Two separate crash incidents in 2013 resulted in significant fires involving the Model S one in Washington (Oct. 13) and one in Tennessee (Nov. 13). Both incidents involved active suspension equipped vehicles operating at highway speeds and reduced ride height running over debris in the roadway.

In both incidents, NHTSA recounted, the struck objects penetrated the aluminum pan at the forward area of the battery, damaging the lithium ion cells of the HVB. Resulting thermal occurred in the battery cells; the fires destroyed the vehicles but did not result in injuries.

In the Tennessee incident, the object struck by the SV was determined to be a three-ball hitch that apparently fell from another vehicle. Tesla performed a series of tests reconstructing this incident and determined that a similar shaped object contacting the forward edge of the HVB could be “tripped” and potentially penetrate the HVB case.

As the object’s opposite end digs into the pavement, vehicle momentum causes the object to impart upward force into the case, described by Tesla as a “piking effect”.

Tesla’s testing reproduced damage similar to that seen in the Tennessee incident, and also showed that a change in ride height strategy, which was implemented in Nov. 2013 via a software update, mitigates the risk of battery compartment penetration when a three-ball hitch is struck.

The object struck in the Washington incident was not identified. More severe damage to the incident vehicle and the unknown shape of the object raised concerns regarding the effectiveness of raising the ride height for objects other than a three-ball hitch.

In a 10 March meeting with ODI, Tesla stated it would conduct a free-of-charge service campaign to modify the the Model S with an underbody triple shield system consisting first of a transverse aluminum bar; then a titanium shield; and finally a shallow angle solid aluminum extrusion.

The low-hanging transverse member and underbody plate are to be mounted forward of the battery case; the third element overlaps the leading edge of the case. Testing conducted by Tesla demonstrated that these modifications improved protection from debris impacts, NHTSA noted.

According to Tesla, the Model S fleet had accumulated ~90 million miles of service at the time it revised the ride height strategy. No further incidents have been identified, and Tesla reports the Model S fleet has traveled an additional ~90 million miles.

ODI believes impacts with road debris are normal and foreseeable. In this case, Tesla’s revision of vehicle ride height and addition of increased underbody protection should reduce both the frequency of underbody strikes and the resultant fire risk. A defect trend has not been identified. Accordingly, the investigation is closed. The closing of the investigation does not constitute a finding by NHTSA that a safety-related defect does not exist, and the agency reserves the right to take further action if warranted by new circumstances.

—NHTSA ODI