Consumer Reports: Prius Plug-In Kit Gets Nowhere Near Maker's 100+ MPG Claim

By Scott Doggett January 7, 2009

hymotion400x267.jpg By Scott Doggett, Contributor

Real-world testing by Consumer Reports showed the best-selling plug-in conversion kit for the Toyota Prius did not come close to meeting its manufacturer's fuel-economy claim of a possible 100+ miles per gallon, the magazine says in its February issue.

Moreover, "our Prius conversion to plug-in power cost more than you could ever expect to recoup in gas savings," the magazine said of A123 Systems' Hymotion L5 conversion kit, which retails for $10,000 to $11,000, including installation.

As we reported two months ago, it didn't matter that Wall Street was spiking like an EKG and the global economy teeters on the brink of a magnitude-9 collapse, Prius owners couldn't hardly wait to plunk down the big bucks for the mileage-extending product.

And according to the half-dozen companies nationwide that sell and install the conversion kits for A123 Systems, sales remain brisk. Indeed, demand for the product -- a lithium-ion battery pack that collaborates with the Prius's factory-installed nickel-metal hydride battery pack -- continues to exceed supply.

But the Consumer Reports article, available only to subscribers and published online this week in advance of it February print edition, will likely give pause to many Prius owners who've placed hefty deposits on the kits but have yet to receive them.

'Actual Mileage Will Vary'

To be fair, A123 Systems doesn't say everyone who buys a Hymotion L5 kit will get fuel economy exceeding 100 miles per gallon. Rather, it says on its Hymotion Website that the conversion modules "can convert your Prius hybrid into a plug-in hybrid capable of 100+ mpg for 30-40 miles."

Those would be the first 30 to 40 miles following a full charge, which is when the vehicle would be powered by the Hymotion L5 lithium-ion battery pack. After that, the Prius's less efficient  factory-installed battery pack would take over.

A123 Systems claims its 100+ fuel-economy figure is based on independent testing performed at Argonne National Labs and Idaho National Labs, and it comes with a disclaimer: "Actual mileage will vary based on each individual's driving style, route, traffic, climate conditions, terrain and other factors."

Much Lower Mileage, Actually

That said, after conducting its own tests on the Hymotion L5 kit, Consumer Reports reported the following:

"We measured the Prius's gas mileage on our standard fuel-economy test cycle, which involves a mix of highway, rural, and city driving. While the converted Prius operates the electric motor and gas engine as needed, as in a standard Prius, it spends more time running on electricity and relies less on gas. Still, when running solely on electricity, any moderate acceleration or mild grade makes the gas engine kick in.

"We could usually drive an average of 35 miles before the new battery was depleted; unlike the original battery, it can't be charged by the gas engine. After that, the plug-in Prius returns to its normal operation. But the extra weight of the new battery drops its gas mileage from 42 to 40 mpg overall and from 34 to 29 mpg in the city.

Hymotion-L5-Inset.jpg "The 67 overall mpg we got during the first 35 miles is a 60 percent improvement over the original Prius Touring. But that's based only on measuring gasoline consumption. A full recharge took about 6 hours and consumed about 5 kilowatts of electricity. That's about 55 cents at the national average of about 11 cents per kilowatt. With help from the Argonne National Laboratory, we calculated that by adding this energy into the equation, overall fuel-economy is equivalent to 53 mpg.

"If gas prices were to hit $4 per gallon again, the cost to operate the standard Prius Touring version would be about 10 cents per mile; the converted version, about 8 cents per mile.

"Because the converted Prius gets its best mileage while using the lithium-ion battery, drivers who can recharge frequently -- at work, for example -- will be able to extend their travel distance beyond the 35-mile sweet spot while getting optimum fuel economy. But finding outlets while on the go could be a challenge.

"People who use fuel-efficient driving techniques will probably get better gas mileage than the average in our tests."

Ouch, Ouch and Ouch

The salient points here include: 53 mpg versus a possible 100+ mpg; a mere 2-cents-a-mile savings when gas costs $4 per gallon; the drop in mileage due to the weight of the conversion kit. The last point begs the question, how much does the conversion kit weigh?

According to Consumer Reports, the kit added 187 pounds to the weight of the vehicle. That's a dramatic increase, when you consider that the Prius has a maximum load capacity of 800 pounds. That's 800 pounds of occupants and cargo.

So with the second battery pack, the maximum load capacity falls to 623 pounds. That means that four adult occupant could easily overload it Indeed, all it would take is four riders weighing 156 pounds on average to exceed the safe load capacity of the plug-in Prius.

And, because the new battery is placed in the plug-in Prius's spare-tire well, the spare tire is placed in a tray and strapped on top of the cargo-area floor, which reduces cargo space. That space is at a premium when the backseat is being used by passengers.

But Still Very Green

Consumer Reports noted that vehicles that run longer on electric power can dramatically cut climate-changing tailpipe emission. But producing power still generates emissions somewhere.

In New England, where the magazine's test track is located, about 42 percent of electric power is generated by the burning of natural gas, which has cleaner emissions than those from gasoline-powered car engines.

Other areas depend heavily on coal to produce electricity, which releases a lot of carbon dioxide into the atmosphere.

That said, a 2007 study by the Natural Resources Defense Council and the Electric Power Research Institute concluded that deploying plug-in hybrids on a national scale would reduce automotive carbon-dioxide emissions by about 45 percent compared with conventional vehicles and would result in a net reduction in air pollution, especially in urban areas.

That's well worth remembering if you've invested in -- or are considering investing in -- a Hymotion L5 plug-in conversion kit for your Prius.

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jkp1187 says: 6:04 PM, 01.07.09

It would've been helpful if they'd run the numbers to show how they look assuming a driver doesn't drive more than 35 miles before recharging (which is typical for most Americans and underpins GM's assumptions concerning the design of the Volt.)

And it should be noted that CR's highest MPG rating was 87 MPG.

billt9 says: 10:27 PM, 01.07.09

"The 67 overall mpg we got during the first 35 miles ... overall fuel-economy is equivalent to 53 mpg."

The 53 mpg figure IS driving no more than 35 miles, while the lithium battery is still operative.

As they mentioned, the gas engine still is used during the first 35 miles, consuming 67 mpg. The gas engine doesn't sit silently for the first 35 miles.

steve_ says: 9:59 AM, 01.08.09

Consumer Reports obviously forgot to refill the Prius batteries with HHO water.

benniewi says: 12:20 PM, 01.08.09

that's what you get,
when you retrofit.

seriously, it's still a parellel hybrid system--not going to be that advantageous unless you can allow extra regen braking when you're driving around that the normal prius battery couldn't accept (due to charge limits).

A series system is better suited for a plug in, since you only run the engine to charge, and in this case, it would have two batteries to charge at one time than one, with a much broader charge range.

A123 systems was bidding for the Volt--probably same technology.

We'll be there soon, but the general public will have to understand a little physics before they are ready for plug-ins. There is no magic battery or hybrid system. You cannot just drive any way you like, anywhere you like and get superior mileage.

Unfortunately, that's how Marketing perceives everything--take the highest possible mileage achieved in only .5% of the time, and broadcast it to the world.

That's just misleading, ESPECIALLY when it comes to batteries.

grannyfranny says: 10:02 PM, 02.01.10

I'm planning to try the Enginer plug-in conversion on my 2006 Prius: The rechargable batteries are used to recharge the hybrid battery, rather than to power the car directly - this may reflect the "series" system rather than 'parallel'mentioned above. The battery pack fits in the space under the cargo floor, but the spare stays in it's space. And, the cost is less than a third of Hymotion: The 2W system is $2K with installation, and should fit with my limited driving on most days. The 4W system, installed, is $3200. And, later this year they are adding a flexible solar screen that will let you recharge while out in the parking lot . I'd love to hear from anyone else who can evaluate this system.

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