Breath in, breath out how healthy are the batteries on Mars and Venus Express?

摘要

I. Introduction Mars and Venus Express are operational since 2003 and 2005, respectively. Combined, this is more than 14 years in flight of two similar spacecraft, yet operating in very different environments. Both S/C had their mission extended multiple times up to now and this process continues. The spacecraft budgets were designed for the original duration (around 1000 days for each mission), and estimates on battery capacity degradation logically cover only that period. To be able to assess the current health state of the batteries, and to gather trends for predictive models, the Flight Control Team had to develop its own empirical methods based on data available from telemetry. A thorough assessment resembling laboratory conditions is not possible due to several factors: no calibration of the sensing equipment possible in-flight, no possibility to remove the battery from the circuit, no possibility to control the discharge rate, no possibility to control the thermal environment. The method developed and used consists of fitting a model of the battery voltage (based on the 'state-of-charge vs. EMF' curve) to the voltage measurements as obtained from telemetry. The model optimizes the initial energy capacity degradation factor and the losses due to internal resistance dissipation. To obtain significant data for analysis the operators had to plan for dedicated longer discharges, as data available from routine eclipses was not sufficient to obtain a significant excursion of the EMF curve outside its linear initial part. Following recommendations from industry, ESA specialists and documentation, the teams also decided to keep MEX and VEX batteries at a lower State of Charge (SoC) outside eclipse periods. This is done by forcing the End-of-Charge voltage which yielded (temporary) battery capacity recovery. This paper explains the methodology used and discusses the results obtained and compares the Mars Express (MEX) and Venus Express (VEX) results and degradation predictions.