Hunter Strong checks out the battery pack in his family’s Prius.(Courtesy photo)

Pedestrians are getting used to the silence of an approaching Prius in its short-phased electric mode, but listeners would have to trail Steven and Marilyn Strong’s converted Prius more than 50 miles before hearing it switch over to conventional gasoline engine sounds.

Last summer, Steven Strong, an international expert on the integration of renewable energy systems in buildings, and his son Hunter, a 1993 Bromfield graduate and an electrical engineering major at Worcester Polytechnic Institute, converted the family’s 2008 Prius hybrid to run with an all-electric option, using a rechargeable plug-in battery.

The conversion was a natural step for the Strongs, who have been involved with renewable energy for more than 30 years. Both their Solar Design office on Ayer Road and their Harvard home use photovoltaic solar arrays for electricity. A good day of sunshine can provide 100 percent of their daily electricity use—including transportation.

To show that it could be done and to get further away from fossil fuels, (and maybe because it was an enticing project), Strong wanted to add an electric-only option to their Prius. The standard Prius hybrid has a conventional gasoline engine, a permanent magnet AC electric motor, and a nickel-metal hydride (Ni-MH) battery pack. It can only run about half a mile on its electric motor before reverting to the gasoline engine, which, combined with the Prius’s regenerative braking system, recharges its 274-volt Ni-MH battery. Strong’s electric conversion gives the Prius a third mode, running on electricity from a LFP (LiFePO4) battery pack that is recharged from a standard electrical outlet using a regular three-prong power cord.

Before starting, both Strongs did extensive research, including visiting Dr. Andrew Frank, professor of mechanical engineering at the University of California at Davis, who has been called the “father of plug-in conversions.” The research gave them the confidence to continue in the face of the complex system.

The Strongs’ license plate says it all. (Courtesy photo)

First, they chose to use the common 15 amp/120 volt connection because of its widespread availability. Even though a full charge takes seven hours, Steven preferred the availability of 120-volt outlets over the less common 220-volt outlets, which could recharge the batteries in one-fourth the time.

The next step was to find a sufficiently large lithium-ion battery pack. Steven’s first-choice source, Boston-based manufacturer A123, would not sell them anything larger than a four kWh pack, the same package A123 offers in its Hymotion conversion kit, available for installation through Toyota dealers. Instead, Strong bought a 10.5-kWh pack from Chinese manufacturer ThunderSky. Installing the 200-pound battery pack, which measured 2 feet by 3 feet by 8 inches, required removing the back seat and the spare tire, but this was the easy part.

The biggest challenge for the Strongs was circumventing the factory software, which Strong said Toyota guards closely. Hunter’s software and electrical engineering experience was essential to “fooling” the car into thinking it was running in its comfort zone.

Another challenge involved increasing the speed and capacity of the Toyota factory fan to handle the increased heat generated from the larger battery pack.

The converted Prius can go 40 miles on a full charge, but Strong prefers to run only down to 20 percent of the charge, about 32 miles, to prolong the battery pack’s life.

Strong emphasized that though they did their conversion over a weekend, this is really not a simple do-it-yourself project, unless you have the necessary electrical and software know-how and have done the research. “Cars have become computer and power systems on wheels,” he said.

Speaking about his motivation for the conversion, Strong said he found the “transition to electricity-based personal transportation very compelling and unavoidable,” remarking that “Europe is way ahead, with charge posts at parking meter-level in many countries.”

Strong said he wants to put the idea of electric transportation in front of people, hoping they will consider it for their next car. With the American transportation fleet turning over every eight to ten years, compared to buildings with significantly longer life spans, Strong sees transportation as an opportunity to make change rapidly.

“The big picture is that our country is more dependent on foreign energy sources than ever. None of the proposed substitutes for liquid fuels scale at the level that is needed. As much as we want to see policy change at the state and federal level, real solutions will continue to rest on the shoulders of individuals. We can’t wait for government to fix the problems before us.” In his opinion, “hydrogen fuel cells are still massively expensive and there are no refueling options in the foreseeable future, whereas electric power charging capability is already in place in every home.”

According to a recent New York Times article, San Francisco’s building code may soon require wiring for car chargers in any new structures. At Google’s headquarters in Mountain View, Calif., employees can plug their cars into chargers supplied by solar panels covering the carports.

Seeing a car with an electrical cord plugged into its bumper is now a novel sight, but Strong hopes it will become commonplace. He predicts that within two years we’ll be seeing more electric vehicles “as every major manufacturer is working on electric models,” made possible by advanced battery technology. According to industry announcements, General Motors will soon be introducing its electric Volt; Nissan, its electric Leaf; and Toyota, a plug-in hybrid and a battery electric vehicle.

The Internal Revenue Service offers credit for 10 percent of the cost for plug-in electric car conversions or a new plug-in electric vehicle. (More information is available at www.irs.gov/newsroom/article/0,,id=206871,00.html) The conversions are not inexpensive: Strong’s project cost in the neighborhood of $10,000.

Strong has long been ahead of his time. In 1979 he installed solar panels on the White House, after which President Carter said, “this solar heater can either be a curiosity, a museum piece, an example of a road not taken, or it can be a small part of one of the greatest and most exciting adventures … as we move away from our crippling dependence on foreign oil.” The next administration removed the panels, the country returned to an era of cheap oil, and many of the American renewable energy patents were picked up by German and Japanese companies.

In 2002, the U.S. Park Service, which is responsible for maintaining the White House, commissioned Strong’s firm to install two solar thermal systems and a 9kWh photovoltaic solar system on a White House maintenance shed roof that needed replacing, in keeping with the Park Service’s mandate to incorporate environmentally-friendly design whenever possible.

Strong was a young engineer working on the Alaskan pipeline in the midst of the oil embargo of the early 1970s when he attended the first world conference on solar electricity. “For a nerdy engineer, it was like falling in love,” he said. In this case, the love object was solar cells that could silently convert sunlight directly into electricity—indefinitely, without pollutants, and without consuming resources. The love affair continues.