Battery operations for the TET-1 spacecraft

Abstract

The TET-1 satellite serves as a technology demonstrator within the On-Orbit Verification (OOV) program with the goal of providing German space companies and research institutions with the opportunity to test their equipment on actual spacecraft. It was launched on July 22, 2012, by a Russian Soyuz/Fregat into a Sun-synchronous orbit with an LTAN of 11:27 UTC. The satellite is powered by nickel-hydrogen batteries during eclipse. The operations team became aware of several issues shortly before and after the launch which created some challenges for battery operation. Some battery cells had undergone reversal prior to launch which seems to have created an imbalance between them. Also, the overall temperature within the satellite and that of the batteries turned out to be higher than predicted and the battery voltage slightly exceeded the limit for payload operations. Regulating the battery temperature was at first attempted by lowering the amount of charge put into the battery. However, this could not be done indefinitely due to worries of battery damage (memory effect) and the constricted charge available for payload operations. Hence, other approaches were employed. The Sun Pointing Rotate Mode (SPRM) was one of the methods used. The satellite was reoriented in such a way that the bus radiator was pointed away from the Earth and the Sun by more than 90 •. Consequently, its ability to radiate heat away into deep space was improved. Another method was a different charging scheme called "ratchet charging", known to have been successfully employed on the Hubble Space Telescope (HST). In this method the battery is charged more than once during a Sun phase and the End-of-Charge (EoC) value is raised every week. A software update during the latter stage provided more flexible control over the solar panel strings, allowing the satellite to fly without a pitch offset which was used earlier to lower the voltage. This paper describes in detail the circumstances in which these issues arose, wherever possible their causes and how they were dealt with. The capacity of the TET-1 spacecraft's batteries is found to have increased from 12 Ah to about 14 Ah without any significant increase in temperature. Data gained at a later point in time also suggest a positive effect of the reconditioning.