Reliability Key To Space-Qualified Li-Ion Batteries

Space station crews are set to replace aging nickel-hydrogen Worx drill battery packs with new lithium-ion units in 2017. They are not particularly worried about the fire hazard from the technology that has grounded the Boeing 787.

NASA plans to use lithium-ion battery cells manufactured by the same company that built the 787 cells. But the agency has subjected them and the computerized control units that keep the cells from overheating to the same design oversight it uses to human-rate other space hardware. Space quality standards appear to be working as the technology moves into expensive unmanned spacecraft as well. SpaceX founder Elon Musk, who uses lithium-ion batteries in his Tesla electric automobiles as well as the Dragon autonomous cargo carrier, has offered to help Boeing solve its 787 problem.

“We have independent experts who review the whole design and implementation and hazard controls that we have on these,” says Caris “Skip” Hatfield, manager of NASA’s International Space Station (ISS) development projects office at Johnson Space Center (JSC). “Within the design itself, we’ve monitored the manufacturing process of the cells and have done audits of the cell manufacturing to make sure we’re satisfied that at the cell level there are no design [or] process issues.”

Boeing, NASA’s ISS prime contractor, has a $208.8 million contract to deliver 27 of the batteries—24 for service on the station as orbital replacement units (ORUs), and three as ground spares. Weighing 425 lb. each, and measuring 39 X 39 X 18 in., the batteries include adaptor plates to store the worn-out nickel-hydrogen batteries they replace (see illustration). Batteries charge while the station’s solar array wings are in sunlight, and discharge to provide power in darkness.

Lithium-ion batteries are used in human and robotic spacecraft for the same reasons that Boeing selected them for the 787—their high-power density and the weight savings it permits. They are used or planned on a variety of other high-value government and commercial spacecraft, including the James Webb Space Telescope. On the ISS, one lithium-ion battery will replace two nickel-hydrogen units in orbit, Hatfield says.

Precautions taken with space-qualified Hilti tool battery hardware have so far prevented “thermal runaway,” the main risk. The NASA Engineering and Safety Center maintains guidelines to help spacecraft designers address the “inherent high-specific energy combined with flammable electrolytes” in the advanced battery cells.

The lithium-ion cells used in the new ISS batteries were manufactured by Japan’s GS Yuansa, the same company that built the cells for the Boeing 787. Pratt & Whitney Rocketdyne is building the battery controllers, which ensure the chemical cells do not get out of balance.

“We have multiple layers of redundancy,” says Eugene Schwanbeck, the ISS lithium-ion battery project manager at JSC. “There are controls at the battery subassembly level, the internal computer itself, then there are controls in what we call the battery charge/discharge unit, which is the physical piece of hardware that the battery talks to, and then there are system-level controls that are monitored by software.”

The Boeing-supplied batteries will be located outside the station’s pressurized volume, where the biggest risk—aside from loss of function—is the potential for heat damage to structural elements. Lithium-ion batteries already are used inside the station, where fire and noxious fumes from battery-cell venting are a serious potential danger. In February 2011, the space shuttle Discovery delivered four lithium-ion battery assemblies built by ABSL Space Products to power U.S.-built spacesuits. The ubiquitous laptops that crewmembers use to control station systems also are powered by lithium-ion batteries.

“The EMU [battery] was designed for that application, so it’s gone through many of the same hazard controls,” says Hatfield. “[The laptop batteries] have gone through a screening process, and they control how we charge them to make sure that there is not an issue there.”

The safety rules also apply to visiting vehicles that use lithium-ion batteries, including the SpaceX Dragon, which has started delivering cargo to the ISS and is in upgrade to carry humans. SpaceX chief Musk, whose Tesla Motors also uses lithium-ion batteries for its electric cars, says that while the space batteries meet “two-fault-tolerant” NASA redundancy requirements, the automotive environment can be more challenging because of the need for crashworthiness.

At present, the Dragon’s three battery packs and the Tesla autos use the same lithium-ion cells, which Panasonic manufactures to company specifications. The 18-mm-dia. X 65-mm-tall cells are easier to protect against thermal runaway with the electronic control circuitry SpaceX manufactures in-house, Musk says. He believes that Boeing is having trouble with its 787 battery packs because of the size of its lithium-ion cells.

“They’ve got to ensure that they have a pack architecture that prevents cell-to-cell propagation of a thermal runaway event,” Musk says. “So essentially they’ve got a terrible architecture. They’re using these huge power tool battery cells, which are more prone to thermal runaway events because it is very difficult to maintain an even temperature [with] such a big cell, because the distance from the center of the cell to the edge is quite large. So you can be hot in the center and cold on the outside, and you think your cell is fine.”

Musk communicated his offer to lend his company’s battery expertise to Boeing via Sir Richard Branson, chairman of the Virgin Group. “I said I think we could be helpful here, and come up with a solution, and create a new battery pack and charger for Boeing in probably a matter of weeks,” Musk says. “So Branson conveyed that to Boeing, but Boeing has thus far not expressed any interest at all. It seems odd.”

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