Functional description

The following functions are included within the remit of the battery management electronics (SME):

• Monitoring the condition of the high-voltage battery

  That means: determining the state of charge and condition (State of Health) of the high-voltage battery

• Refrigerant shutoff valve control (with expansion valve) to cool the high-voltage battery

  The high-voltage battery is cooled by refrigerant. For this reason the air conditioning refrigerant circuit is extended to include the high-voltage battery unit. The two expansion valves for the passenger compartment climate control and for the high-voltage battery are switched in parallel. The battery management electronics use a pulse-width modulated signal (PWM signal) to activate the refrigerant shutoff valve (with expansion valve) for the high-voltage battery unit. Owing to the request from the battery management electronics, the IHKA control unit activates the air conditioning compressor if it is not already running.

• Controlling the switch contactors for switching the high-voltage system on and off

  The high-voltage battery is connected and disconnected from the high-voltage electrical system by electromechanical switch contactors inside the high-voltage battery unit. The electromechanical switch contactors are activated by the battery management electronics. The rear power distribution box supplies the switch contactors with vehicle voltage (12 V) through the safety battery terminal.

• High-voltage interlock loop

  A test lead for the high-voltage interlock loop runs through all connectors of the high-voltage components. A bridge is installed in some connectors. The test lead is arranged in a ring shape (similar to the MOST ring). The battery management electronics (SME) generate the test signal. If the high-voltage electrical system is started up, the battery management electronics feed the test signal into the test lead.

  The following control units in the ring evaluate the test signal (square wave signal with defined frequency) of the test lead:

  • Electrical machine electronics (EME)
  • Battery management electronics (SME)

  If the test lead circuit is interrupted, the EME control unit or SME control unit switches off the voltage supply for the high-voltage electrical system. Only once the test lead circuit is re-closed can the high-voltage electrical system be supplied with voltage again.

  The EME control unit and the SME control unit evaluate the test signal. If the test signal received deviates significantly from the test signal transmitted (signal level, short circuit to ground or to B+), the EME control unit or SME control unit switches off the voltage supply for the high-voltage electrical system.

• Insulation monitoring

  The battery management electronics (SME) monitors the insulation at regular intervals while the high-voltage system is active. This is to determine whether the insulation resistance between the active high-voltage components (e.g. high-voltage cables) and ground is above a required minimum value. If the value drops below the minimum, there is a risk that the vehicle components may be subject to a hazardous voltage. Ground serves as the reference potential. Without taking any additional measures, this method can only identify local insulation faults in the high-voltage battery unit. However, it is just as important to identify insulation faults in the high-voltage cables to ground inside the vehicle. For this reason, all electrically conductive housings of high-voltage components are connected to ground.

  Insulation faults can thereby be identified in the entire high-voltage electrical system. The insulation monitoring responds in two stages. Even if the insulation resistance drops below the first threshold value there is still no direct risk to people. The high-voltage system therefore remains active. No Check Control message is output. However, the fault status is recorded in the battery management electronics fault memory.

  A fault code entry is stored if the insulation resistance drops below a second, lower threshold value. A Check Control message is also output.