Plug-in Toyota Prius Hybrid Test Drive: Is 100 MPG Possible with A123 Systems Hymotion Kit?

Ann Arbor, Mich. — As General Motors ramps up for the 2011 Volt and Toyota readies the long-awaited, factory-built plug-in Prius, customers wait-and wait. And wait. Even when these cars hit the market, some 24 months from now, demand will be very high. It could take many more years for production plug-in hybrids to reach mainstream garages in America.

But why wait? Prius owners can turn their cars into plug-ins today with a $10,395 L5 plug-in conversion module from Hymotion. Plug-ins allow owners to charge their batteries with a household electrical outlet overnight, then drive on electrical or mostly electrical power until the battery is depleted. When there's no more battery capacity left, the gas engine takes over. The range of pure electric travel depends on battery capacity and driving style, but for reference, Chevy expects the upcoming Volt to go 40 miles on electric power. Hymotion claims its kit will deliver between 30 and 40 miles of electric-only range. Current production hybrids like the Toyota Prius go less than a mile on electric-only operation.

To find out whether the Hymotion claims are valid, we slid behind the wheel of a fully charged plug-in Prius and drove it for two days. Do plug-ins live up to the hype? Let's find out. —Larry Webster

The Specs

The centerpiece of the Hymotion module is a 5-kilowatt-hour lithium-ion battery pack manufactured by Boston-based A123Systems. The battery pack and its associated electronics supplement, but do not replace, the standard Prius nickel-metal-hydride battery pack and fits in the spare-tire well of the Prius. So while installation of the kit means there's no place for the spare tire, the Prius's generous cargo area is preserved.

The kit is mostly not a do-it-yourself affair. A buyer will have to take his Prius to one of six authorized installation centers spread across the country (you can locate the shops here). Installation is included in the purchase price.

In addition to the battery pack, the module includes a three-prong plug that's mounted in the rear bumper, a dash switch that turns the extra battery on and off, and a reprogramming of the Prius's computer so the readout displays the charge state of both the A123 and the standard Prius battery packs.

As with any electric car, the battery pack is the key to the system. The A123 unit is unique, the company says, because it can be completely depleted without sacrificing longevity. Current batteries, like the stock one in the Prius, use only a small portion of the available energy capacity. That helps to extend the life of these expensive packs. GM engineers say the Volt will use only half of its 16-kwh hour rating-it will be charged to 80 percent of its capacity and drained to about 30 percent. But A123 engineers say they've tested single-cell versions of their batteries and after 7000 cycles, the battery still retains over 80 percent of its full capacity. If we figure one charge per day, the A123 battery could potentially last for almost 20 years. Trials for multicell packs such as the one in the L5 module are ongoing, but engineers expect similar robustness.

While there are many lithium-ion batteries on the market, the internal chemistry of the batteries varies widely. It's this "pixie dust," as one engineer put it, that separates the batteries. Cost is of course a major issue. Current estimates put the latest li-ion batteries at about $1000 per kilowatt-hour. Even if that price tag is reduced by half, these plug-in hybrids will carry a hefty premium over their gas-powered siblings. The widespread acceptance of plug-ins will ultimately come down to oil prices.

The Drive

We used a standard extension cord to connect our garage-wall outlet to the bumper port of the Prius. The car's brake lights illuminated to inform us that the juice was indeed flowing and then dimmed to indicate that the batteries were charging. After roughly six hours, the brake lights turned off, which meant the pack was fully charged.

The dash switch that engages the A123 battery pack has a light that glows when the system is engaged and battery has a charge. As soon as the supplemental battery is depleted, the light turns off so we would know exactly when it was drained.

We pressed the main power button on the Prius and immediately the gas engine kicked on. While we had enough electrical power on board to move the car easily, the engine still runs every now and then to warm up. It was a chilly, 46-degree morning and if we had wanted to heat the cabin, the engine would have continued running. But it was our job to see how we far we could travel on electric power so we shut off the climate control.

Our in-town loop consisted mostly of stop-and-go driving with peak speeds less than 50 mph. We quickly realized that a subtle right foot is required to keep the gas engine from coming to life and we practiced our best hypermiling techniques. Anything more than a quarter-throttle and/or uphill grades caused the engine to kick on.

It was possible to pull away on pure electric power and accelerate up to roughly 35 mph. Above that speed, the engine would cycle on and off and we knew from a drive the previous day that the engine would sometimes turn off once the car was cruising-even at 65 mph.

Our best indication of the engine's operation was the Prius's energy-flow screen, which shows the power flowing from the engine and batteries to the front wheels and conversely displays reverse flow when coasting or braking. One thing we noticed was that the Hymotion computer reprogramming wasn't perfect. The battery icon, which is supposed to show the state of charge of both batteries, flickered between green and blue, making it almost impossible to tell how much charge was left.

Functionally, however, the car felt like a standard Prius-the difference was that it was in electric-only mode far more often. Since the Prius's electric motor is only good for 67 hp, the gas engine had to assist from time to time, but careful driving kept that to a minimum.

After 24 miles the light went off, indicating that the A123 battery was depleted. We charged up the battery again that night. The next day we drove a different loop that had less traffic lights- this time, the Prius went 25 miles. Once the A123 battery is drained, the Prius behaves like the standard-issue Toyota product.

We also measured the fuel economy during our short drive-it averaged 54 mpg. That figure is not accurate, however, because the amount of fuel we added was about a gallon and filling the gas tank to the exact same point both times proved impossible. A little extra gas on the refill makes a dramatic swing in the mpg figures. The Prius's onboard computer registered 70 mpg, which is likely closer to the real number. It's also important to note that "miles per gallon" is a misleading number here, because we didn't increase efficiency, we just used electrical energy in place of chemical energy.

The Bottom Line

Financially, the Hymotion system makes little sense. Sure, it cost less than 50 cents to charge the battery each night, but the A123's 25-mile range only saved about a half-gallon of gas. At current gas prices it would take over a decade to save enough money to pay for the kit.

On the other hand, saving money isn't the whole point. The idea is to find another way to power our cars that uses less oil. And for a lot of folks, that's all that really matters.

Even though our electric range wasn't over 30 miles, other users, who are likely more skilled at stretching every electron, have achieved better than 30 miles on a charge. The Hymotion L5 appears to be a solid conversion. Other than the flickering battery display, the car felt like it could have come straight from the Toyota factory.


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