REESS Thermal Propagation Test
1. Objective/Proposal:
The purpose of this procedure is to assess the risk from a thermal event which initiates inside of
the REESS battery case. The procedure will initiate a credible thermal event inside of the battery
case and evaluate the results at the vehicle level.
Risk areas and Safety needs:
A thermal event inside of a REESS battery can occur as a result of several types of events. The
most common events are internal cell short circuit, external short circuits, or abuse conditions.
These initiating events have been observed in the battery failure of the Boeing Dreamliner from a
internal short of a single cell, external short circuits resulting from debris intrusion on transit bus
battery systems, and vehicle fires in Tesla Model S vehicles resulting from striking road debris.
The resulting thermal event can produce expulsion of effluents and toxic gases along with heat
and fire which can potentially harm the vehicle occupants. The design of the battery pack
determines the management of thermal propagation, thermal containment, rate of energy release,
and management of gaseous material venting from cells.
Performance Criteria, Justification, and Pass/Fail Criteria:
This test procedure is applicable to all RESS-equipped HEV, PHEV and EV vehicles. Specific
guidance has been provided for application of the procedures to Li-ion based RESS systems as
Li-ion cell chemistry is the dominant chemistry in RESS at the time of this writing, however, the
approach provided could be applied to a range of other cell chemistries.
The test procedure described is composed of three parts:
a. Selecting an appropriate thermal initiation methodology,
b. Verifying the thermal initiation methodology in coupon or module level tests; and
c. Full scale in-vehicle testing to assess whether a thermal event within a RESS will pose a
significant hazard to the vehicle’s occupant or the surrounding environment.
Pass/Fail Criteria
The current GTR draft does not address the safety critical events resulting from a thermal
runaway event. The pass/fail criteria are still being researched for this test but the final
requirements should both protect the vehicle occupants for a sufficient period of time
after initiation of the thermal runaway, and ensure that the vehicle does not present a
hazard to the surrounding environment.
Thermal Initiation Method Selection Process
A thermal initiation method must be selected. NHTSA has current a current research
program to develop and validate thermal initiation methods.
The use of nail penetration has been determined as insufficient to capture the thermal
performance of a battery failure. It does not provide a good representation of the thermal
event that would occur as a result of a cell internal short. (The Dreamliner battery
failures are prime examples of underestimation of the true cell thermal output from an
internal short resulting from reliance on nail penetration test performance.)
The thermal initiation method should be appropriate for the type and size of cell being
tested.
Vehicle Level Test
The vehicle level test requires the initiation of a thermal event in the battery installed in a
full vehicle

The vehicle will be prepared by installation of the initiation method into the
battery pack.
 Temperature, effluent gas, and battery instrumentation will be installed as
specified in the procedure.
 The thermal runaway event will be initiated.
 Cabin Tenability Requirements
o The cabin must remain tenable for sufficient time to allow safe egress of
vehicle occupants after they have perceived that a serious failure has
occurred with the battery pack, or for 1 hour after initiation of a thermal
event that does not produce a condition that provides significant warning
properties to occupants. See Section Error! Reference source not found.
for further discussion.
o The cabin temperature must remain tenable, assuming vehicle windows
are closed, and HVAC system is not operating.
o The cabin air must remain free of significant inhalation hazards, assuming
vehicle windows are closed and the vehicle HVAC system is not
operating.
 Hazards to the Surrounding Environment
o The vehicle must not pose an ignition or mechanical hazard to the
surrounding environment
o The vehicle should not ignite as a result of thermal propagation testing.
o Vent gases emitted by the vehicle as a result of testing should not ignite.
o There should be no explosion as a result of testing.
o
DRAFT Test Procedures
See EVS-06-35 for current Test Procedure. (Procedure is still being refined for initiation
method)