This section deals with the selection and layout of the secondary power system. This system has to store the required power for eclipse times and needs to be functionally over the entire mission lifetime. The battery pack is used to store power generated when the satellite GranaSAT is in the sunlight. It should be noted that the battery is only used as a power source during periods of high power demands or when the satellite is in an eclipse. The battery used is made by SAFT and consists of one lithium-ion cell. This battery has a prismatic shape, making it easier to fit within the cubic confines of the satellite.
Analysis and Evaluation of different Batteries for Cubesats
Before we determine the needed capacity of the battery we will have a look on the various types of battery cells available and used for space-application.
If we have a look in the website of the most important suppliers of CubeSat systems:
|Clyde Space||Lithium Polymer|
|The CubeSat Cookbook||Lithium Ion|
Lithium Ion is the most common battery used for space missions. But before we analyze the Lithium Ion batteries we are going to study Lithium Polymer in order to compare both.
Lithium Polymer Batteries
Laptop computers, cellular phones, and other personal electronics use LiPo technology. These batteries offer a number of distinct advantages:
High Energy Density
These batteries are capable of storing a proportionately very high amount of energy compared to other tehonologies such as NiCd or NiMH.
Flat Voltage Curve
LiPo are considered fully charged at 4.2 volts and discharged at 3.0 volts. This allows for a very flat voltage dischar curve, providing a relativity constant performance throughout the discharge cycle.
Low Self Discharge
Unlike NiMH and NiCd batteries, Lithium Polymer cells experience a very low rate of self-discharge when not in use. This means that users can now fully charge their Lithium Polymer batteries one day and then use them weeks later, at which time they will still have nearly a full charge.
Continuous Discharge Rating (“C Rating”)
The continuous discharge rating in terms of the pack’s capacity «C» are listed on the notation on the Redline battery packs.
No Memory Effect
Lithium Polymer cells do not develop a memory effect from being only partially discharged and then charged again unlike the NiCd cells. The cells may be partially charged and discharged without damaging their performance so long as they are kept within their normal operating voltage parameters.
The same chemical material is utilized in Lithium ion and lithium polymer cells. As a result, they perform similarly in terms of capacity, cycle life and talk time. However, there are some differences.
Primary, Lithium polymer chemistry differentiates itself from Lithium Ion in the type of electrolyte used (a plastic-like film that does not conduct electricity but allows ion exchange). As a result, LiPo batteries are lighter than Li-Ion.
However, there is a very important and decisive difference: Li-Ion has been the most used technology for CubeSAT missions up to now. When a spacecraft is designed, reliability is more important than using the latest and most powerful technology because of the need to make sure it works.
Rechargeable lithium-ion battery MP 176065
Once GranaSAT-I’s battery technology has been selected, we can start searching for an appropiate model for our mission. The following graphics show a comparison between some Li-Ion batteries:
The MP 176065 Lithium Ion cell has been selected for use on the GranaSAT-I. Prior to selecting this cell it will be undergo a number of tests to verify its performance in a space environment.
Translation and proofreading by Felisa Sánchez Aragon