Hard-charging electric vehicles? Do the math!

Glaskowsky challenges innumeracy in media reports of a new battery technology.

A couple of people sent me links to an Associated Press story, "Texas Startup Says It Has Batteries Beat."

In the story, AP writer Grant Slater presents the claim by Austin, Texas, start-up EEStor (the company has no Web site yet) that "a motorist could plug in a car for five minutes and drive 500 miles roundtrip between Dallas and Houston without gasoline."

Now, c'mon. This is just not practical. I can say this even though I know nothing about EEStor's technology beyond news reports that it's some kind of high-density capacitor.

It takes about 10 horsepower to keep a regular passenger car moving at highway speeds, overcoming wind and rolling resistance. The energy for the trip works out to 10 times 750 watts times 30,000 seconds (the duration of the trip at 60 mph) or 225 megajoules. Let's just ignore all the efficiency issues-- losses in the capacitor, losses in the electric motors, losses in the cables--and think about that figure.

Putting 225 megajoules into a car in five minutes (300 seconds) requires a power supply of 750 kilowatts. That's a thousand times the power of a high-end gaming PC or a waffle maker.

Let's consider what kind of current this would require. In a U.S. household, the highest voltage available is usually the 240V service for ovens, clothes dryers, and the like. This service is usually limited to about 60 amps...but fast-charging an electric car as we've described would require 3,125 amps. Ready to plug your car into 52 outlets at once?

Any inefficiencies in the system start cranking up that number, and it's already impossibly high. So 240V isn't going to work for anyone, and 480V service--as high as you'll probably ever see in a garage--wouldn't make enough of a difference.

The overhead wires in your neighborhood might be carrying "7200/12470Y" power--three-phase AC with each phase carrying 7,200 volts referenced against a common neutral. The effective voltage is that 12,470 kilovolt figure. Now we're talking, huh? With that kind of service running into your garage...well, you'll never have that kind of service in your garage; it's just too dangerous...With that kind of service available at the neighborhood filling station you could make do with just 60 amps of current on a three-wire cable that would actually bend a little...but switches and connectors for 7,200-volt power are not very practical.

So either we're talking about insanely heavy copper cables or insanely high voltages. I'm sure this is all well-known to the people at EEStor, but they've obviously been seduced by the siren call of the sound bite. That's fine, we know companies sometimes say unbelievable things.

But reporters are supposed to disbelieve unbelievable things. If EEStor said its technology would allow an ordinary automobile to fly from Houston to Dallas, Slater surely would have demanded some backup. This 5-minute charge concept is just as crazy, but he passed it along unquestioned.

This is the one area where gasoline is simply a lot better than electricity. Gasoline contains about 130 megajoules per gallon, and it takes only about 10 seconds to pump a gallon of gas. Even with the very low efficiency of gasoline engines, you can fill a tank a lot faster than a battery or capacitor with the same driving range.

That's an advantage for hybrid cars, but how important is it, really? As long as the range of an electric car is much larger than the distance of a daily commute, and larger than the distance between hotels out on the highway, electric is still a reasonable alternative. A 30-minute charge after a 30-minute commute is easily accomplished with the power from a 240V outlet.

But don't get your hopes up for 5-minute, 500-mile recharges. That's just marketing hype.

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About the author

    Peter N. Glaskowsky is a computer architect in Silicon Valley and a technology analyst for the Envisioneering Group. He has designed chip- and board-level products in the defense and computer industries, managed design teams, and served as editor in chief of the industry newsletter "Microprocessor Report." He is a member of the CNET Blog Network and is not an employee of CNET. Disclosure.

     

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