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Prius Hymotion Conversion

I’m thinking about buying a Prius and they offer something called Hymotion Conversion which turns a Prius into a plugin, and theoretically raises the MPG to 100 miles for the first 30-40 miles. Anyone know anything about this?

1) Does it really work

2) Does the battery retain the charge for a long time, or will it lose it’s ability to charge effectively after a year or so.

3) Does this destroy my ability to maintain the car (I know it voids the warrantee, but I’m thinking about getting a used Prius anyway, so not worried about that part of the tradeoff).

Thanks everyone,


The standard Prius battery is not big enough to power the car for very far or very long. Most of these modifications involve installing a fairly large supplemental battery bank in the trunk to provide the required energy. You must ask yourself are the benefits worth the drawbacks…

I would not do it, the money you spend will never be recovered in fuel costs, and these conversions have had spotty reliability. Say you go from 50 to 100 mpgs. In 50,000 miles that’ll save you 500 gallons of gas, worth $1000 these days. How much do they want for the conversion? Oops, I checked - $10,000!!! This is not green. Thing of all the resources used to make it.

You’re asking the wrong crowd. Ask a rep from the company to let you speak to some references. You’ll get not much more than conjecture here.

Sorry, $10,000 to save 500, even 1000 gallons is not conjecture.

It kinda works. Basically, they install some beefier batteries and so you plug it in overnight and drive on only battery power for a while. Expressing it in MPG terms isn’t the most useful because obviously you’re getting infinity miles per gallon when you’re driving on battery power, but once they’re drained the gas engine has to kick on and it’ll take a lot of gas to recharge those big batteries. The trouble is that the advertised distance is questionable-- you might be able to get 30 miles if you are driving very slow, but more typical driving will result in a far shorter range. For comparison, with the standard Prius batteries you can go about a half mile at 45 MPH and the bigger batteries are not lightyears ahead by any measure.

Plus these conversion are extremely expensive so there’s no way in heck you’re going to make it up financially and the environmental benifits are questionable. If you happened to have a very short commute (and for some reason you couldn’t ride your bike or some other more eco-friendly option) it might result in some minute reduction of C02 or whatever your environmental pet cause of choice is, but in reality most people just do the conversions for bragging rights, hence the totally irrelevant “mine’s bigger than yours” MPG measurement that gets trotted out.

Consumer Reports recently did just this conversion, with these results:

"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 kWh of electricity. That’s about 55 cents at the national average of about 11 cents per kWh. 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. "

Doesn’t sound like it makes any sense to me. Wait for the real plug ins coming out in a year or two.

For ten grand it’s ridiculous to even consider this. Here’s what Popular Mechanics had to say about it.

At least a few dozen people around the country have now done one-off conversions of hybrids like the Toyota Prius and Ford Escape to transform them into plug-in vehicles. For those who may not have the hardware hacker gene in their DNA, the first commercially available plug-in conversion kit is now on the market from A123 Systems subsidiary HyMotion. Popular Mechanics Detroit Editor Larry Webster recently had the chance to sample a converted Prius with the HyMotion kit for a couple of days. The kit consists of a 5 kWh lithium ion battery and associated electronics that are claimed to give the Prius a 30-mile electric range. As Larry found out this isn’t exactly true. The pack will give the Prius up to 30 miles of driving before the battery is depleted. However, the Prius is not designed as an electric vehicle and the 67 hp electric motor is insufficient to let it operate on electrons alone at all speeds. With a very light foot on the gas pedal you can get up up to about 35 mph without the engine. Even at higher speeds, Larry found the engine would occasionally shutoff allowing the car to run on batteries. At lower speeds however, the engine will occasionally start up depending on the driving condition so that 30-mile range is really mixed mode driving. During Larry’s time with the Prius he got about 24-25 miles before the lithium battery was depleted. Given the $10,000 price tag of the kit, this looks likes its strictly for the environmentally-minded rather than those trying to save money.

Bottom line is that no matter what anyone in the battery industry claims or insinuates, battery technology for the long haul is nowhere in sight yet.

Sorry, but that doesn’t answer any of the questions that person has which is precisely why I suggested they ask somewhere else. The people on this board are all too happy to give 100 reasons why they would not do something, leaving the OP’s original questions unanswered.

OP asked ‘does it really work’. The costs (both $$ and pollution impact of making all those batteries) are so far out of balance with the small benefits (see below) that the answer is no, it doesnt’ work in terms that consider the overall benefit of the conversion.

Okay, well, I think we’ve pretty well covered question #1. So the answer to question #2 is that the batteries should last at least as long as the regular Prius batteries would, but the plug-ins haven’t been running around long enough to tell. As for #3, things like oil-changes and maintenance on the non-hybrid components will be unchanged, but you will be beholden to the company that did the conversion for any service necessary on the new parts and many of the original hybrid parts that will be modified.

But considering how expensive the conversion is, worrying about battery life or service costs are probably not going to be of great concern to anyone seriously considering it.

It is a good idea on paper until you do a break even analysis. Like most new technology, it is very expensive when it is new. Expect prices to come down as battery technology improves. I would wait until it makes sense financially, which means it will pay for itself in five years.

I have crunched some of the numbers, and at the current price of gas ($2), it will take you more than 40 years to recover your $10,000 if you average 12,000 miles/year, and that doesn’t include the cost of replacing the batteries when they die. Even if the price of gas goes back up to $4/gallon, you won’t recover your $10,000 in saved gas until you have averaged 12,000 miles/year for about 20 years, if the batteries last that long. Even if gas goes back up to $4/gallon, it isn’t a sound investment.

Increasing your miles per gallon (MPGs) from 40 to 67 may sound like a lot, but it isn’t. You only save about 100 gallons per every 10,000 miles. In comparison, increasing fuel economy from 10 MPGs to 12 MPGs will actually save more gas (167 gallons per 10,000 miles). So the folks who developed this Hymotion Conversion kit could save a lot more gas if they developed technology to help trucks and SUVs save fuel.

Obviously the op is asking if the system functions reliably and as advertised. Greasy Jack answered the question well.

You’re asking the wrong crowd. Ask a rep from the company to
let you speak to some references. You’ll get not much more
than conjecture here.

I have to agree with Texases on this one. It’s not conjecture at all. He’s providing some basic math to help the OP understand where the break-even point might be.

If the OP doesn’t want to use Texases’ “close enough” figures, then he/she can find and insert the exact cost and mileage figures. But at least the OP has the info now to make an educated decision (which wasn’t apparent from the OP’s post).

OP said “theoretically raises the MPG to 100 miles for the first 30-40 miles. Anyone know anything about this? … 1) Does it really work?”

Obviously, it doesn’t work, in the context of 100 mpgs. It is not ‘green’ to waste money and resources. I’m not negative on plug ins, just on wasteful plug ins.