components are extensively involved in high-voltage on-board electrical system energy management:

 Electric machine

 High-voltage batteries

 Battery management system control unit

 Alternating current charger for high-voltage battery

 Drivetrain control unit

 Electric machine control unit

 230 V feed socket

In detail the energy management encompasses the following functions:

 Vehicle network support

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

 Interlock circuit

 Insulation monitoring

 Energy distribution

 Drive programs "C", "E", "E+"

Vehicle network support

All implemented control units are supplied via the on-board electrical system battery (G1) of the 12 V on-board electrical system. The on-board electrical system battery sits in the seat base of the front passenger seat. When the vehicle is stationary and the ignition is not switched on, all consumers are supplied by the on-board electrical system battery.

On-board electrical system support ensures that the on-board electrical system battery is charged at all times. If the charge level of the on-board electrical system battery or the outside temperature during operating status do not reach a threshold value, they are supercharged via the DC/DC converter control unit (N83/3). The DC/DC converter control unit converts the high voltage from the high-voltage on-board electrical system to the voltage level of the 12 V on-board electrical system to supply the 12 V consumers and to charge the on-board electrical system battery. To do this the DC/DC converter control unit receives a voltage specification for the 12 V on-board electrical system from the signal acquisition and actuation module (SAM) control unit (N10/1).

After the ignition has been switched on, the available voltage of the on-board electrical system battery is checked and, as required, supercharged and supported via the DC/DC converter control unit by the high-voltage batteries (G1/18, G1/27, G1/28). The consumers of the 12 V on-board electrical system continue to be supplied by the on-board electrical system battery.

If the charge level of the on-board electrical system battery or outside temperature during operating status do not reach a threshold value, a cut-in of the high-voltage on-board electrical system takes place, even after prior testing, to support and, if necessary, charge the 12 V­on-board electrical system, including the on-board electrical system battery.

By connecting the charging cable to the 230 V feed socket (X58/23) and to the private power network (stationary service line) or a public power network (charging station), the high-voltage batteries are supercharged externally.

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

Depending on the driving condition, e.g. charging, service, or accident, the high-voltage on-board electrical system can and must be switched on or disconnected. For this purpose, the high-voltage output of the high-voltage batteries can be disconnected from the high-voltage on-board electrical system by contactors actuated by the powertrain control unit.

In the event of an accident, a security system recognizes the severity of the crash and decides whether the high-voltage on-board electrical system must be switched off reversibly or irreversibly.

 Reversible: A small amount of minor damage without triggering the airbag or impairing the high-voltage on-board electrical system. The high voltage buffer is discharged within a few seconds. The vehicle can be re-started via the ignition.

 Irreversible: The power supply of the contactors is interrupted by a pyrotechnical element. The high voltage buffer is discharged within a few seconds. The vehicle cannot be started again by the operator.

S08.00-A022-75

S08.00-A015-76

Manual power disconnect of high-voltage on-board electrical system

Two high-voltage disconnect devices are available:

 High-voltage disconnect device (Service Disconnect):

The high-voltage disconnect device (S7) in the engine compartment serves the workshop for the vehicle power disable process.

It is located in the load line of the contactors, is connected in series, and can be removed for service. The high-voltage disconnect device is located on the front right module bracket (3) in the engine compartment.

 Alternative high-voltage disconnect device for an emergency (rescue separation point):

The alternative high-voltage disconnect device is located in the seat base of the front passenger seat. It is identified with an information label (2). During this process, it is necessary to cut through the electric line at the marked separating point. The electric line is not located in the high-voltage on-board electrical system.

The standardized rescue separation point serves the emergency workers through irreversible shutoff of the high-voltage on-board electrical system during rescue and recovery operations.

 The alternative high-voltage disconnect device must be actuated only if the high voltage disconnect device (Service Disconnect) cannot be actuated in the engine compartment because this step is irreversible.

 For the rescue services, there are two special adhesive labels with the QR Code® as a link to the rescue card. It is located on the interior of the vehicle socket flap as well as on the B-pillar on the front passenger side. They mark the position of the components of the high-voltage on-board electrical system as well as the pyrotechnical equipment and disconnect devices.

Interlock circuit

If the interlock circuit is interrupted, e.g. by disconnecting a connector coupling, this leads to opening of the contactors in the high-voltage batteries due to interruption of the power supply. Contactor opening leads to power-down of the high-voltage on-board electrical system.

Active rapid discharge is also integrated in the high-voltage components, since the high-voltage on-board electrical system is not de-energized immediately after opening the contactors due to capacitors in the high-voltage components.

If the interlock circuit is interrupted during operating status, the shutoff of the high-voltage on-board electrical system takes place only after the next vehicle stop if the selector lever is set to position "N" at vehicle stillstand or the key is turned to the "OFF" position.

 To be able to perform service work without any hazard due to electric shock, the high-voltage on-board electrical system's voltage must also be switched off. For this purpose, the front right high-voltage disconnect device (S7) in the engine compartment can be actuated; this results in the interlock circuit also being interrupted.

Insulation monitoring

Insulation faults in the high-voltage on-board electrical system are detected early on due to the high-voltage components' insulation monitoring. A sensor is integrated in each high-voltage battery and constantly measures the resistance. The measured value is transmitted via the hybrid CAN (CAN L) by the battery management system control unit (N82/2) to the powertrain control unit and is evaluated there. If a certain resistance value is not reached, neither charging of the high-voltage batteries nor operating status is possible.

Energy distribution

The operation strategy in the vehicle is designed to control the flow of energy dynamically. Therefore the necessary energy is prepared for the arising driving condition in good time. To this much information flows in the intelligent controlling, which evaluates this information and allows the respective drive torques to be provided. Information includes the driver request, the requests on the vehicle and the environmental conditions.

Also the achievable service life of the energy storage system very much depends on the operating strategy. This in particular requires, in a heating case, an intelligent energy management, that uses all available energy sources, in order to increase the range of the vehicle. In this case, the energy distribution to the high voltage components, is limited depending on the current driving situation (12 V on-board electrical system, heating and climate control functions).

Drive programs "C", "E", "E+"

Three drive programs are offered. At the start, the base drive program "E" ("Economy") is active. The two programs "C" ("Comfort") and "E+" ("Economy Plus") can be selected via the drive program button (N72/1s6) in the center console.

The drive program "E+" is designed for driving as economically as possible. This includes limiting the type-related maximum speed, adaptation of the accelerator pedal characteristic as well as fixed setting of the maximum regenerative braking stage.

There is also an acceleration boost as a time-limited function which is triggered by the kickdown functionality of the accelerator pedal.

In Economy mode, the heating or air conditioning switches to an energy saving program.

 Further information for this is contained in the separate function description "Elektrofahrzeug Funktion" (Electric vehicle function).