With space’s extreme environments, there is never a one-size-fits-all battery. Nor is any space mission ever the same, so customized batteries for each operation are essential. Saft, has been powering outer space for more than 50 years. Saft’s first battery was launched into space in 1966 aboard the D1A “Diapason”, which was powered with nickel cadmium (Ni-Cd) technology. To date they are the only battery manufacturer to supply all battery technologies used in space: Nickel (Ni-Cd, Ni-H2), primary lithium (Li-SO2, Li-SOCl2 and Li-MnO2) and rechargeable lithium or lithium-ion (Li-ion).

The Saft 48997 Li-ion battery system is typical of those used to power satellites and spacecraft. (Image: Saft)

Following Diapason 1A success, Saft completed numerous Ni-Cd battery contracts for low earth orbit (LEO) observation satellites such as the Spot family, Helios military observation, and DSP / DMSP US, as well as satellites in geostationary orbit (GEO) telecommunications such as Telecom 1, Marecs and Meteosat. These satellites provide us with the images necessary to make meteorological predictions.

The Solar and Heliospheric Observatory (SOHO), which is positioned close to the sun, allowing hundreds of astronomers to study and analyze photographs of solar eruptions, has been powered with Ni-Cd batteries. Other well-known satellites such as Envisat, ERS, ISO and XMM (infra-red telescope Rx) also used Ni-Cd type batteries.

Starting in 1987, Saft developed and qualified the Ni-H2 technology, a combination of a Ni-Cd battery and a fuel cell system. A pressure vessel with Inconel material and significant hydrogen pressure allowed for a hydrogen electrode using platinum oxide. Exhibiting double the specific energy as per Ni-Cd, the Ni-H2 technology has represented a substantial portion of the battery market from end of the 1990’s to the beginning of 2000. This technology was progressively replaced by Li-ion in mid-2000.

This diagram explains the advantages Li-ion has versus Ni-Cd and Ni-H2. (Image: Saft)

Li-ion solutions for space require meticulous quality, testing and documentation for critical missions. Since the late 1990’s, Saft has engaged in the qualification of Li-ion batteries following regulations from the European Space Agency (ESA) and NASA. Each piece of satellite equipment, including the battery, is subjected to robust tests on ground to ensure it can sustain harsh space environments.

Harsh Environments

As part of the qualification test plan, batteries must pass the electrical performances characterization in large temperature and current range. Outside satellite temperatures can reach -180°C to +200°C. Mechanical tests with high level vibration and shock testing are completed with margin factors to simulate mechanical constraints during the launcher takeoff. These severe conditions are reproduced on the ground with specific test equipment simulating the environment the batteries will see. Space batteries must sustain very low pressure vacuum conditions, as well as high photon and ion radiations to demonstrate long-term cycle-life. A cycle performance test simulates the orbit profiles with mission durations that can last up to 18 years with more than 80,000 charge/discharge cycles to fulfill low earth orbit (LEO) missions.

The Philae lander and Rosetta space probe in orbit. (Image: CNES)

The cost to place one kilogram in orbit is in the range of $20/g to $60/g depending the on type of satellite orbit. The weight is the driving parameter and should be the lowest possible when asking for a highly specific energy battery. Specific energy is the unit representing the amount of energy delivered by one kilogram of battery (Wh/kg). The higher the specific energy of the battery, the higher the weight savings are at the battery level. This enables the satellite to embark with a larger payload with items such as scientific instruments, specific detectors, telescope, or additional antennas/emitters and transponders.

Since no repairing is possible in space, satellite batteries must provide power during the whole life of the satellite with margin and demonstrated high reliability figures. The batteries are designed and qualified to withstand very high reliability, high specific energy, extreme temperature ranges, high vibration/shock levels, radiation doses, and long operational requirements. Compared to previous qualified batteries such as Ni-Cd and Ni-H2, the main advantage of Li-ion is its substantial weight savings, which decreases a system’s overall size. The weight saving is given directly by the specific energy increasing (from 50 Wh/kg for Ni-H2 to 150 Wh/kg or more for Li-ion), but also by lower heat dissipation and high energy efficiency reducing the satellite’s weight by limiting the size of radiators and solar panels.

Modular Design

The Li-ion voltage slope during charge and discharge enables cell parallel assembly, providing modularity. Ensuring safety requirements, Saft’s “all-in-one” modular package provides balancing, by-pass and disposal functions. Designed for adaptations, solutions are adjustable for each satellite’s requirements.

Created for parallel and series assemblies, modular P-S batteries for space allow for numerous collections for up to 12 cells in parallel with VES140, VES180, VL48E, and VL51ES and up to 24 cells in a series. These designs are adapted to geostationary orbit (GEO) and medium earth orbit (MEO) platforms anywhere from 3 kW to 25 kW, and they have been used onboard more than 110 satellites placed into GEO orbit. The first GEO telecommunication satellite launched powered by Li-ion technology was the Eutelsat satellite W3A. The battery system consisted of two blocks, 6P11S and VES140, placed into orbit more than 12 years ago. The energy degradation after 12 years was less than two percent; compare that to systems that exhibited more than 40 percent degradation for the same mission.

With a special negative electrode, the small VES16 cell offers ultimate efficiency with its significant charge current capability, which is the key point for LEO missions. Batteries with a wide range of S-P configurations (4 to 60P) are largely used for LEO observation, military or constellation satellites. The unique structure helps reduce the weight of the satellite, successfully addressing needs of cycling in LEO missions. VES16 withstands more than 60,000 cycles up to 12 years with a depth of discharge (DOD) up to 40 percent. Other technologies are limited to 15-20 percent DOD.

Challenging space missions encompass GEO telecommunication satellites, MEO global positioning satellites, and high power telecommunications, observation, and defense LEO satellites. The main challenge is the demonstration of cycle life when mission duration reaches 15 to 18 years, because it is nearly impossible to run life testing for this duration on the ground. Accelerated life tests are performed to speed up the cycle duration.

Saft relies on its Li-ion Model (SLIM), compatible with GEO, MEO and LEO satellites, to simulate battery performances throughout the long term mission’s entirety. SLIM, macroscopic electrochemical model calculates optimum solutions and forecasts impending performance. Electrochemical characteristics (energy, capacity, EMF, internal resistance and end of charge voltage) or mission figures and profiles (power, duration, depth of discharge, end-of-charge voltages, and temperature during eclipse and solstice and cell failures) assist with the ideal construction of the battery solution.

Li-ion rechargeable battery solutions are generally coupled with solar panels on space applications to provide secondary power, extending mission life. However, when the sun obstructs solar panels during eclipses, Li-ion is the sole power source. From multiple times a day, to numerous months at a time within a year, eclipses make Li-ion rechargeable technology crucial for the success of critical space missions.

Satellites, Probes and Rovers

In today’s space market, satellites are the main consumers of rechargeable Li-ion technology. However, primary lithium technology is used to power space vehicles including probes and rovers for specialized missions. Primary lithium technology provides one-off missions with long-lasting power in extreme temperatures. Lithium sulfur dioxide (Li-SO2) batteries have powered the Mars Exploration Rover, Mars Lander, Deep Impact, and Stardust. Lithium thionyl chloride (Li-SOCl2) batteries have powered the Mars Pathfinder, Atlas Centaur, and Philae Lander. In fact, Saft’s Li-SOCl2 battery is credited for the success of the 2014 Philae mission onto the Tchouryomov-Gerasimenko comet, completing more than 90 percent of planned experiments and ensured telecommunications between the lander and Earth. The vehicle traveled for 10 years in temperatures below -60°C and was operational immediately to power the Philae lander after separation from the Rosetta probe.

Aside from extreme temperatures, some space missions require unique architecture to address individual conditions. The ExoMars Rover vehicle required Saft to customize a Li-ion battery solution supporting cleanliness protection of Mars. The mission’s purpose is to explore past or current signs of life on the planet. Knowing potential organisms on the plant may include bacterial cultures, the ESA emphasized the importance of “planetary protection.” To prevent infecting possible complex organic molecules, the vehicle and its instruments must carry out the mission in an extreme clean fashion. However, the critical technical choice for this mission stays the compatibility with the wide range of temperatures. The selected battery will be composed with MP XTD (for “extended temperature range”) cells, the only electrochemistry able to sustain Martian harsh conditions.

Just like Earth, conditions in space constantly shift requiring extraordinary battery solutions to power critical space missions. With no operation identical, the design, testing, and functionality of each battery solution must be customized to meet the application’s demands.

This article was written by Yannick Borthomieu, Space and Defense Product Manager, Saft (Île-de-France, France). For more information, Click Here .