A Battery On Wheels Turns A Diesel Semi Electric Without Touching The Engine

Fully electric semi trucks can cut operating costs for fleet operators, but they routinely cost two to three times what a diesel equivalent does, and they come saddled with range limits and charging headaches. San Francisco startup Revoy has a clever way around that, turning existing diesel semis into hybrids with a swappable, plug-in electric dolly. The setup also buys more life out of the diesel trucks a fleet already owns, rather than making it replace them outright.

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Revoy was founded in 2020 by CEO Peter Reinhardt and CTO Ian Rust. Rust, who previously worked at autonomous driving startup Cruise, said the idea grew out of collaborative robotics, closer in principle to an exoskeleton for humans than to a conventional EV drivetrain. “Instead of removing the driver, we’re using robotics to switch the fuel source,” he told Bloomberg.

What Actually Bolts On

The product is a self-powered unit that connects directly between the tractor’s fifth-wheel hitch and the cargo trailer. The electric add-on measures 13 feet (3.96 meters) long and tips the scales at 22,000 lbs (9,979 kg). Inside sits a 575 kWh lithium-iron-phosphate battery that feeds an electrically driven axle, which is where all the real work happens.

When the truck driver hits the throttle, the system reads the load demand and engages the electric motor to shoulder the heavy lifting, keeping the diesel engine from straining. Under braking, the motor flips to regenerative mode, topping up the battery while helping slow the truck. The dolly also runs active safety systems that guard against rollovers, flipping, and jackknifing, and it supports blind-spot detection and automatic reversing.

The Benefits In Fuel Economy And Emissions

With the add-on fitted, a standard semi hauling an 80,000 lbs (36,287 kg) gross vehicle weight can cover more than 200 to 250 miles (322 to 402 km) on electric power alone, which is enough to reshape how a lot of regional routes get run.

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The company says average fuel economy on the diesel hybrid can climb from the usual 6 to 8 mpg (29.4 to 39.2 lt/100 km) to 20 to 35 mpg (6.7 to 11.8 lt/100 km), and touch 40 mpg (5.9 lt/100 km) on short-haul runs under 150 miles (241 km). Those are real savings at a moment when diesel prices have spiked because of the ongoing war with Iran.

Reinhardt says US retail diesel still sits about 40 percent above the pre-war level of roughly $3.80 per gallon, and fuel usually accounts for more than a fifth of a truck owner’s operating costs. At the same time, carbon emissions drop by as much as 85 percent.

Rather than parking a rig for hours to charge that enormous battery, Revoy built the dolly to be swapped. The company is putting together a network of highway hubs where a truck can trade a drained unit for a fresh one, with the whole exchange wrapping up in under five minutes.

According to Bloomberg, the 3 million registered heavy-duty freight trucks in the United States each cover roughly 60,000 miles (96,561 km) per year. That adds up to a $900 billion market that throws off around 300 million tons of carbon dioxide annually, or 6.7 percent of all US greenhouse emissions. An earlier report from FleetOwner pegged the potential savings at an average of $25,000 per truck each year in fuel costs, which is the kind of number that gets a fleet manager’s attention.

Dolly by Subscription

Instead of selling the hardware outright, Revoy hands the electric dolly to fleet operators as a service with a subscription fee. The company secured regulatory approval from the National Highway Traffic Safety Administration in 2023, then wrapped a yearlong pilot with freight company Ryder System Inc. in 2024. Its first commercial operation starts in 2026 in Oregon, deploying an initial fleet of four dollies built from Chinese components. That run will cover a 200-mile stretch near Portland using second-hand diesel trucks bought by Revoy’s partners, with the company training those partners’ drivers to operate the dollies.

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Revoy raised $27 million in its latest funding round, pushing total funding to $41 million, reports Bloomberg. With that money, the company is developing a cheaper second-generation dolly that pairs a smaller battery with the same 200-mile EV range. The plan is to scale production and have dozens of second-gen units running US shipping routes by late 2027. For now Revoy is focused on the US, though Rust hopes the technology can eventually reach other markets.