It is true the National Highway Traffic Administration (NHTSA) closed its inquiry into two Tesla battery fires Mar. 26, but contrary to reports, Tesla was not unequivocally “cleared.”
“The closing of the investigation does not constitute a finding by NHTSA that a safety-related defect does not exist,” wrote NHTSA, “and the agency reserves the right to take further action if warranted by new circumstances.”
Instead, the closing appears more like a measured compromise because Tesla made changes, and despite any reporting that suggests Tesla acted purely above and beyond the call of duty, NHTSA has not clearly stated this.
As we reported last Friday, Tesla announced it had begun equipping Model S sedans since Mar. 6 with an improved triple shield made of aluminum, titanium, and with an alloy bar to sweep away items as big as three-ball trailer hitches.
This latter road debris and another unidentified piece of debris in two separate incidents last year in Tennessee and Washington state respectively were confirmed by NHTSA as having caused fire in the two Model S sedans.
Once the battery compartment was punctured said NHTSA in its official report, the cars were soon disabled. As their drivers sought a place to pull over, one traveled approximately 0.8 miles, the other 1.8 miles as the cars’ software alerted the drivers and began to shut the vehicles down.
A fire then ensued due to the flammable nature of the batteries.
“Thermal runaway occurred in the HVB [high voltage battery] cells,” wrote NHTSA in its report. “The fires destroyed the vehicles but did not result in injuries.”
The battery chemistry from the Panasonic “automotive grade” li-ion cells used in the Model S is believed to have a greater degree of flammability than used in the Chevrolet Volt, or Nissan Leaf, for example. In exchange, it has very high energy density which accounts for the Tesla’s outstanding range and performance.
The Tesla battery cell structures are encased with a fire retardant in modules which while maybe having some effect, was not enough according to NHTSA to have stopped the thermal runaway – a term for fire spreading.
Earlier this year, after consulting with NHTSA’s Office of Defects Investigation (ODI), Tesla also issued a software update that prevented Model S drivers from lowering their vehicles to the degree that they previously had. About two-thirds of Model S sedans are equipped with an optional air-assisted suspension system which actively controls ride height.
Both vehicles involved in the fire were equipped with this system, which automatically lowered the vehicle at higher speeds.
So far Tesla has relatively few Model S sedans on the road – maybe somewhere around 30,000 more or less. NHTSA said it does not detect a trend at this point.
That is what NHTSA said, but a trend was essentially observed by the conservative watchdog group Center for Automotive Safety which has been known to hold NHTSA to task.
The Washington-based non-profit’s director, Clarence Ditlow explained his view last November that two strikes and two essentially identical fires in so short a production run “is not an outlier.”
“You clearly have a large flat target. So the potential for road debris hitting the vehicle … is higher than in, say a gas-tank vehicle,” said Ditlow suggesting a vulnerability at the time, and prior to Tesla’s upgrade last month.
Further, “road debris is a common hazard” he said, and this NHTSA said also in closing the case.
“The fact that you had two impacts shows it’s not an exceedingly rare event like being hit by a meteor so you need to upgrade the shield,” said Ditlow.
And Tesla did upgrade the shield short of being forced to do a recall, which might have been damaging to the new company’s reputation, and unknown is whether this possibility was ever discussed with NHTSA.
NHTSA did say Tesla has reported about 90 million fleet miles have accumulated since the last incident. The Washington incident occurred October and a month later in November the Tennessee fire occurred. At the time Tesla said the fleet had 90 million miles, so the fleet mileage has doubled since then.
NHTSA said all things considered it is satisfied with Tesla’s two fixes – the ride height adjustment and the shielding which Tesla is making optional. Present owners are not required to have the upgrade if they do not want to, but all future Model S sedans will be so equipped.
Tesla said the triple shield would diminish range by .1 percent, and have no meaningful impact on aerodynamic lift.
When specifically asked last week, NHTSA three times avoided direct answers to direct questions alternately by phone and e-mail as to whether it collaborated in the triple shield engineering.
NHTSA does say Tesla did the testing, but did not answer as to its involvement, or whether it advised, or urged, or informally ordered the triple shield upgrade.