When Elon Musk guaranteed that the new Tesla semi-truck would last 1 million miles without breaking down, experts assumed he was talking about the drive train, not the ultra-sensitive battery. But a person familiar with the truck tells Axios that he meant the battery, too.

Why it matters: A battery going that far would have multiple times the longevity of any commercial vehicle battery in use now or announced for release, and would help make Tesla's Semi competitive with diesel-burning competitors (since the battery is estimated to cost $170,000 on top of the $100,000-plus cost of the truck itself).

Musk announced his truck with typical showmanship (see photo above), but fleet owners who buy cargo trucks are not typically given to whims of cool and style. "They decide on the total cost of ownership," John Rapaport, a co-founder of Repower, a consultant to truck fleet owners, tells Axios. "They are very sophisticated buyers. They understand how to model out all of the variables."

By the numbers: In his Nov. 16 unveiling of the Semi, Musk said the truck will travel 500 miles on a charge when it goes on sale starting in 2019, which is a lot; Mitsubishi's new eCanter has 62.5 miles of range on a charge. Given that the standard lithium-ion battery lasts 1,000 charge-discharge cycles before replacement is recommended, that would add up to about 500,000 miles.

But a typical tractor-trailer in the U.S. will travel 100,000 miles a year or more, according to industry data. Meaning that, if Musk's truck operates like most electrics including Tesla's own passenger vehicles, it would deliver only about five years of use before the battery needs replacing.

in the U.S. will travel 100,000 miles a year or more, according to industry data. Meaning that, if Musk's truck operates like most electrics including Tesla's own passenger vehicles, it would deliver only about five years of use before the battery needs replacing. Truck drivers are also likely to use Tesla's 30-minute fast-charging technology almost every time their battery runs out, and not the slower recharging system, thus subjecting the pack's physics to regular, enormous stress, said Venkat Viswanathan, a professor at Carnegie Mellon University.

to use Tesla's 30-minute fast-charging technology almost every time their battery runs out, and not the slower recharging system, thus subjecting the pack's physics to regular, enormous stress, said Venkat Viswanathan, a professor at Carnegie Mellon University. Musk has not disclosed his planned sticker price, but experts estimate that the Semi could be $300,000, about twice the cost of new diesel trucks. Very few fleet owners would spend that kind of money if the battery needs replacing so soon, even if they do save about $100,000 in diesel over the five years of ownership, as Tesla estimates.

That Musk instead plans to guarantee 1 million miles of travel, as we have been told, suggests his team has tinkered with the truck in unprecedented ways so that the battery can undergo 2,000 charge-recharge cycles, twice the usual number, Viswanathan said.

In a series of tweets, Nikola Motor Company, a rival truckmaker, yesterday speculated that Musk is modeling his battery pack not on his passenger vehicles, but on his stationary batteries — the Powerwall that he markets for use in buildings and homes. Such a configuration — installing many more battery cells than actually required, and running them at relatively low charging voltage — would make the system much more durable, Viswanathan said.

In an email exchange, Tony Seba, who teaches at Stanford University, adds that self-driving technology will make the Semi more efficient, and suggests that drivers may not be permitted to fully discharge the batteries. "If you don't fully charge and discharge a battery, it's going to last far longer than if you do," Seba said.