1. Thermal Runaway Initiation and Propagation – Recommendations toward a test procedure
Steven Recoskie
Research Council Officer
Energy, Mining and Environment – Ottawa
Co-authors : NRC : Dean MacNeil, Sebastien Touchette, Giulio Torlone, Joel Perron, Brody McLeod; Transport Canada : Kyle Hendershot
January 23-24th, EVS 17 - GTR Detroit, USA

2. Thermal propagation requirement in current GTR draft
“5.4.12: Thermal Propagation: For the vehicles equipped with a REESS containing flammable electrolyte, the vehicle occupants shall not be exposed to any hazardous environment caused by thermal propagation which is triggered by an internal short circuit leading to a single cell thermal runaway…” As stated in : we should consider the second case, unless first case is ubiquitous and soundly proven for a given design based on field history, documentation, prequalification test or other acceptable metric (as suggested in C3 of the thermal propagation whitepaper).
1. Cell level protection,
system detection and intervention,
or insufficient heat to initiate/sustain a reaction
Internal
Short
Circuit
Heat generation
2. Single Cell
Thermal Runaway
Heat generation
Thermal
Propagation

3. B1 - What should be simulated?
No chain reaction
No chain reaction
Internal
Short Circuit
Heat generation
Single Cell
Thermal Runaway
Thermal
Propagation
Difficult to define heat generation time/energy (dependent on SC, materials, cell construction, etc.)
There’s no mechanism to conclusively identify OR ability to measure these internal properties from field data, especially after a thermal event.
Can be theorized and numerically modelled, but no validation data exist
Even internal short circuit devices have chosen characteristics that require validation (contacting surfaces, surface area, resistance, etc.).
The resultant of an ISC that leads to TR is locally applied heat generation.
TR response for a given cell type is more easily defined and reproduced than SC response.
One option: Heat generation time/energy of TR can be characterized by adiabatic ARC tests on single cells.
Does not require the definition of SC and is inclusive of the “inconclusive” causes that lead to vehicle fires.

4. B2 – Test method equivalency and technology neutrality
Suggestions and considerations:
Impossible to establish equivalency between different types of initiation methods (mechanical, electrical, thermal) and between different experiment scales (cell, module, pack, vehicle).
Ideally, one single method could be applied to all vehicle designs to reduce the challenge of finding equivalency between methods.
Since ISC conditions will be different for each cell type, the test method should be allowed to be tailored based on the cell properties (ex. capacity, format, chemistry). These adjustments can be established through single-cell characterization.
Test methods must consider how they could be implemented at the vehicle level (unless being proposed as a prequalification test) and significant modifications to BMS, REESS seal integrity, or thermal management system should not be permitted.

5. B3 – Which cell should be selected as the target cell?
In practice, an internal short circuit could occur in any cell in the pack.
A central interior cell may not be the worst case:
Unfortunately, the location of the worst-case target cell is not trivial and multiple target locations (and tests) may be required.
ALL external trigger methods will have some level of invasiveness, and so some minimal level of modifications must be tolerated.

6. B3 – Which cell should be selected as the target cell?
Suggested cell selection procedure:
Perquisite: Selection must be applicable for a vehicle-level test.
Top priority: cell selection must not impede functionality of the original design safety:
Does not disable cooling
Does not disable BMS
Does not change pack gas permeability
Minimize modifications to thermal insulators and structure (based on volume reduction)
Second priority: choose a cell that has the worst-case conditions for thermal propagation:
The best heat transfer to at least one adjacent cell (ex. thinnest spacers/gaps/barriers or vent direction toward an adjacent cell)
After satisfying “a”, the fewest heat sinks and “non-productive” thermal pathways (ex. edge cell with fewest number of adjacent cells and with the largest adjacent air space)
This necessitates an opportunistic approach and will require a different cell location selected for each pack design.
Work with OEMs to determine a few suitable locations; have final selection decided by external review

7. E1 - What should be evaluation (pass-fail) criteria?
All vehicle designs incorporate safety elements in different ways (cell/module/pack/vehicle levels).
Necessitates a vehicle-level test (at minimum) such that all safety mechanisms are engaged and method is not design restrictive. (BMS, thermal management, vehicle alert systems).
Suggested criteria (similar as current GTR phase 1 draft):
“Sufficient egress/warning time (ex. 5 minutes) between the time of activation of a notification of a potential hazard1 and the occurrence of a hazardous environment inside and near the vehicle2.”
Warning indication (active notification)
When the vehicle is in “active driving possible mode” i. Notification to the driver via telltale/audio warning
When the vehicle is not in “active driving possible mode” i. Notification in the form of horn/flashing exterior lights (similar to theft alarm)
Based on high temperatures, low oxygen concentration and acute toxicity (i.e. health risks) within the vehicle cabin and surrounding the vehicle (ex. within 2m perimeter).

8. E1 - What should be evaluation (pass-fail) criteria?
Sufficient egress/warning time (ex. 5 minutes) between the time of activation of a notification of a potential hazard1 and the occurrence of a hazardous environment inside and near the vehicle2.
Warning indication (active notification)
Based on high temperatures, low oxygen concentration and acute toxicity (i.e. health risks) within the vehicle cabin and surrounding the vehicle (ex. within 2m perimeter).
This one criteria is inclusive of the other pass cases (no TR, no TP, full containment) and does not require discretization (prequalification tests at smaller scales should be evaluated separately).
This one criteria is technology neutral (a basic requirement for vehicle occupant safety assuming single-cell thermal runaway is possible).
This one criteria is independent of the (single-cell) initiation method given that the manipulations to the original design do not influence the outcome (can be quantified by testing in different locations).

9. H1 - Key test conditions – Rapid heating
Ideal Condition
NRC Current Conditions
Reasoning
Heater
Thickness (mm)
< 5
1.2
To minimize additional foreign volume
Area (cm2)
< 25
5.6
To concentrate heat to a smallest feasible area on the cell surface
Heating Rate (°C/sec) 50
Similar to SC. To minimize unproductive heat transfer and adjacent cell preheating
Maximum heater temperature (°C)
~700°C (needs to be adjusted
Selectable °C, depending on method
700°C represents a typical TR temperature. Dependent on realistic vs optimized objective.
Heat Flux (W/cm2)
> 1 x 103
> 1 x 106
To ensure concentrated localized heat
Ratio of total input energy to cell energy (%)
< 20
< 10, typically < 5
Minimize the addition of additional energy to the system
Total heating duration (seconds)
< 180
< 180, typically < 60
Dependent on realistic vs optimized objective
Acceptable neighboring cell temperature rise (°C)
<10
This creates result bias and is unwanted
Target cell location
Multiple positions (3)
Least invasive position
Single cell TR can occur anywhere, Worst-case scenario is non-obvious
Pack modifications
None
Holes for TRIM connections
BMS and thermal management should be active. Manipulation of thermal barriers should be minimized.
Ambient temperature (°C)
Max. operating temperature
22°C +/-5°C,
typical
Higher ambient temperatures will be worst-case scenario
Test instrumentation
BMS response and/or the voltage/temperature of target cell (minimum)
Temperature and voltage of every cell or module (for research purposes)
Using BMS response only would be minimally invasive, but external voltage/temperature of target cell required for validation. NRC adds many external sensors for research studies.

10. Research update
Full-scale vehicle-level test has been postponed until Spring 2019, on account of the cold climate in Canada (outdoor test facility).
Working on new EV module and pack level tests with active cooling
Applying TRIM method to additional cell types from other applications
Collaborating with JRC to evaluate TRIM method. If other contracting parties or OEMS are interested in evaluating this method, please contact us.
Working with vehicle OEM to interpret the CANbus data collected from pack-level testing.
If interested, we welcome any OEM participation in our testing.

11. Acknowledgements
The authors gratefully acknowledge financial support for this project from Transport Canada through its Motor Vehicle Standards - Research and Development Branch, ecoTechnologies for Vehicles Program and the National Research Council through its Vehicle Propulsion Technologies Program.
Thank you for your kind attention!
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