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How Tesla May Beef Up Its Model S Battery Protection System


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

Enlarge Photo

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

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(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_

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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

Enlarge Photo

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

Enlarge Photo

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.


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