System Level Modelling of Batteries in Spacecraft Power System in Multiple Source and Multiple Load Configuration

Abstract

In present day spacecraft applications, power system uses solar arrays as the primary source of power generation and rechargeable Lithium ion batteries as secondary source of power. In normal orbits, during sunlit period, solar arrays support the power requirement from all the subsystems and payloads along with the charge power requirement of batteries. During eclipse period, entire load should be supported by batteries. In addition to the eclipse support, starting from launch pad to till deployment of solar panels, all loads will be on batteries. Contingency loads should also be taken care by batteries. So, battery sizing is very critical part in designing the power system of spacecraft. In multiple source and multiple load configuration where two or more batteries are connected to same Spacecraft power bus, current sharing between the batteries is expected, which needs to be considered for optimized sizing and designing of batteries. The scenario becomes complex with additional constraints namely some loads can come on one battery and not on others or vice versa. Even in a configuration where potential difference exists between batteries, current sharing will start as soon as this difference vanishes. A system level simulation model is developed, using network equations, to analyze in-circuit performance of batteries in multiple source and multiple load power system. The simulation model uses actual current sharing and predicts State of Charge (SOC) of each battery from which Open Circuit Voltage (OCV) is derived. This OCV of batteries is used periodically for finding actual current sharing which runs in a loop for analyzing battery performance in multisource system. Currently a configuration in which two batteries are connected to two buses, which are paralleled and having different loads, is simulated. Optimized battery sizing, actual current sharing and SOC of each battery are derived. This model can also be extended to a system where more than two batteries are present. Two different approaches are used for simulation and results are compared. The developed generic model has features to use in multiple battery configuration. He has served as Project Manager and Deputy Project Director for several spacecraft projects. He steered the development of modular design of high energy lithium ion batteries, Nickel hydrogen and Nickel cadmium batteries. He has been involved in Design, fabrication, testing and on orbit Management of Satellite Batteries and power system since Sept 1985. Sankaran M, currently Outstanding Scientist and Deputy Director, Communications and Power Systems Area, U R Rao Satellite Centre, Indian Space Research Organization. He joined ISRO in 1986 after his M.Sc., degree in Physics from RSGC, under Bharathidasan University, Tiruchirappalli. During his past 32.5 years in ISRO, he has contributed mainly in the area of solar arrays, power systems and RF communication systems for spacecraft. He has served as Project Manager and Deputy Project Director for several spacecraft projects. He steered the...