Lithium Ion Battery Management Strategies for European Space Operations Centre Missions

摘要

Effective battery management on a space mission is one of the key factors in ensuring mission success and longevity. Given the reliability of modern spacecraft, the unavoidable ageing of batteries can become a critical life-limiting factor. To improve this, it is necessary to have a strategy for management and monitoring of spacecraft batteries that is tailored to both the mission profile and the battery technology in use. This paper will focus on several missions flown from the European Space Agency's (ESA) European Space Operations Centre (ESOC) in Darmstadt, Germany. The main case studies in this paper focus on missions that regularly use their Lithium Ion batteries, although a summary of other missions that contain Lithium Ion batteries will also be presented. Lithium Ion batteries are currently the prevailing battery technology in use on current and future European Space Agency missions. The paper will begin with an overview of the Lithium Ion battery technology that has largely replaced all others for modern space batteries. Their proper management requires different techniques compared to previous space battery technologies; for instance compared to the previous Nickel-Cadmium technology, Lithium Ion battery deep discharges should be avoided where possible - which increases the risk of using deep discharges to measure degradation. The paper will describe the characteristics and influencing factors of Lithium Ion battery degradation, along with an overview of research aimed at prolonging lifetime of the batteries. The paper will also summarise methods available in order to measure the absolute or relative degradation of Lithium Ion batteries and the limitations of these methods based upon the capabilities of each spacecraft and the mission profile. The paper will then detail the actual operational implementation of this information on two representative ESA missions. The first case study will be Mars Express, which has been flying three Lithium Ion batteries for ten years and using them for prolonged eclipse seasons 2-3 times per year. The power demand of the spacecraft is high and the available margin in the power system is low, therefore modelling and management of the batteries is critical to the mission. The second case study will be ESA's CryoSat-2 mission, which has been flying a single Lithium Ion battery for 4 years. The battery is younger, and the power system has more margin but eclipse seasons are an almost constant feature of the routine mission (albeit with varying duration eclipses). In addition, the satellite flies in a non-sun-synchronous orbit, which makes the assessment of the expected state of battery charge more difficult. An overview of the techniques used on other flying ESOC missions will also be presented (Herschel, Planck, GOCE, Venus Express and Rosetta). The paper will describe new operations that have been introduced to manage the degradation of the batteries, including specially designed settings that, while respecting the allowed usage profile of the battery, modify the charge and discharge management strategies and other flight operations to almost halve the rate of degradation compared with the worst-case design assumption. In addition, the methods used by each mission to assess absolute and/or relative battery degradation in flight will be discussed. The paper will conclude with an overview of the lessons that have been learnt so far at ESOC from missions flying Lithium Ion batteries. These lessons could be used as a model for current and future operators of spacecraft with Lithium Ion batteries on how to best manage their batteries for longevity, mission reliability and success.