A Model S driven in Kent, Washington, near Seattle, on Oct. 1 struck metal debris on the road.
Musk said in a statement on the company's Web site that the object appears to be a "curved section" that fell off a semi-trailer.
The car's warning system alerted the driver to pull over and exit the vehicle, after which a fire broke out, Tesla said.
"The geometry of the object 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," Musk said in the statement. "Had a conventional gasoline car encountered the same object on the highway, the result could have been far worse."
News of the fire, the first such incident involving a Tesla vehicle, triggered a 10 percent drop in the company's shares over two days, reflecting concerns that the incident may tarnish the automaker.
"For consumers concerned about fire risk, there should be absolutely zero doubt that it is safer to power a car with a battery than a large tank of highly flammable liquid," Musk said at the end of the note.
Tesla shares have soared fivefold this year as the electric-car maker posted its first quarterly profit and started expanding to Europe and Asia.
The Washington State Patrol and local authorities are investigating the incident. A review by U.S. safety inspectors isn't likely soon because they are among furloughed workers during the U.S. government shutdown.
Musk: "For consumers concerned about fire risk, there should be absolutely zero doubt that it is safer to power a car with a battery than a large tank of highly flammable liquid."
The force of the collision punctured the battery pack's aluminum casing, creating a 3-inch hole, Musk said.
The fire caused by the impact was in the front battery module, one of 16 in the car, and "was contained to the front section of the car by internal firewalls within the pack," he said.
The Model S battery casing offers more protection than that of gasoline-powered autos, said Musk, who is also Tesla's biggest shareholder.
"A typical gasoline car only has a thin metal sheet protecting the underbody, leaving it vulnerable to destruction of the fuel supply lines or fuel tank, which causes a pool of gasoline to form and often burn the entire car to the ground," he said.
Tesla shares rose 4.4 percent to $180.98 at the close of trading Friday in New York. bloomberg.com