PHOTO: Ray Ng

The Subtle Difference: You’d never realize this Prius was a plug-in unless you knew where to find the 110-volt plug: built neatly into the right-hand license-plate light.

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 4.5‑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.