Unless your name is Brinks,  you rarely hear the words "armor" and "car" mentioned in the same sentence.

But two recent widely-publicized fires in the Tesla Model S electric car have put the spotlight on how its battery pack is protected from severe impacts underneath the car. 

MORE: Tesla Fires: What We Know, And What We Need To Find Out

Both fires were caused by high-speed collisions with road debris, which apparently struck the underside of the cars, where the battery is located under the floorpan.

Tesla Motors - Model S lithium-ion battery pack

Tesla Motors - Model S lithium-ion battery pack

(A third fire in Mexico came after such a violent collision that a gasoline car may well have suffered the same fate, or worse.)

Hole punched

In the first incident, in Kent, Washington on October 1,  a Model S ran over a large metal object that, according to Tesla,  "...caused a powerful lever action as it went under the car, punching upward and impaling the Model S with a peak force on the order of 25 tons.

"Only a force of this magnitude would be strong enough to punch a 3-inch diameter hole through the quarter inch armor plate protecting the base of the vehicle," the company said.

That penetration triggered a battery fire that, while limited to one small segment of the battery, eventually consumed the front end of the car in a blazing inferno.

In the second incident, the Model S driver reported that he ran over a  "rusty three-pronged trailer hitch" on the highway at about 70 mph.

Third Tesla Model S catches fire after hitting road debris. Photo via Twitter user @NASHVILLAIN_

Third Tesla Model S catches fire after hitting road debris. Photo via Twitter user @NASHVILLAIN_

He continued driving for about 2 minutes after the impact, then pulled over when the car warned him that he should pull over immediately, because it was about to shut down. The fire started in the nose of the vehicle soon after the driver got out of the car

Thus far, there's no information on where (or whether) the trailer hitch penetrated into the car's battery. Photos show the fire well ahead of the front edge of the main battery, so it's possible the battery was not involved in the conflagration at all. 

Heavy metal

The Model S battery pack--about nine feet long, five feet wide, and  four inches thick--essentially forms the entire underside of the car between the axles.

The exterior surface is a smooth quarter-inch thick metal plate designed to protect the battery against impacts from below.

Tesla has not publicly responded to questions asking what material the quarter-inch "armor plating" is made of.

Tesla Model S battery pack, from U.S. patent 8286743 for Vehicle Battery Pack Ballistic Shield

Tesla Model S battery pack, from U.S. patent 8286743 for Vehicle Battery Pack Ballistic Shield

About two weeks ago, I e-mailed a Tesla spokesperson asking that question, but got no response. (Other journalists have had the same experience.) Questions about the construction and testing of the battery pack also went unanswered.

A few days ago, a  technician at my local Tesla service center--not an official spokesman--assured me that the armor plating was steel. 

However, Car and Driver recently reported that an unnamed Tesla source had said that the Model S battery armor is in fact aluminum.

The penguin test

Who's right? Meeble knows.

Meeble is my 17-year-old daughter's beloved penguin refrigerator magnet, a prized possession since she was eight.

In a flash of inspiration, I grabbed Meeble off his Frigidaire perch and took him out to the driveway. There I placed him against the the bottom of my 60-kWh Model S.

He immediately fell to the ground.  

Okay. It's aluminum. (Or possibly some form of non-magnetic stainless steel, although that may be somewhat less likely given that the Tesla body shell is roughly 97 percent aluminum.)

Weight watcher

For maximum performance and efficiency, aluminum certainly makes sense.  A  quarter-inch steel plate big enough to cover the bottom of the Model S battery pack would weigh some 400 pounds. A similar aluminum plate would weigh only about 150 pounds.

2012 Tesla Model S body-in-white

2012 Tesla Model S body-in-white

But aluminum is a comparatively soft metal, with less impact resistance than steel. Is it strong enough to be armor? The U.S. Army apparently thinks so.  Its M113 armored personnel carrier uses aluminum armor.  

But, according to the Army Guide,  the M113's aluminum protection is essentially considered "Armor Lite."

"Unfortunately, as the welded armored plating used on the M113 has a maximum thickness of 35mm," says the Guide, "it can only serve as protection against shell splinters and small calibre projectiles."

A thickness of 35 mm is about 1.4 inches--almost six times thicker than the Model S battery shield. So characterizing quarter-inch aluminum as "armor" may be a bit of a stretch.

A rock and a hard place

A couple of days ago,  when I inspected the bottom of my Model S while it was on a lift, the battery armor certainly looked impervious to me.

I had my car on the lift to inspect its underside  after a minor impact incident of my own. Rolling along an unfamiliar unpaved driveway at about 5 mph,  I inadvertantly bottomed out on an embedded rock.

Tesla Model S battery pack, from U.S. patent 8286743 for Vehicle Battery Pack Ballistic Shield

Tesla Model S battery pack, from U.S. patent 8286743 for Vehicle Battery Pack Ballistic Shield

It made quite a substantial clunk/thump. The car behaved normally afterward, but with all the hooh-hah about fires and underside damage lately, I decided to have Tesla service inspect the battery casing.

The technician quickly found a light scuff mark, about two feet long, near the rear of the battery case. There was no deformation, no gouging. 

Isolated incidents?

For such a major thunk against an embedded rock, it seemed to be an almost trivial amount of damage. The scuff was barely visible. Clearly, in my case the battery's protective armor had done its job perfectly, no sweat.

2013 Tesla Model S electric sport sedan [photo by owner David Noland]

2013 Tesla Model S electric sport sedan [photo by owner David Noland]

But twice in the past six weeks, the Model S aluminum armor has apparently failed to do the job. 

Should these two impact-related fires be considered minor isolated incidents? (No one was hurt, after all.)

Or does Tesla need to beef up its battery-protection armor to prevent future incidents?

Possible armor upgrade 

If Tesla does decide to upgrade, there's a blueprint for improvement already in place. 

A Tesla patent, unearthed by faithful reader John C. Briggs, outlines a detailed plan for "an improved protection system for a battery pack mounted between the passenger cabin floor panel of an electric vehicle and the driving surface."

The application, number 13311435, subsequently became patent 8286743, granted to Tesla on October 16, 2012, for a "vehicle battery pack ballistic shield."

The application was filed December 5, 2011 and published on June 28, 2012, just a week after the first production Model S rolled off the assembly line.

The name on it is "Rawlinson," referring to Peter Rawlinson, Tesla's chief engineer in December 2011.

Peter Rawlinson, Tesla Motors vice president and chief engineer

Peter Rawlinson, Tesla Motors vice president and chief engineer

Carbon fiber and crush zones

Because Tesla closely guards technical information, it's unclear whether the "improved protection system" describes the current battery shielding system or a future update.

It's not unreasonable to assume that the new design in the patent could be considered  "improved" over the one used in current cars--likely the one in the granted patent.

The patent describes a "ballistic shield....formed of a relatively light-weight metal (e.g., aluminum, aluminum alloy, etc.) or a composite such as a carbon fiber/epoxy composite."

A carbon fiber/epoxy composite shield would be even lighter than an aluminum one, but far more expensive.

The document goes on to describe a strategy of building in a "crush zone" of space between the outer shield and the inner battery case. Citing drawing reference numbers, the document reads,

In order to prevent road debris that impacts shield layer1203 from transferring that energy directly into the lower surface 603 of the battery pack, layer 1203 is spaced apart from (the) pack.  As a result, when an object impacts layer 1203 (the outer shield), the battery pack remains undamaged even if layer 1203 is deformed.

Compressible material

The patent suggests possible spacings from 10 mm  (about half an inch) up to 50 mm, about 2 inches. It recommends 50 mm as the most desirable spacing. Structural members called stand-offs are used to separate the ballistic shield from the battery proper.

The document continues: "A layer of compressible material may be interposed between the ballistic shield and the battery pack enclosure.....The compressible layer may be fabricated from foam or (injection-molded) plastic."

Tesla Motors - Model S lithium-ion battery pack

Tesla Motors - Model S lithium-ion battery pack

An "egg crate" shape is suggested for the compressible material between the shield and the battery case.

The document also describes a thermal protection layer, or firewall, on top of the battery and below the interior floor of the car. It is designed to  prevent or slow the spread of a battery fire into the passenger compartment.

Various levels of thermal conductivity are described for the firewall, the safest of which would withstand continuous temperatures of about 1,800 degrees F.,  and 2,500 degrees F. for one second.

Your move, Tesla

We don't know precisely how the current Model S ballistic shield is constructed, only that it is made from quarter-inch aluminum.

But the prospect of a carbon-fiber ballistic shield, backed by 2 inches of  crush space filled with compressible foam, is an intriguing one. And we like the idea of an 1800-degree F firewall on top of the battery, too.

Will the hot glare of media, investor, and public scrutiny lead Tesla to improve its current aluminum shield? Or has it already built these improvements into its latest cars?

Stay tuned.


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