There can be a number of reasons to do an Emergency Shutdown of a battery pack and a number of approaches to this. Hence interesting to layout and discuss this topic.
It should be noted that we are discussing the BMS opening the contactors.
The main reasons for an emergency shutdown are:
- Cell Temperature
- Cell Voltage
- Over Current
- Impact
- Disconnected HV
- User request
Each of these reasons needs to be explored in terms of the scenario and in terms of the possible mitigations or lead up to the final open contactors.
This post has been built based on the support and sponsorship of: AVANT Future Mobility, Quarto Technical Services, TAE Power Solutions, h.e.l group and The Limiting Factor.
Cell Temperature
This could be because a cell is too hot or that the cells are too cold and being charged.
Before this point a request to increase the coolant flow and to reduce the charge/discharge rate would be requested. However, if the temperature is still rising then there will be very few options other than to disconnect the battery pack.
Cell Voltage
Over or under voltage could result in damage or complete failure of a cell. Similar to cell temperature an initial request to reduce the charge/discharge rate would be requested.
Over Current
If the is too high for too long it might be possible to detect and open the contactors before more significant damage is done. However, opening contactors under full current can cause damage. Normally there is a number of full load opening events that the contactors are qualified against.
Also, a very high current scenario is likely to be associated with an external short circuit and hence the current spike is most likely to blow the fuse first.
Impact
This will be an external request from the VSC or a direct input to the BMS from the crash sensor.
Disconnected HV
HVIL, or high-voltage interlock loop, is the low voltage loop that connects along with the HV system. If this gets broken due to damage or a disconnected connector it will get detected and the ultimate response is the emergency shutdown of the battery pack.
User request
This would be via the VSC and it would issue the open contactor request to the BMS.
See also: Contactors, Contactor Weld Detection, Renault Fireman Access
HVIL Open Circuit means that there is a risk that a person may come into contact with HV. Either serious consequences. But this is another example of where the decision of what to do when an HVIL Open Circuit is detected should be left to the vehicle level system control.
The fault is only a hazard when it is actually possible for someone to touch the HV contacts. Given that the HV system is not accessible from inside the passenger compartment then there is zero risk if the vehicle is moving. Therefore there is no need to isolate the HV in this circumstance.
Checkout my patent 11673472 and think it all through fully for your application.