"The peak power requirements can hit 150 watts during
transmission times, when the rovers relay data or pictures to Earth," said
Richard Ewell, project element manager of MER power subsystems at JPL. The two
batteries aboard each rover also recharge during the day at periods when more
solar energy is collected by the rover than can be immediately consumed, he
added.
Ewell told SPACE.com that the lithium-ion batteries
were responsible for powering each of the rovers at liftoff and will provide the
power necessary to deploy their parachutes during the descent to the Martian
surface.
Spirit and Opportunity are the first NASA spacecraft
to carry the new batteries, which weigh about 16 pounds (7.15 kilograms) a piece
and 19 pounds (8.77 kilograms) when combined with the support struts and heating
units necessary for the planetary journey. Similar, but smaller, batteries were
designed and tested for the Mars Surveyor 2001 probe set to launch in mid-2001.
However, NASA canceled that mission before they could be flown. The Surveyor
2001 battery tests were handy for the engineers building the rechargeable
lithium-ion units for the MER missions, JPL researchers said.
The hardy batteries should last the entire duration
of each rover's 90-day mission, although dust accumulation on MER solar panels
could eventually hamper the recharging process. Even so, a 90-day mission will
completely surpass the accomplishments of Mars Pathfinder in 1997. That
mission's lander used compact, rechargeable silver-zinc oxide batteries that
shut down after about a month. Sojourner, Pathfinder's little rover that could,
used non-rechargeable lithium-vinyl chloride batteries that eventually ran dry,
leaving only the solar panels to fill its power needs.
Building a better battery
Even before NASA looked into Air Force power systems,
AFRL researchers were developing new batteries capable of surpassing those
commercially available in terms of energy output.
AFRL researchers have been working on lithium-ion
technology since the mid-1990s, though the original concept was invented in
Japan at the start of that decade, said Stephen Vukson, an AFRL chemical
engineer and manager of the laboratory's lithium-ion battery project. NASA
officials began working with the Department of Defense on the batteries in 1997
as part of a joint lithium-ion development program.
"We use lithium because it's a light metal and
carries a high voltage when compared to other power systems," Vukson told
SPACE.com. For example, he added, a car battery produces about two volts of
electricity, while a lithium-ion version of the same size can produce more than
four volts.
In a traditional battery, electrical energy is
produced when ions move from a negative electrode to a positive electrode
through an electrolyte solution that is typically water-based.
The AFRL's lithium-ion battery, however, uses a
negative electrode composed of lithium ions encased in a carbon matrix, which
prevents volatile reactions by the highly reactive lithium metal. The lithium
ions flow through an electrolyte composed of organic solvents and lithium salts,
since a water-based solution would also react adversely with lithium metal. The
positive electrode is typically made of cobalt or nickel, explained AFRL
officials.
Military space
Vukson said the AFRL did have space, but not Mars, in
mind when it launched its initial lithium-ion battery project. "We were
typically thinking more about satellites, as opposed to rovers," he said.
Since rechargeable lithium-ion batteries are lighter
and smaller than nickel-cadmium or nickel-hydrogen units, they cut down on a
military satellite's weight. That savings could mean lower spacecraft launch
costs or allow satellite designers to add more sensors and equipment that would
otherwise be sacrificed for weight concerns.
In military aircraft, such as the B-2 Spirit stealth
bomber, a 30-pound (13-kilogram) lithium-ion battery could serve the same power
needs as a 100-pound (45-kilogram) nickel-cadmium version and still be only half
the size.
The smaller batteries generate less heat, reducing
the size of radiators required to keep a satellite from overheating. They also
don't require as large solar panels either, since they can convert solar power
into energy more efficiently.
Something smaller
While rechargeable lithium-ion batteries offer some
improvements over their nickel-based counterparts, there is still room for
improvement.
AFRL researchers have said they are working to
increase the temperature range in which the batteries can operate. The MER
batteries, for example, function between -4 degrees Fahrenheit (-20 degrees
Celsius) and 140 degrees Fahrenheit (40 degrees Celsius). But military aircraft
require battery performance at both colder and hotter temperatures.
"You also can't use pure lithium metal in these
systems," Vukson said, adding that the carbon matrix used to contain lithium
ions and limit reactivity contributes greatly to the system's weight. "If you
could get rid of that, the battery may weigh half as much; but it's no small
chore."
Meanwhile, Ewell said that NASA engineers are
considering whether to use a rechargeable lithium-ion system in future Mars
missions, such as the Mars Science Laboratory - a proposed 2009 mission still
under study. At the same time, other established battery systems, such as
nickel-hydrogen for one, have not yet been ruled out, he
added.
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