Aviation authorities in the U.S. and abroad have grounded the new Boeing 787 Dreamliner because of problems with big batteries onboard. Batteries failed on two planes operated by Japanese airlines, including one battery that burned while the plane was on the ground.
These lithium-ion batteries are new to jetliners. They're powerful and lightweight, and, unfortunately, they're also fragile.
"Everyone's carrying a lithium ion battery in their pocket today," says chemical engineer Vince Battaglia at Lawrence Berkeley Lab in Berkeley, Calif. They're in cellphones, laptops, cameras — small electrical devices thrive on them. "And there's not a big issue" about them going up in flames now and then.
Lithium-ion batteries produce twice the voltage traditional batteries do. And when they go bad, that power turns into heat. The reason it's not such a big deal, Battaglia says, is that the batteries are usually smaller than pinkie. "The energy is small. It's easy to get the heat out of that little battery"
A small battery loses its heat quickly. But if you supersize that battery, up to, say, the size of a big suitcase, it's harder to get rid of the heat.
"When you go to these bigger cells, you've got all the heat in one cell," Battaglia says. "You've got to get it out."
You do that by using fans or circulating a coolant around the battery. That works fine so long as the battery is intact. But chemical engineers know that lithium-ion batteries have an Achilles heel for two reasons.
One is the fluid inside the battery, called the electrolyte. Positive and negative ions flow back and forth through the electrolyte, which creates electricity. In normal batteries the electrolyte is an acidic water. In lithium-ion batteries, it's a flammable organic compound.
Donald Sadoway, an engineer with MIT, notes that "if you do get up to temperature, you've got fuel there. It'll burn."
And you can get high temperature because of the second weakness. It's a thin membrane inside the battery that it regulates the flow of ions in that electrolyte fluid. If that membrane breaks, the flow of ions gets out of control, and the fluid heats up.
"You breach that membrane," says Sadoway, "and now you've got two compounds that want to react with each other violently and there's nothing to prevent them from doing so."
If it gets hot enough, the electrolyte fluid vaporizes and escapes. Think of that brandy you pour into a hot skillet and light up for a little flambe.
Laptop batteries sometimes burn like this. And one of Chevrolet's Volt electric cars did. After a controlled crash test, the battery's electrolyte got loose and burned the car to a crisp.
Berkeley's Battaglia says these batteries are pretty delicate. He says he'd drive a car with lithium-ion battery, but he worries about what happens when that internal membrane gets damaged. "If I got into an accident, I wouldn't take it into my garage and plug it in," he says.
But Sadoway at MIT says these batteries certainly can be used safely. "We know what the possible failure modes are, and they are defendable," he says. "You can make these lithium ion batteries absolutely bullet proof, safe and everything else, but the question is, what's your price point?" Who's willing to pay the cost, compared to other batteries?
That price point may be too high for the auto industry. John Hanson, Toyota's environmental and safety manager, says the company uses them in its all-electric vehicles. The company trusts them. But Toyota is looking for alternatives.
"The only thing we have against lithium is that in order to go to the next step in developing vehicles that the mass market will buy," he says, "they have to be less expensive and they have to deliver more range, and we can't get that from lithium."
Their future in airplanes, however, likely hangs on what engineers learn from the two incidents on the Dreamliner.