Evaluation of maximum safe voltage for nickel-cadmium cells

摘要

The electrical power subsystems of earth-observation spacecraft such as ESA's Envisat and Metop and CNES's Spot 5, employ a differernt battery management scheme from the earlier (ERS, Spot 1 to 4) programmes in order to improve the overall power subsystem efficiency. Since this is expected under certain conditions, to put an increased stress on the batteries during charge, a test programme was started to evaluate the maximum safe charge voltage allowable with 40 Ah SAFT Nickel Cadmium cells under LEO cycling conditions, before irreversible pressure build up, due to hydrogen evolution occurs. The tests are underway at the Euopean Space Battery Test Centre (ESBTC) at ESTEC. As the voltage limit is expected to be sensitive to battery ageing, tests of cells at beginning of life are augmented by a test of cells which have already undergone extensive cycling. Cells are fitted with pressure gauges and gas sampling valves which allow the compositionof the gas within the cells to be periodically measured by mass spectrometry. Results so far available are described.