Small Satellite Power Systems

Batteries

Batteries need to meet the power requirements that can not be covered by solar panels. In very rare cases primary cells are the only power source on-board, but more commonly rechargeable cells are employed. Batteries are therefore generally employed in eclipse, or to meet short term power peaks. Rechargeable batteries suffer from degradation with use, and so both the frequency and the amount of charge taken out of the battery become factors in limiting lifetime. Generally fewer cycles and less charge taken out in each cycle lead to a longer battery life time. The choice of technology is therefore closely related to the type of orbit. Low Earth Orbit may mean that a satellite clocks up over 14 charge/discharge cycles per day, or over 5000 per year. In higher orbits, eclipses are rarer and so a greater depth-of-discharge can be tolerated for the same lifetime. Some technologies are more suited to many charge/discharge cycles, whereas others are better at handling large energy cycles. In addition to the lifetime issues, battery technology choice is also based on the volume and mass required for the energy storage, and in some case safety considerations are important for instance if the spacecraft is carried on man-rated launchers such as the Space Shuttle. 

The primary battery technologies for Geostationary spacecraft are Nickel-Cadmium and Nickel-Hydrogen. The latter is increasingly being placed in a Common Pressure Vessel (CPV) in order to improve its energy density. Small satellites generally launch into Low Earth Orbit and have employed Nickel Cadmium (NiCd) in most cases. Nickel-Metal-Hydride batteries seemed promising technology for a while until Lithium Ion (Li-Ion) batteries appeared which is superior in many respects. Beyond that Lithium Polymer and even full Polymer batteries are promising technology.

For small satellites, NiCd is still the basic choice as heritage and low cost are desirable characteristics in many cases. Where mass is of the essence, or to demonstrate new technology Li-Ion is being proposed into missions, and so far has flown on three small satellites (STRV-1c/d and PROBA)

Links

Sony    A good set of pages explaining the basic battery technologies (albeit for terrestrial use)
Sanyo    Battery products
SAFT    Battery products, including space

Solar panels

Solar panels are generally used in missions where power must be generated beyond the capabilities of primary battery cells. Solar panels produce electrical power with Voltage determined by the cell technology and current by the level of incident light. There are inherent efficiency losses, and with typical solar flux of 1358W/m², the solar cell efficiency determines how much electrical power is generated by the total area of solar cells (not the panels!). Both Voltage and Current change with temperature and radiation dose, and as a result panels are less efficient at higher temperatures, and degrade with time due to total radiation dose. A technology  with a long heritage is Silicon, but its advantages over Gallium-Arsenide have gradually eroded over time. The current technology of choice for most  satellites is Gallium-Arsenide, and cells with efficiency of up to 26% are available, with 30% efficiency cells on the drawing board.  

Silicon is seeing a comeback in thin-film solar panels, which have a low efficiency near 10%, but have the advantage that they can be manufactured onto thin flexible sheets. Cells sold for terrestrial applications are typically also low efficiency Silicon, and have been successfully employed on very low cost missions.

Micro-satellites

This list contains data on some select power systems of various small satellites.

Please refer to the key to the tables if you are not using frames.

Mini-satellites