A System Engineer’s Thoughts on Contactor Control

As a vehicle level systems engineer who spent many years working with an HV battery team, I naturally have some thoughts about opening and closing contactors. Here is my starter for 10 on the topic of ‘Control contactors’ 😇

Contactor Control is another one of those areas where responsibility is divided between a vehicle’s system level control and the battery management control. From the systems engineering perspective;

• The vehicle system should decide when the contactors are requested to open or close
• The battery management should decide if it can open or close the contactors

Whenever there is a division of responsibility between the system and component teams, there must be an early discussion and agreement so it is clear to all what needs doing and who is responsible for making it happen. One party making assumptions or unilateral decisions is a good way of failing to meet legislative or operational requirements!

The vehicle system is aware of the overall vehicle status and the driver’s intensions.
The vehicle system knows about and understands many things including;

• Ignition status
• What the driver is trying to do; stop or drive off
• HV system isolation status
• Contactor weld status — the battery will have shared the status
• Connector status obtained by either HVIL or other means
• HV Battery capability
• The currents flowing in the HV system
• The voltages at different points in the HV system
• Vehicle crash status

Using this information, the vehicle system control can make an informed decision to request the contactors to close or open. The vehicle system has enough information to know if it is safe to close the contactors even if certain HV system faults are present such as loss of isolation.

The battery management system may know quite a lot about the HV system. For example, it probably hosts the HV isolation monitoring function or the HVIL function. It will certainly know if any of its contactors are welded or stuck open. It will also know if it is;

• safe to close the contactor considering the voltages either side of the contactor.
• safe to open the contactor considering the current flowing through it

The battery management system is not aware of everything else that is going on in the rest of the vehicle.

In ‘normal operation’ closing the contactors needs a sequence something like this;

1. Deciding if the vehicle operating state needs the contactors closed
   • Clearly a vehicle system level decision.
2. Getting the HV bus voltage close enough to the HV battery voltage to close the contactor without risk of welding it
   1. A particular vehicle may have several ways of increasing the HV bus voltage to match the HV Battery voltage. For example, the traditional pre-charge resistor implemented in the HV battery, a switch mode power supply from the 12v system, using the back EMF from the traction inverter or BSG (Belt Starter Generator)
   2. Whatever mechanisms are available, there must be an early discussion and agreement between the vehicle system and battery teams about how, when and who chooses which will be used
3. Deciding if the bus and battery voltages are close enough to close the contactors without risk of welding
   1. The vehicle system can send the close request and the battery management decides when to close
   2. The vehicle system can monitor the voltages and only send the close request when the voltages match well enough
4. Providing power for the contactor coil to close the contacts
   1. Usually a battery management function
5. Deciding that the contactor is closed and publishing the status to the vehicle system
   1. Usually a battery management function

Similarly, in ‘normal operation’ opening the contactors needs a sequence something like this;

1. Deciding if the vehicle operating state needs the contactors open
   1. Vehicle system decision
2. Reducing the current through the contactor to a low enough level to open without the risk of damage caused by arcing
   1. Vehicle system managed
3. Deciding if the contactor current is low enough to open the contactor without risk of damage due to arcing. There are 2 options;
   1. The vehicle system can send the close request and the battery management decides when to open
   2. The vehicle system can monitor the current and only send the open request when the current is low enough
4. Removing the contactor coil power to open the contactor
   1. Usually a battery management function
5. Deciding that the contactor is open and publishing the status to the vehicle system
   1. Usually a battery management function

In ‘abnormal operation’ it may be necessary for the battery management system to make a unilateral decision to open the contactor, for example when;

• Vehicle crash is detected
• HV overcurrent is detected
• HV Battery over temperature is detected
• Loss of communications is detected

However, such a unilateral decision can happen when very high currents are flowing through the contactors. The resulting arc can weld the contactors defeating the objective of trying to open them!

Working closely with the systems team can help mitigate this weld problem. Analysing the communication path for signals carrying the fault status and understanding the rest of the system response may mean that the battery management can wait for the vehicle system to reduce the contactor current before opening the contactors.

Conclusion
Making the decision to close or open the contactor safely is complicated. Lots of factors should be considered, communication timings, what the vehicle system can do to help, what the battery needs and what the legislation actually means all require detailed understanding. Close working between the system and battery management teams are the best way to ensure success😊