Plugging Away in a Prius

Jonathan Sawyer spent $30 000—and voided the warranty—to add a plug to his Prius hybrid

10 min read
Plugging Away in a Prius
Photo: RayNg

Ten years ago, Jonathan Sawyer wanted an all-electric car badly enough to lease a General Motors EV1 from his sister’s address in Arizona, one of the few states where GM marketed the car. He smuggled it into his hometown of Boulder, Colo., on the back of a flatbed, and periodically returned it to Tempe, Ariz., for maintenance the same way—until his dealer refused to service it, noting that his radio presets weren’t local, the car’s garage-door opener was useless in his sister’s carport, and a photo of his EV1 had appeared in a Boulder newspaper.

Later, relations with GM improved; Sawyer even got the company to lease him two more EV1s in Colorado when Arizona demand proved low. But that low demand gave him an insight: if even environmentalists in Boulder weren’t going for EV1s, what hope did the car have in the mass market? GM was probably right, he reluctantly concluded, when the company apparently decided in 2003 that the market wasn’t ready for a two-seater with a 110-kilometer (70-mile) range and an 8-hour recharge time.

Now Sawyer is one of the first people in the world to own a car conceived and designed precisely to overcome the range problem: a plug-in hybrid electric vehicle, or PHEV. An electrical engineer by training and cofounder of FreeWave Technologies, a start-up that makes radio-­telemetry ­equipment, Sawyer has pursued new and alternative technologies for years. His house has both photo­voltaic panels and a wind turbine. Before the PHEV, his main ride about town was an all-electric Toyota RAV4 EV. So last year, the 52-year-old single father of two school-age daughters (Allison, 12, and Melanie, 10) opted for the state of the art in automotive ­environmentalism.

In October, Sawyer paid US $25 000 for a brand-new black 2008 Toyota Prius. But compared with his RAV4 EV, it was a gas guzzler, going only 1 or 2 km electrically before switching on its internal combustion engine. So Sawyer wrote a check for the car, then drove it directly to Hybrids Plus [see our sidebar, "Getting on the Grid"], also in Boulder, where he wrote another check, for $32 000—to have his shiny new Prius converted into a PHEV. (The radio-telemetry business has been very good to Sawyer.)

A plug-in conversion service either replaces the car’s original battery pack with one having far higher energy capacity, as Hybrids Plus does, or supplements it, as many other conversion companies do. The car can then travel, in this case, up to 50 km in all-electric mode without switching on the engine. The conversion also adds a charging system that lets an owner recharge that pack by plugging into a standard household electrical outlet.

For trips beyond the 50-km pure-EV range, the vehicle carries its own recharging system—the car’s original gas-powered engine. The converted car consumes much less fuel than a standard hybrid. A PHEV’s mileage varies with differing driving styles and geography, but plug-in owners like to quote figures like 3.4 to 2.9 liters per 100 km (70 to 80 miles per gallon). PHEVs running in electric mode cost far less to operate—at a typical cost for nighttime grid electricity, roughly $0.02 per mile, against roughly $0.14 per mile for gasoline-fueled travel.

Someday, millions of cars will come off assembly lines as plug-inhybrids. Analysts are mostly agreed on that. But for now, in the whole wide world fewer than 200 PHEVs roam the roads. Plug-in conversions are a cottage industry in North America, offered in kit or turnkey form by perhaps a dozen businesses and organizations. Over the past two years, the budding industry has been aided and abetted by a remarkable and vocal network of plug-in owners, advocates, and fans. They swamp Internet bulletin boards and e-mail their government representatives, arguing for a vast R&D effort aimed at producing cars that get 100 miles per gallon or more, powered as much as possible by grid ­electricity. The key enabler is recent advances in large-­format lithium-ion batteries [see “Lithium Batteries Take to the Road,” IEEE Spectrum, September 2007].

One influential advocacy group is the California Cars Initiative, a Palo Alto–based nonprofit start-up of entre­preneurs, engineers, environmentalists, and consumers. CalCars maintains a popular wiki on all ­topics having to do with plug-in hybrids, particularly public policy and technology developments ( By stirring demand, the organization hopes to “encourage automakers to produce 100+ MPG ‘no-sacrifices’ high-performance, clean hybrid cars,” to quote its mission statement. Or as Carl Lawrence, lead founder and CEO of converter Hybrids Plus, says, “It’s about creating the perception that this can be done—that if you’re not talking 60 or 80 miles per gallon, then you’re wasting your time.”

It’s quite a big tent of proponents, with some of them worried more about national security than ecology. R. James Woolsey Jr., former director of the CIA, told an IEEE symposium last fall that he views plug-ins as one way to help “destroy oil as a strategic commodity.”

Right now, though, if you want a plug-in, converting an existing conventional hybrid-electric vehicle is the only way to go. It will be three years or more before any of the major automakers sells a car designed from the ground up as a PHEV. Even then, the industry will most likely have to subsidize the cost of the battery packs for years. It’s widely assumed that Toyota has subsidized its hybrid vehicles, which make up almost 80 percent of the hybrids sold worldwide since the first-­generation Prius was introduced in 1997.

The second-generation Prius , introduced in 2003, is the highest-volume hybrid vehicle ever made. Toyota has now built more than half a million of them. Last year, the company sold 181 221 Priuses in the United States alone; the model’s distinctive wedge profile suggests “hybrid car” the same way the distinctive radiator shell of a Rolls-Royce intones “luxury.” The Prius also makes a surprisingly good PHEV, considering that it was never intended to be one. And to explain why, we’ll start by tearing apart a stock Prius.

Toyota’s Hybrid Synergy Drive system seamlessly shuffles power among the combustion engine, two electric motor-generators, and a battery pack. The main 50-kilowatt (67â¿¿­horsepower) motor-generator does just one thing: it drives the front wheels through a reduction gear, which reduces the motor-­generator’s rotational speed to the wheels’ lower speed. The secondary motor-generator serves several masters. It recharges the 1.3-kilowatt-hour nickel-metal-hydride (NiMH) battery pack, and it supplements the power from the main electric motor when more propulsion is needed. The secondary motor-generator also quickly starts the gas engine. To save fuel, the Prius, like most conventional hybrids, shuts off its gasoline engine whenever the car is not in motion—for example, at a stoplight—and then restarts it when the car gets back up to about 25 km/h or whenever the driver steps down hard on the accelerator.

The mechanism by which the system shuffles power, and the heart of Hybrid Synergy Drive, is known as a planetary gear set. In this set, a central gear (the  sun”) connects to the drive shaft of the secondary motor-­generator. Its “planet” gears are in turn surrounded by a ring gear that drives, or is driven by, the main motor-generator—which is also connected to the differential that turns the wheels. The carrier for the planet gears is connected to the engine’s output shaft.

By varying the speed at which the planet gears spin, this arrangement allows the system’s control software to alter the power split between primary and secondary motor-generators. More speed means more power going to the main motor. All three components—the ring gear, the planetary carrier, and the sun gear—work in unison to control the torque output through the ring gear to the wheels.

In a Prius, then, engine power may either turn the wheels or recharge the battery pack. The car’s control software makes those decisions, splitting motive power between engine and electric motors, recharging, and regenerating power on braking.

Depending on driving needs, the primary electric motor can provide up to 78 percent of the car’s total torque of 515 ­newton meters (380 foot-pounds) or spin the shaft of the secondary motor to recharge the batteries. The system adjusts relative power levels without changing the mechani­cal load on the engine. The driver needs only to brake and accelerate; software makes all the necessary decisions. Unless a driver pays close attention to the graphic shown on the car’s dashboard display, it’s possible to drive a Prius without ever knowing it’s a hybrid—though it’s occasionally very quiet for a car.

Under certain driving conditions, an unmodified Prius can run as fast as 65 km/h (40 mph) on electric power alone, but only for a kilometer or two. Priuses sold in Europe and Japan (but not in the United States) have an “EV Mode” switch on the dashboard. It commands the car to power itself purely on electricity for a short period, drawing more energy from the batteries than the car’s power-shifting algorithms would otherwise permit. Its all-electric range, however, is 1 to 2 km at most at neighborhood speeds. The EV-mode switch was removed for the U.S. market, by the way, because cars sold there must guarantee that all elements of their emissions-control systems will function properly without maintenance for 10 years or 220 000 km, whichever comes first. Using the EV-mode switch increases the demand on the stock Prius’s NiMH batteries and in so doing makes the U.S.-market lifetime requirements more of a stretch.

To maximize the chances that the standard Prius battery pack will survive 10 years under any conceivable operating conditions, Toyota rigorously keeps the pack’s state of charge—expressed as a percentage of the full-capacity charge—between 50 and 80 percent. Toyota does not disclose exact details, but some engineers say the band is even narrower under most operating circumstances.

Hybrids Plus replaced the original 1.3-kWh NiMH battery pack in Sawyer’s new Prius with a custom-built 4.5-kWh pack. The new pack, which fits inside the exact same opening in the trunk floor that the old pack did, turns the car into a PHEV-15, the number indicating its all-electric range of 15 miles (24 km). But like most of the company’s 10 Prius customers to date, Sawyer opted for an extension pack with another 4.5 kWh, making the car a PHEV-30. That secondary pack, which is mounted unobtrusively under the cargo-area carpeting, occupies roughly 45 liters (less than 2 cubic feet) of space. While it reduces load space somewhat, that’s the tradeoff for 24 more electric kilometers.

How do you more than triple the capacity of a ­battery pack without greatly altering its volume? In this case, you go from nickel-metal-hydride to lithium-ion. The 4.5â¿¿kWh battery pack contains roughly 600 lithium-ion cells manufactured in China by A123 Systems of Watertown, Mass. A123 says that its cells, which use lithium-ion nano­phosphate for the cathode, will retain much of their energy capacity over 10 years, performing far better than the cobalt-oxide chemistries used in mobile phones and laptops. Laptop ­batteries generally last fewer than five years before their ability to recharge has declined enough—40 or 50 ­percent, say—to require replacement. A123’s cells were designed to do much better, but the company’s oldest cells—for power tools—date only to late 2006, so their life expectancy in the field remains unproven.

Although complicated in its own right, the battery swap is not the trickiest part of the conversion. The tougher challenge is figuring out what data to transmit to the Prius’s vehicle- and engine-management controllers so that they never “realize” they are working with a battery pack with triple or sex­tuple the energy capacity of the original.

“We don’t modify anything of the original vehicle’s controllers,” says Lawrence of Hybrids Plus. What the new battery-­management system does do is send altered data to those systems—the most important of which is data on the battery’s state of charge. With the converted car running in electric-only mode, this state will vary from 90 ­percent of capacity to less than 40 ­percent. But a stock Prius is programmed to charge the battery if the state of charge falls below 50 percent and to shed energy by helping to spin the driveshaft when the state rises above 80 ­percent. So to keep the car running in electric-only mode, a micro­processor in a new “pack controller” that monitors and controls the replacement battery pack keeps sending data within those limits. This allows the vehicle to keep going in pure-­electric mode for many miles, telling the controller the state of charge is above 50 percent even as it drops significantly below that.

The added pack controller runs software that’s custom written by Hybrids Plus. Besides the software, which lies to the Prius’s existing control systems, Hybrids Plus makes hardware modifications. For instance, the ­lithium-ion packs generate less heat than the stock NiMH pack, Lawrence claims, so they don’t require the forced-air cooling provided by an electric fan mounted above the right wheel. The company leaves the fan in place but disconnects it from the vehicle controller.

A standard Prius has many other operating parameters and system checks to ensure the health and longevity of its battery pack, all of which had to be reverse engineered to ­accommodate the much larger, lithium-ion pack and its peculiar characteristics and also to let the car operate for a few dozen kilometers as a pure EV. Hybrids Plus declined to give specifics on any other software modifications, which it considers its core intellectual property.

Caveats abound. A July 2007 Toyota memo expressed concerns about increased hydrocarbon emissions after conversion and cited increased potential for injury after an accident and other risks, such as fire. It also notes that the car’s rear-crash safety may be affected by the larger, somewhat heavier battery pack. So far, Hybrids Plus has not performed crash tests on a converted car. The Toyota memo also notes that conversion “voids the express warranty provided by Toyota at the point of sale.”

The tailpipe emissions of the converted car may, paradoxically, exceed those of the standard car, perhaps even the maxi­mum limits on the U.S. Environmental Protection Agency’s test cycles (the converted car is still “street legal” because those limits apply only to the sale of new cars). Researchers from Argonne National Laboratory, in Illinois, tested two Prius plug-in conversions and presented their data at ­various conferences last year. Neither of those conversions was done by Hybrids Plus—one was by Hymotion Canada, of Concord, Ont., the other by EnergyCS, of Monrovia, Calif. Researchers found that the cars’ emissions of hydro­carbons and nitrous oxides equaled or exceeded those of the ­standard Prius.

In the car’s first two months, it ran more than 1000 miles and still had gas sloshing in the 45-liter tank

The bulk of the emissions occur mainly in the 20 to 40 ­seconds after ignition, before a car’s catalytic converter heats up to the temperatures at which it removes pollutants most effectively. A standard Prius keeps its catalyst at full operating temperature, which makes sense because it operates in pure electric mode for only a fraction of any trip. A PHEV, on the other hand, uses the engine so infrequently that the converter cools down, meaning an emissions spike occurs if the engine has to be switched on suddenly, as when going up a long, steep hill.

But enough nits. Is the car a winner? After driving it for nine weeks, Sawyer says he is delighted. His only complaint is the relatively low speed at which the car automatically switches on its gas engine. He’d like to be able to cruise at 65 or 70 km/h without the engine kicking on. That said, he proudly pointed out that in the car’s first two months of operation, it ran more than 1000 miles, and he still had some gas sloshing around in the 45â¿¿liter (11.9â¿¿gallon) tank from his first and only fill-up. That works out to roughly the magical 100â¿¿mpg figure (2.35 L/100 km), obtained by religiously plugging in the car every night and driving it mostly in urban areas and on short trips.

That regimen does take a toll on the Sawyer family’s household electricity use, which dad and younger daughter Melanie measured as part of a science project. Recharging the Prius, as expected, was the single largest draw, using an average of 3 to 4 kWh per day. That much energy cost less than $0.50 at Colorado’s average rate of $0.093 per kWh in November 2007, when the Sawyers made their calculations. Still, compared with what the gasoline would cost—at the time, about $3.00 a gallon—that’s a savings of a few dollars every day when the car is used up to its 48-km range.


Embarrassment of Riches

Over the course of two years, the photovoltaic cells on the Sawyers’ house have generated 2000 kilowatt-hours more than the family used; they sold the excess to their utility.

The savings are actually higher for Sawyer because much of his electricity comes from his home’s photo­voltaics and its wind turbine. Even so, getting a full payback on the conversion would require Sawyer to drive half a million kilometers, or 12 times around the equator, just on electricity. (Good luck finding a plug.)

So what’s the family’s verdict on their plug-in Prius?

“My daughters love the silence and smoothness of plug-ins and electric vehicles,” says Sawyer with a grin. “I hope each of them will be able to take her driving test in one.”

With six years until the youngest daughter takes the test, that’s not out of the realm of possibility. It all depends on how long those lithium-ion batteries last.

UPDATE: John Voelcker details a new Hymotion/A123 plug-in conversion kit in our Tech Talk blog.

To Probe Further

See our slideshow, “Getting the Grid.”

See our sidebar, “Automakers’ Plug-In Plans

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