Marshall Brain's Blog

The previous post on Toshiba's new batteries (with one-minute recharge times) prompted a question that then led to an email exchange. The question is, "Can't we now use these batteries to create electric cars that recharge in a minute or two?"

The answer to that question is "probably not." One funny thing about electric cars is that the recharge time might still be high even with batteries that take only one minute to recharge.

A typical car uses about 15 horsepower to cruise on a flat freeway. The power is needed to cut through aerodynamic drag, overcome rolling resistance, run the air conditioner and other appliances in the car and so on. Let's say an electric car is more efficient, so it needs 10 HP. A HP equals about 750 watts. If you assume normal inefficiencies in the batteries, the converter, etc. let's say it's 1,000 watts per HP. That means the car needs about 10 kilowatt-hours per hour to go down a flat highway at a reasonable speed.

If you want the car to have a three hour range, what this means is that you need to be able to pump 30 kilowatt-hours of electricity into the car to fully recharge it.

The biggest circuit breaker in a typical house might be 50 amps at 220 volts (probably for the air conditioner). That's roughly 10,000 watts. So, if you plug an electric car directly into the biggest circuit breaker in a typical house, it would take about three hours to fully recharge the car. It has nothing to do with the batteries -- it has to do with the amount of time it takes to move electricity across a wire into the car.

Even if you plug an electric car directly into the 200-amp main coming into the house, all you can pump is 40 kilowatt-hours of electricity per hour into the car. It would still take perhaps 45 minutes to recharge an electric car with 150 to 200 mile range. And you would not be able to turn on anything else in the house while recharging the car.

I don't ever think we will have quick-recharge electric cars. It is always going to take at least an hour or so to recharge an electric car with any kind of range if "plugging it in" is the way you do the recharging.

It is when you think about this that you realize the miracle of gasoline. When you pump 20 gallons of gas into your car in 5 minutes, you are transferring an amazing amount of energy into the gas tank. A gallon of gas, if you could convert it completely to electricity in a 100% efficient process, contains about 36.6 kilowatt-hours of energy. Twenty gallons of gas therefore contains about 730 kilowatt-hours of electricity. And it all comes into the car in 5 minutes.

As you can see, gasoline has a good energy density that it makes it easy to move a fair amount energy into a car's fuel tank quickly. Is there anything with a better energy density? One year ago today I wrote an April-Fool's article on a vehicle called the NCar. The NCar is powered by enriched uranium, and it is such an appealing idea. A pound of highly enriched uranium contains the energy equivalent of about one million gallons of gasoline. If we could somehow tap into that incredible energy density in a safe way using a small (big as a typical car engine, say) reactor, we would be set. You would never have to refuel your car. If we could make small nuclear-powered AA batteries, they would potentially last for decades in a cell phone or laptop without ever needing recharging.

Alas, it does not appear that we will be seeing uranium batteries any time soon...