The problem with plug-in hybrids? Their drivers.

Plug-in hybrids are supposed to be the best of both worlds—the convenience of a gas-powered car with the climate benefits of a battery electric vehicle. But new data suggests that some official figures severely underestimate the emissions they produce. 

According to new real-world driving data from the European Commission, plug-in hybrids produce roughly 3.5 times the emissions official estimates suggest. The difference is largely linked to driver habits: people tend to charge plug-in hybrids and drive them in electric mode less than expected.

“The environmental impact of these vehicles is much, much worse than what the official numbers would indicate,” says Jan Dornoff, a research lead at the International Council on Clean Transportation.

While conventional hybrid vehicles contain only a small battery to slightly improve fuel economy, plug-in hybrids allow fully electric driving for short distances. These plug-in vehicles typically have a range of roughly 30 to 50 miles (50 to 80 kilometers) in electric driving mode, with a longer additional range when using the secondary fuel, like gasoline or diesel. But drivers appear to be using much more fuel than was estimated.

According to the new European Commission report, drivers in plug-in hybrid vehicles produce about 139.4 grams of carbon dioxide for every kilometer driven, based on measurements of how much fuel vehicles use over time. On the other hand, official estimates from manufacturers, which are determined using laboratory tests, put emissions at 39.6 grams per kilometer driven.

Some of this gap can be explained by differences between the controlled conditions in a lab and real-world driving. Even conventional combustion-engine vehicles tend to have higher real-world emissions than official estimates suggest, though the gap is roughly 20%, not 200% or more as it is for plug-in hybrids.

The major difference comes down to how drivers tend to use plug-in hybrids. Researchers have noticed the problem in previous studies, some of them using crowdsourced data. 

In one study from the ICCT published in 2022, researchers examined real-world driving habits of people in plug-in hybrids. While the method used to determine official emissions values estimated that drivers use electricity to power vehicles 70% to 85% of the time, the real-world driving data suggested that vehicle owners actually used electric mode for 45% to 49% of their driving. And if vehicles were company-provided cars, the average was only 11% to 15%.

The difference between reality and estimates can be a problem for drivers, who may buy plug-in hybrids expecting climate benefits and gas savings. But if drivers are charging less than expected, the benefits might not be as drastic as promised. Trips taken in a plug-in hybrid cut emissions by only 23% relative to trips in a conventional vehicle, rather than the nearly three-quarters reduction predicted by official estimates, according to the new analysis.

“People need to be realistic about what they face,” Dornoff says. Driving the vehicles in electric mode as much as possible can help maximize the financial and environmental benefits, he adds.

It’s important to close the gap between expectations and reality not only for individuals’ sake, but also to ensure that policies aimed at cutting emissions have the intended effects. 

The European Union passed a law last year that will end sales of gas-powered cars in 2035. This is aimed at cutting emissions from transportation, a sector that makes up around one-fifth of global emissions. In the EU, manufacturers are required to have a certain average emissions value for all their vehicles sold. If plug-in hybrids are performing much worse in the real world than expected, it could mean the transportation sector is actually making less progress toward climate goals than it’s getting credit for.

Plug-in hybrids’ failure to meet expectations is also a problem in the US, says Aaron Isenstadt, a senior researcher at the ICCT based in San Francisco. Real-world fuel consumption was about 50% higher than EPA estimates in one ICCT study, for example. The gap between expectations and reality is smaller in the US partly because official emissions estimates are calculated differently, and partly because US drivers have different driving habits and may have better access to charging at home, Isenstadt says.

The Biden administration recently finalized new tailpipe emissions rules, which set guidelines for manufacturers about the emissions their vehicles can produce. The rules aim at ramping down emissions from new vehicles sold, so by 2032, roughly half of new cars sold in the US will need to produce zero emissions in order to meet the standards.

Both the EU and the US have plans to update estimates about how drivers are using plug-in hybrids, which should help policies in both markets better reflect reality. The EU will make an adjustment to estimates about driver behavior beginning in 2025, while the US will do so later, in 2027.

New York City’s plan to stop e-bike battery fires

Walk just a few blocks in New York City and you’ll likely spot an electric bike zipping by.

The vehicles have become increasingly popular in recent years, especially among delivery drivers, tens of thousands of whom weave through New York streets. But the e-bike influx has caused a wave of fires sparked by their batteries, some of them deadly.

Now, the city wants to fight those fires with battery swapping. A pilot program will provide a small number of delivery drivers with alternative options to power up their e-bikes, including swapping stations that supply fully charged batteries on demand. 

Proponents say the program could lay the groundwork for a new mode of powering small electric vehicles in the city, one that’s convenient and could reduce the risk of fires. But the road to fire safety will likely be long and winding given the sheer number of batteries we’re integrating into our daily lives, in e-bikes and beyond.

A swapping solution

The number of fires caused by batteries in New York City increased nearly ninefold between 2019 and 2023, according to reporting from The City. Concern over fires has been steadily growing, and in March 2023 Mayor Eric Adams announced a plan to address the problem that included regulations for e-bikes and their batteries, crackdowns on unsafe charging practices, and outreach for delivery drivers.

While batteries can catch fire for a variety of reasons, many incidents appear to have been caused by e-bike drivers charging their batteries in apartment buildings, including a February blaze that killed one person and injured 22.

The city’s most recent effort, designed to address charging, is a pilot program for delivery drivers who use e-bikes. For six months, 100 drivers will be matched with one of three startups that will provide a charging solution that doesn’t involve plugging in batteries in apartment buildings.

One of the startups, Swiftmile, is building fast charging stations that look like bike racks and can charge an e-bike battery within two hours. The other two participating companies, Popwheels and Swobbee, are proposing a different, even quicker solution: battery swapping. Instead of plugging in a battery and waiting for it to power up, a rider can swap out a dead battery for a fresh one.

Battery swapping is already being used for some electric vehicles, largely across Asia. Chinese automaker Nio operates a network of battery swapping stations that can equip a car with a fresh battery in just under three minutes. Gogoro, one of MIT Technology Review’s 2023 Climate Tech Companies to Watch, has a network of battery swapping stations for electric scooters that can accommodate more than 400,000 swaps each day.

The concept will need to be adjusted for New York and for delivery drivers, says Baruch Herzfeld, co-founder and CEO of Popwheels. “But if we get it right,” he says, “we think everybody in New York will be able to use light electric vehicles.”

Existing battery swap networks like Nio’s have mostly included a single company’s equipment, giving the manufacturer control over the vehicle, battery, and swapping equipment. That’s because one of the keys to making battery swapping work is fleet commonality—a base of many vehicles that can all use the same system.

Fortunately, delivery drivers have formed something of a de facto fleet in New York City, says David Hammer, co-founder and president of Popwheels. Roughly half of the city’s 60,000-plus delivery workers rely on e-bikes, according to city estimates. Many of them use bikes from a brand called Arrow, which include removable batteries.

Convenience is key for delivery drivers working on tight schedules. “For a lot of people, battery charging, battery swapping, it’s just technology. But for [delivery workers], it’s their livelihood,” says Irene Figueroa-Ortiz, a policy advisor at the NYC Department of Transportation.

For the New York pilot, Popwheels is building battery cabinets in several locations throughout the city that will include 16 charging slots for e-bike batteries. Riders will open a cabinet door using a smartphone app, plug in the used battery and take a fresh one from another slot. Based on the company’s modeling, each cabinet should be able to support constant use by 40 to 50 riders, Hammer says.

“Maybe it leads to an even larger vision of battery swapping as a part of an urban future,” Hammer says. “But for now, it’s solving a very real and immediate problem that delivery workers have around how they can work a full day, and earn a reasonable living, and do it without having to put their lives at risk for battery fires.”

A growing problem

Lithium-ion batteries power products from laptops and cellphones to electric vehicles, including cars, trucks, and e-bikes. A major benefit of the battery chemistry is its energy density, or ability to pack a lot of energy into a small container. But all that stored energy can also be dangerous.

Batteries can catch fire during charging or use, and even while being stored. Generally, fires happen when temperatures around the battery rise to unsafe levels or if a physical problem in a battery causes a short circuit, allowing current to flow unchecked. These factors can set in motion a dangerous process called thermal runaway.

Most batteries include a battery management system to control charging, which prevents temperatures from spiking and sparking a fire. But if this system malfunctions or if a battery doesn’t include one, charging can lead to fires, says Ben Hoff, who leads fire safety engineering and hardware design at Popwheels.

Some of the delivery drivers who attended a sign-up event for New York’s charging pilot program in late February cited safety as a reason they were looking for alternative solutions for their batteries. “Of course, I worry about that,” Jose Sarmiento, a longtime delivery worker, said at the event. “Even when I’m sleeping, I’m thinking about the battery.”  

Battery swapping could also be a key to safer electric transit, Popwheels’ Hammer says. The company has tight control over the batteries it provides drivers, and its monitoring systems include temperature sensors installed in the charging cabinets. Charging can be shut down immediately if a battery starts to overheat, and an aerosol fire suppression system can slow a fire if one does happen to start inside a cabinet.

The batteries Popwheels provides are also UL-certified, meaning they’re required to pass third-party safety tests. New York City banned the sale of uncertified batteries and e-bikes last year, but many drivers still use them, Hammer says.

Low-quality batteries are more likely to cause fires, a problem that can often be traced to the manufacturing process, says Michael Pecht, a professor at the University of Maryland who studies the reliability and safety of electronic devices.

Battery manufacturing facilities should be as clean as a medical operating room or a semiconductor facility, Pecht explains. Contamination from dust and dirt that wind up in batteries can create problems over time as charging and discharging a battery causes small physical changes. After enough charging cycles, even a tiny dust particle can lead to a short circuit that sparks a fire.

Low-quality manufacturing makes battery fires more likely, but it’s a daunting task to keep tight control over the huge number of cells being made each year. Large manufacturers can produce billions of batteries annually, making the solution to battery fires a complex one, Pecht says: “I think there’s a group who want an easy answer. To me, the answer is not that easy.”

New programs that provide well-manufactured batteries and tightly control charging could make a dent in safety concerns. But real progress will require quick and dramatic scale-up, alongside regulations and continual outreach to communities. 

Popwheels would need to install hundreds of its battery swapping cabinets to support a significant fraction of the city’s delivery drivers. The pilot will help determine whether riders are willing to use new methods of powering their livelihood. As Hammer says, “If they don’t use it, it doesn’t matter.”