Getting Amped

A Plug-in Hybrid Conversion

Inside this Article

The author and his plug-in hybrid.
The author and his plug-in hybrid.
Paul Guzyk and Daniel Sherwood, the founders of 3Prong Power
Paul Guzyk and Daniel Sherwood, the founders of 3Prong Power, strip the Prius’s hatchback to its core, revealing the stock 1.4 kWh NiMH battery, usually hidden from view behind the rear seats.
The stock 1.4 kWh NiMH battery
The stock 1.4 kWh NiMH battery, usually hidden from view behind the rear seats.
Technicians prepare the wiring behind the dash for the new switch.
Technicians prepare the wiring behind the dash for the new “electric vehicle” switch.
Connecting the new battery pack to the old increased the capacity by 143%.
Connecting the new battery pack to the old increased the capacity by 143%.
The new battery and charging module drops in above the spare tire.
The new battery and charging module drops in above the spare tire.
The module placement makes accessing the spare tire more difficult
The module placement makes accessing the spare tire more difficult, but allows a clean-looking installation.
The male AC receptacle and its cover
The male AC receptacle and its cover are the only external signs of the installation.
After the installation, not even a trace of the battery modification is evident.
Voilà: After the installation, not even a trace of the battery modification is evident.
The author and his plug-in hybrid.
Paul Guzyk and Daniel Sherwood, the founders of 3Prong Power
The stock 1.4 kWh NiMH battery
Technicians prepare the wiring behind the dash for the new switch.
Connecting the new battery pack to the old increased the capacity by 143%.
The new battery and charging module drops in above the spare tire.
The module placement makes accessing the spare tire more difficult
The male AC receptacle and its cover
After the installation, not even a trace of the battery modification is evident.

The drizzle turned to a downpour when I pulled my Prius into the funky Berkeley, California warehouse serving as the digs for 3Prong Power, a startup company that converts conventional hybrid-electric vehicles into cars that can charge their larger batteries at home with 120 VAC. Within minutes, Paul Guzyk and Daniel Sherwood, the founders of 3Prong, had stripped the Prius’s hatchback to its core, revealing the stock 1.4 kWh nickel metal hydride battery (NiMH), usually hidden from view behind the rear seats.

Seconds later, they had removed the rear seat backs and several plastic panels, clearing a spot where the spare tire usually sits, for an auxiliary 2 kWh lithium-ion (Li-ion) pack. The ensuing several hours of clearing, wiring, and fabricating were to turn my Prius into a high-mpg, plug-in hybrid. The goal was to cut my annual gasoline consumption in half, by increasing the battery capacity and adding a switch to drive in all-electric mode for shorter trips, the most common use for vehicles.

Heading to Hybrids

The plug-in hybrid movement has come a long way in 10 years. When I bought my first hybrid, a 2003 Honda Civic hybrid, it was a technological breakthrough. I was amazed that it got about 45 mpg in mixed city/highway driving. But back then, hybrids were a curiosity. Fast-forward to 2010, less than a decade later: the ability for a gas-powered car to juggle energy between an internal combustion engine and a battery-fed electric motor has become a commonplace reality.

The rapid growth of the hybrid market—though still less than 3% of all new car sales—was the smelling salts that global carmakers needed to wake up to the world’s environmental and energy realities. There are now more than 1.5 million hybrids on U.S. roads, with more than 20 models available to consumers. And now the auto industry is buzzing with excitement about the next steps: plug-in hybrids and pure electric cars that can greatly reduce or entirely eliminate the use of petroleum.

The good news is that auto executives have started to pay more attention to hybrids and electric cars. The bad news is that plug-in hybrids will be considerably more expensive than conventional cars—and they will only begin to trickle out in late 2010. It could be several years before a salesperson can “put you in this car today” and have you silently drive off the lot using “electric” fuel.

Extending the EV Moment

As most hybrid drivers will tell you, the real fun of driving a hybrid car—besides stretching out the time between fill-ups—is the brief periods when the gas engine goes dormant and you’re gliding down the road on a magic carpet of battery power. Hybrid engineers have been able to extend those brief “EV moments” in the latest hybrids, especially in the 2010 Ford Fusion Hybrid and 2010 Toyota Prius. But even with those recent improvements, all-electric range usually maxes at a half-mile or so. That leaves folks like me wanting more battery storage—some way to do most of my daily driving on electricity and only occasionally use the gas engine for longer trips or for accelerating and climbing hills.

I have been tracking aftermarket conversion packages and services ever since Felix Kramer, founder of CalCars, hacked his Prius in 2004 and turned it into a plug-in hybrid. Conversion systems, many using the CalCars methodology, have been available for a few years, but they’ve been pricey—up to $12,000. That’s why Detroit-based Enginer’s system—$2,500, which includes installation—caught my eye. (Enginer charges $1,000 more to California customers to help defray costs of the required 5-year warranty.) It’s by far the least expensive system I’ve spotted so far. Could this aftermarket plug-in conversion system mean that plug-in hybrids are ready for prime time? The folks at 3Prong Power asked me to evaluate the system, using my own car as the test platform. I couldn’t resist.

My wife Angie and I primarily run a taxi service for our two kids—back and forth from school, music lessons, martial arts, and soccer games, as well as the usual errands to the market, bakery, and bank. (I walk or ride my bike to my office.) Our most common trip is about 10 to 15 minutes, with stops every block or two along the way. A hybrid’s gas engine usually takes about 10 minutes to warm up before it will go dormant and allow the electric motor to do more of the work. For our driving, that’s just about the time we’re pulling back into our driveway, which means we are getting lower fuel mileage than for slightly longer trips.

While the Enginer’s 2 kWh battery pack is a pipsqueak compared to the upcoming Chevy Volt’s 16 kWh or the all-electric Nissan Leaf’s 24 kWh, it might be exactly what we need for our quick trips—a chance to extend those all-electric trips from a few blocks to a couple of miles. A plug-in hybrid offers many of the benefits of a pure electric car without the biggest drawback—limited range. When the batteries drain on a plug-in hybrid, the car reverts back to being just a standard hybrid vehicle by running the engine when needed.

Plug-In Particulars

“Electric transportation has zero tailpipe emissions,” says Sherwood. “Most studies show that greenhouse gas emissions for electrics are better than for biofuels.” Sherwood, an electrical engineer, also installs photovoltaic systems. “You can put solar panels on your house, and you can run your car on sunshine.” (See Ask the Experts in this issue.)

The Enginer system, developed by auto engineer Jack Chen, is one of about four different flavors of aftermarket plug-in conversion packs on the market (see “Plug-In Hybrid Conversion Systems” sidebar). Unlike some other systems, Enginer doesn’t hack or “spoof” the Prius computer system and it doesn’t remove or replace the existing hybrid batteries.

“We don’t even open the hood of the car. We hook into the car’s electric system under the dash,” said Guzyk. Installers don’t cut any factory wiring. The whole system connects to the car with only five wires—three under the dash and two in the trunk area, along with the original Prius traction battery.

The system uses the additional 2 kWh battery pack (an optional 4 kWh pack is also available) to continually feed juice to the original Prius battery. A “PHEV” switch is added, which fits into an existing spot on the dashboard. The switch awakens the “try EV mode” software that resides in the 2004 to 2009 Prius computer. Toyota installed that trigger wire as a standard feature of Priuses in Japan, but left it dormant for North American Priuses. (The EV button/mode is standard on the third-generation 2010 Toyota Prius—although the battery pack is still small at 1.4 kWh.)

Enginer’s PHEV switch turns on the battery pack, the battery management system, and a DC-to-DC “boost” converter—the Enginer’s “secret sauce,” according to Guzyk. It takes direct current from four 12-volt batteries and converts it to 240 volts. “The Prius battery charges at approximately 240 volts, but we save a lot on cost by using a 48-volt battery,” Sherwood explains. “It’s a standard battery size. With the converter, you need fewer cells to get a higher voltage.”

As 3Prong was finishing the conversion, I had a 120 VAC power source installed at my driveway, with a retractable industrial extension cord tucked away behind a small hinged-door outlet box on the side of the house. I couldn’t wait to plug in, charge, and then hit the road.

Plug-In Performance

On the first mile-and-a-half of the inaugural trip—a 5-mile jaunt to pick up Chinese food to celebrate the conversion—the car topped out at 99.9 mpg, as high as the Prius’s dashboard display can go. But as expected, the auxiliary battery couldn’t keep up with the all-electric demands, so the car reverted to standard hybrid mode, using the gas engine to power the wheels and recharge the stock Prius battery. Think of it as a bathtub draining about four times faster than you can fill it. By the time I pulled back into my driveway—the crispy Szechuan chicken still warm—the dashboard display read 78.5 mpg. Still pretty sweet.

Of course, the mpg reading on the dashboard only tells you how much gasoline is used, and doesn’t reflect the use of a small amount of household electricity. In effect, the dashboard reading is slightly overstated, but the extra 2 kWh, at $0.11 per kWh, is a small price.

As I drove the next few days, the same pattern continued. The Prius acted like an electric car for a mile or so, but then the auxiliary battery struggled to refill the factory Prius battery. Stoplights were a welcome opportunity for the Prius battery to catch its breath. Sometimes the car wouldn’t go into EV mode at all, despite having a fully charged battery. Like all the other aftermarket conversion products, the Enginer system does its best to predict how the Prius will respond to the extra juice, but it’s not an exact science. Some trips would tally a remarkable 85 or 90 mpg, while others delivered a more modest 50 or 55 mpg.

Even when the car wouldn’t go into electric mode, it clearly benefited from the second battery. Techniques for maximizing mileage with conventional hybrids—such as feathering the accelerator or lifting your foot off entirely, then very lightly easing back in—were even more effective post-conversion.

Was it worth it? Well, that depends on how you see the extra attention required—remembering to plug in the car at night, using fancy footwork on the accelerator, flipping the PHEV off and on to engage the EV mode—and a less accessible spare tire under the new auxiliary batteries. My 10-year-old son Isaac compared the ups and downs of the Prius battery to the “manna bar” in several video games that he plays. “Every time you use special ability or spells, your manna goes down,” explains Isaac. “When you just use normal attacks that don’t cost any manna, your manna will regenerate.” And that’s how I felt. The extra benefits from the conversion felt like manna. Not as predictable as you would want, but great when it works.

After one month of driving, the car’s average mileage jumped from the mid-40s to almost 60 mpg. How does that pencil out in terms of return on investment? To answer that, I asked accountant Bob Susich, based in nearby Moraga, California. He was the very next 3Prong customer, converting his Prius using the 4 kWh Enginer system that sells for $4,500 in California. Susich also has an 8 kW photovoltaic system on his house.

“With today’s prices, I can look at this as a seven-year investment. Run the car 120,000 or 130,000 miles, mainly around town, and it becomes break-even,” says Susich. “If you believe that gasoline in three or four years will be $5 or $6 a gallon, then it’s a very rapid payback.” But Susich isn’t trying to save money with his conversion—he just doesn’t want to lose money. “There’s an ecological dimension as well. If I can have no carbon footprint or reduce it dramatically, well, why not?”

The financial and energy ledger doesn’t quite balance out for me—maybe because I don’t have a PV array yet, but also because the Enginer system, while relatively inexpensive, does not reliably supply the level of electric power I was seeking. However, I’m very encouraged by the growing field of aftermarket systems (which are all more powerful, but pricier, than Enginer’s). That at least four distinct plug-in conversion systems exist indicates that a market is emerging—and plug-in hybrids and electric cars from major car makers are right around the corner. Based on my particular driving cycle, the Enginer system resulted in a 40% to 50% mpg improvement. That’s great. The experience only intensified my desire for a more robust electric-drive car and made me more eager to go solar at home. My conversion is about more than adding batteries to a Prius. It’s a conversion to the belief that cars can and should run on sunshine.


Bradley Berman is the founder and editor of and the forthcoming, launching in spring 2010. He writes about hybrid cars and other advanced fuel-efficient vehicles for The New York Times, the Reuters news agency, the Detroit Free Press, and other publications. Through his writing and media work, Berman promotes a greater understanding of the importance of sustainable mobility.

Comments (2)

Dan Cosgro's picture

This article although good is very dated at 2010. Would like to hear from people who have made the jump with larger battery packs. My Prius is a 2005 and the factory battery pack is worn out. Toyota service and sales people will tell you it never needs to be replaced but almost none of them own a 10 year old Prius. My battery rarely shows a full charge unless I am doing a 15 mile + downhill and will keep it only a few miles despite feathering the accellerator. Would love a 6+kw battery system for electric mode in town and the hybrid system for trips over the Cascades...

Michael Welch's picture
Look up a company called Enginer (not Engineer). They've been in business quite awhile and sell PHEV kits in 2, 4, & 8 KWH sizes.
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