The Boeing 787, billed as a revolutionary design in terms of fuel efficiency, range, and passenger comfort, has spent the last two months with its wings clipped. The FAA grounded the Dreamliner following two separate high-temperature failures of the plane's lithium-ion batteries in January, leaving the 49 planes that Boeing has delivered so far to seven different airlines sitting idle.

Today Boeing executives announced a fix that the company hopes to implement and certify "within a matter of weeks." The new design involves a stainless steel enclosure that will, according to Boeing, prevent any possibility of a battery fire because it will starve the battery of oxygen. Boeing also admitted, though, that it may never find a direct root cause of the failures.

Boeing's choice of lithium-ion batteries for key electrical systems—unprecedented in commercial aviation—had come under fire in the weeks following the grounding because of the inherent risks of Li-ion technology. Rival Airbus announced last month that it will no longer use lithium-ion batteries on the forthcoming A350-XWB, slated to enter service in 2014. Despite Boeing's optimism for their fix, it's really a band-aid solution, since it doesn't actually address the cause of the battery failures in the first place. Many experts are still asking: Is lithium-ion technology the wrong choice for aviation?

A Key Part of Modern Airliner Design

Aircraft batteries, at the fundamental level, play a similar role to automotive batteries. They initiate engine start (by starting the auxiliary power unit, or APU) and provide system power when the engines are not running—when the aircraft is on the ground or during an in-flight emergency.

Traditionally, lead-acid batteries and nickel-cadmium batteries have been the go-to for aviation. General aviation planes and some business jets use lead-acid chemistry, while larger business jets and commercial aircraft typically use Ni-Cad. These are durable and have good low-temperature performance but require regular maintenance, as frequent as every 100 flight-hours, that involves a 24-hour period of aircraft downtime to fully cycle the batteries to eliminate memory effects. And though they pack a better energy-to-weight ration than lead-acid batteries, nickel-cadmium batteries still carry a significant weight penalty compared to more modern battery chemistries.

More Electric, More Efficient

The Dreamliner is a whopping 20 percent more efficient than similar-sized aircraft from previous generations. One big chunk of this improvement comes from improved engines and aerodynamics, but the 787 also greatly benefits from weight and efficiency improvements that owe much to the use of lithium-ion batteries instead of Ni-cad. Li-ion batteries hold nearly three times as much energy with the same amount of weight. Outfitting the plane with Ni-Cad batteries instead of Li-ion would have made each Dreamliner about 200 pounds heavier.

And the 787 uses battery power for much more than its predecessors did. Traditional aircraft use compressed air from the engines to power extensive pneumatic and hydraulic networks that drive flight control surfaces and other critical systems in the plane. The 787, uses electrical power to accomplish many of these tasks, both directly via electric motors and by running smaller, higher-pressure hydraulic compressors. This allows the Dreamliner to use "bleedless" engines, meaning less wasted energy.

Naturally, though, this design shifts more load to the electrical system, increasing both the peak power demands and the energy storage requirements for the batteries. To achieve maximum fuel efficiency despite the weight demands, Boeing decided to use lithium-ion batteries not only for the Dreamliner's Ship's battery—which provides backup power—but also the APU battery, a decision that has now come under significant criticism.

It's well known that Li-ion batteries are inherently less stable than Ni-cad or other battery types. Li-ion batteries are more susceptible to a failure mode called thermal runaway, when one cell overheats (exceeding 130 degrees C), triggering a chain reaction in the neighboring cells and causing a catastrophic failure at high temperature. The root cause can be manufacturing defects, overcharging, or physical damage such as a puncture. There are many well-documented cases of Li-ion battery fires in consumer electronics, and more alarmingly, in aircraft: In 2010, a UPS 747 freighter crashed because of a main-deck cargo fire fueled by a large shipment of Li-ion batteries.

The 787 isn't the first aircraft to use Li-ion batteries. Airbus' A380 uses lithium-ion to power its emergency lighting system, and to do so the batteries had to meet extra certification requirements because they are inherently less stable than Ni-Cad batteries. Still, the scale and operational requirements for the Li-ion batteries in the 787 is vastly different, and, frankly, unprecedented for a commercial airliner.

A Better Way?

Perhaps it's not the use of Li-ion batteries that's the Dreamlier's problem, but rather the way those batteries are put together. That's the idea floated after the 787 mishaps by Tesla Motors CEO Elon Musk (also head of SpaceX), who criticized the design of the 787's two Li-ion batteries, calling it "fundamentally unsafe" in an email to the aviation publication Flightglobal.

Musk may be on to something. The Tesla Roadster's Li-ion battery—which, at 56 kWh, packs more than ten times the 787's combined battery capacity—has a much different design. Boeing's batteries are composed of eight large cells, each of which weighs 6 lbs and can hold approximately 250 watt-hours of energy. By contrast, the Roadster's battery is made up of about 6800 cells, each weighing merely 1.5 ounces and packing just 6.7 W-h. According to Musk, thermal runaway in a big cell releases "a proportionately larger amount of energy" compared to a small cell, which makes it difficult to avoid a domino effect. Tesla batteries also package the cells differently. The 1.5-ounce cells are cylindrical, resulting in less direct contact area with neighboring cells, and the cells are packaged into sheets that are surrounded by a liquid cooling system. The eight cells in the 787 battery are directly adjacent and without any supplemental cooling system.

Musk's comments make it sound like it might be easy for Boeing to redesign the 787's electrical architecture to incorporate a brand new Li-ion battery design with the Tesla-style safety features. In reality, this is the type of change that can take years to incorporate into an in-production aircraft.

Problem Solved?

Boeing is now on track to finally get the 787 airworthy again. The company is working to retrofit the 49 delivered aircraft and the 25 waiting for delivery as soon as possible once the design change is tested and certified.

Still, it's unlikely that the 787 will be back in commercial service within two months. Although Boeing gave no indication that its proposed fix is temporary, the new design is far from ideal. The steel enclosures add about 150 lbs, which removes nearly all the weight savings gained by using Li-ion batteries in the first place. Most likely, Boeing will still be working on a more long-term solution to make the batteries safer and eliminate some of this weight.

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