“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 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.”