Electric Automobiles Electric automobiles sound like a good idea. The news is full of hyperbole about the great new Tesla Roadster and plug-in hybrids. Charge your car by plugging into the wall, and drive around with zero emissions. The Toyota Prius is half-electric, and that's been a big success, right? Moreover, wall-plug power is cheap. A kilowatt-hour costs only 10 cents. At $3 per gallon, the cost of energy from gasoline is about 36¢ per kilowatt-hour. That means that recharging a car by plugging it into the wall socket is equivalent to buying gasoline at under a dollar per gallon! Alas, electric automobiles have very serious problems. The fundamental one is that batteries store very little energy, compared to gasoline. High-quality, expensive batteriesthe kind used in cell phones and portable computersstore only 1% as much energy as gasoline, pound for pound. That's a big factor. It is offset somewhat by the fact that electric energy can be used more efficiently than gasoline, so the disadvantage for batteries is actually more like a factor of 30. For the same range, you can carry 62 pounds of gasoline, or 1860 pounds of batteriesalmost a ton. Because batteries have more pounds per cubic foot than gasoline, they won't take up 30 times the spacebut only 10 times as much. That's why cars like the Tesla Roadster are possible. The cost savings are illusory. High-performance batteries are very expensive and need to be replaced after typically 700 charges. Here is a simple way to calculate the numbers. The computer battery for my laptop (on which I am writing this) stores 60 watt-hours of electric energy. It can be recharged about 700 times. That means it will deliver a total of 42,000 watt-hours, or 42 kilowatt-hours, before it has to be replaced for $130. Put those numbers together to get the battery replacement cost: $130/42 = approximately $3 per kilowatt-hour. That's 30 times more expensive than the 10¢ per kilowatt-hour to charge it. The real expense for fancy batteries is not the cost to recharge them, but the cost to replace them. The same factor will be true for the Tesla Roadster. Driving it will seem very cheap, until the time comes to get new batteries. They are by far the most expensive component of that car. Can't batteries be made cheaper? Yes, but then they become unreliable and typically take fewer charges. That has probably already been your experience if you've purchased Chinese copies of expensive batteries for your computer, cell phone, or digital camera. The reason for the high cost is, in part, that batteries are intrinsically dangerous. Poor quality control leads to batteries that spontaneously burst into flames. Engineers are working hard to solve these problems. Can't the technology be improved to allow more recharges? Yes, and engineers are working on that. The problem is that in a battery a chemical reaction produces the electricity; to charge the battery, requires reversing that chemical reaction while maintaining the physical integrity of the electrodes. It is difficult. If the technology improves so that you can recharge the battery 7000 times instead of 700, then electric fuel will cost only 3 times as much as gasoline, instead of 30. There is one additional problem. Gasoline can be pumped into an automobile in 2 or 3 minutes. Charging a battery typically takes 15 minutes to a half hour. Quick-charging batteries are being developed, but they may be even more expensive. What about the Prius? That's a hybridand it uses a battery. And it's a great success! Doesn't that prove that my numbers must be wrong? Unfortunately, the answer is no. Even the Prius suffers from battery limits, although very few Priuses have been driven far enough to require battery replacement yet. The Prius attempts to minimize the recharge problem by using the gasoline engine most of the time. The battery is recharged primarily during braking, and that helps the car get its good gas mileage. It is also used when accelerating from a stop because that's when the gasoline engine is least efficient. Eventually the batteries will need to be replaced, which may cause the owners consternation when they learn the cost. Some Prius owners have adapted their cars to "plug in," so that they can recharge their batteries at home, and some add circuits that allow them to drive the car in pure battery mode without the gasoline engine turning on. They say they are saving lots of money on gasoline. That's true. But the more they use the batteries, the sooner they will have to replace them. They may be shocked when they have to pay for the new ones, and I don't mean electrically shocked. There is one kind of cheap and reliable kind of battery that is not overly expensive to replace: the lead-acid battery used in standard autos to start the engine. This battery is so inexpensive that it can actually be used for an electric car and will cost no more than $3 per gallon of gasoline equivalent even when the replacement cost is considered. The main disadvantage of such batteries is their poor energy storage capability. They typically contain 1/1000 the energy of gasoline. Combining that fact with the high efficiency of electric motors leads to the conclusion that lead-acid batteries are only 300 times worse than gasoline, pound for pound. That's why they are used for extremely short-range applications, particularly when pollution from carbon dioxide would be a problem. They work effectively for forklifts and wheelchairs but, with the factor of 300, find very little use for normal driving. Who killed the electric car? Expensive batteries did.
"Physics for future Presidents" by Richard Muller (2008), pp. 304-7
