San Francisco startup Revoy has introduced a swappable electric dolly that can turn existing diesel semi trucks into hybrids without touching the engine.
The 575 kWh plug-in module can cut emissions by 85 percent and is available to fleet operators through a per-mile subscription model.
Fully electric semi trucks can cut operating costs, but they cost two to three times more than diesel equivalents and face range and charging issues.
Revoy's solution extends the life of diesel trucks already in a fleet, avoiding the need for outright replacement.
How the Electric Dolly Works
The self-powered unit connects between the tractor's fifth-wheel hitch and the cargo trailer.
It measures 13 feet (3.96 meters) long and weighs 22,000 lbs (9,979 kg).
Inside sits a 575 kWh lithium-iron-phosphate battery that powers an electric axle.
When the driver accelerates, the system reads load demand and engages the electric motor to assist, reducing strain on the diesel engine.
Under braking, the motor switches to regenerative mode, recharging the battery and helping slow the truck.
The dolly also includes active safety systems to prevent rollovers, flipping, and jackknifing, plus blind-spot detection and automatic reversing.
Fuel Savings and Emissions Reduction
With the dolly fitted, a standard semi hauling an 80,000 lbs (36,287 kg) gross vehicle weight can travel more than 200 to 250 miles (322 to 402 km) on electric power alone.
The company says average fuel economy 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 reach 40 mpg (5.9 lt/100 km) on short-haul runs under 150 miles (241 km).
Carbon emissions drop by as much as 85 percent.
Instead of parking for hours to charge, the dolly is designed to be swapped at highway hubs, with the exchange taking under five minutes.
According to Bloomberg, the 3 million registered heavy-duty freight trucks in the US each cover roughly 60,000 miles (96,561 km) per year, contributing to a $900 billion market and around 300 million tons of carbon dioxide annually.
An earlier report from FleetOwner estimated potential savings of $25,000 per truck each year in fuel costs.
Subscription Model and Rollout
Revoy offers the dolly as a service with a subscription fee.
The company received regulatory approval from the National Highway Traffic Safety Administration in 2023 and completed a yearlong pilot with Ryder System Inc. in 2024.
Its first commercial operation starts in 2026 in Oregon, with an initial fleet of four dollies covering a 200-mile stretch near Portland.
Revoy has raised $27 million in its latest funding round, bringing total funding to $41 million.
The company is developing a cheaper second-generation dolly with a smaller battery and the same 200-mile EV range, aiming to have dozens of units running US routes by late 2027.
For now, Revoy is focused on the US, though CTO Ian Rust hopes the technology can eventually reach other markets.
