On 8/11/11 8:46 PM, Gould Smith wrote:
Kenneth Ransom, N5VHO has plotted the battery min/max for the last 8 days. We see that the battery voltage is decreasing at a faster rate than expected. Kenneth's graph can be found on the arissat1.org site under FAQ http://www.arissat1.org/v3/index.php?option=com_content&view=section&...
Disclaimer: what follows is entirely my own personal opinion, not necessarily that of the ARISSat-1 team or anyone else. I might be missing information that would affect my analysis.
Ken's plot is extremely valuable. It strongly suggests battery failure, not a negative power budget as Tony, AA2TX suggests. Note the sharp and consistent *increase* in daylight battery voltage that occurred late on Aug 11, roughly coincident with sharp *decreases* of voltage during eclipse and reports of computer resets and extended low power operation. This is inconsistent with a negative orbit-average power budget. If that were the case, the battery would never reach full charge and the voltage would never rise so high.
This plot suggests that one or more cells in the battery have lost significant capacity. The cell with the lessened capacity reaches full charge very quickly and then goes into overcharge, allowing the overall voltage of the string to rise. Then of course the voltage falls very quickly in eclipse as the impaired cell(s) rapidly discharge.
As Luc Leblanc points out, silver-zinc (Ag/Zn) batteries have long been the battery of choice in the aerospace industry where their high energy density (almost comparable to li-ion), long shelf life and reliability outweighs the obviously high cost.
The Apollo/Saturn program apparently used Ag/Zn batteries exclusively, powering everything from the guidance system in the Saturn V to the lunar module to the portable life support systems the astronauts used on the lunar surface.
But these features are associated with Ag/Zn as a *primary* battery, one that is never recharged. Only the three entry batteries in the command module were ever recharged, and then only a few times during a mission after moderate discharges. Those Ag/Zn batteries had rated cycle lives measured in the single digits; Luc's figures show only modest improvements since then.
Another unusual feature of Ag/Zn batteries is a two-stage discharge process. When the battery is fully charged, the positive plate contains AgO, silver oxide with the silver in the +2 valence. As the battery discharges, the silver is first reduced to Ag2O, with silver in the +1 valence, and then eventually to metallic silver, Ag, with valence 0.
This two-stage conversion of silver means that fairly high voltages are needed to fully charge the battery. During the Apollo 7 mission, the shakedown flight of the Apollo command module in 1968, the entry batteries could not be fully recharged because the battery charger could not lift the battery terminal voltage high enough through the line resistance. Over 39V is needed to fully charge a nominal 28V battery. According to Ken's graph the voltage has never been this high, and we don't know its initial state of charge. This suggests another possible failure mechanism besides exceeding the battery's cycle life: despite a positive power budget, the battery was never fully recharged, and overdischarge of one or more cells caused cell reversal and damage.
-Phil