PowerSource

Jun 18 2008 11:25AM | Permalink | Email this | Comments (1) |
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This post wraps up the last of my Q&A with Bill Yalen, lead project engineer with Yardney Technical Products, supplier of the lithium ion batteries used on the Mars Phoenix lander. Here’s Part 1, and here’s Part 2.

Because of my interest in the chances of the Phoenix surviving the Martian winter, Bill addressed the effect of extreme low temperatures on the battery.

“We have hard-frozen Li-ion cells in the past and verified that they could be thawed and recover. To do that, however, they should be at medium state of charge, whereas the Phoenix battery would presumably have discharged deeply due to lack of solar power… Unless there was some way to command it to drop off the power bus earlier, while it did still have that partial charge… – And then there would have to be a way to bring it back on later, in the spring. That’s a pretty tough path to follow. Also, to survive freezing, the cells would have to be completely unloaded – any electrical load at all on the battery during the freeze would cause damage and make it unrecoverable. So, theoretically – perhaps something could be designed to do that. Practically, for Phoenix – not likely.

So just how cold is “hard frozen”? Is it the freezing temperature of Co2 (-78.5 C)? Again, he gave a detailed explanation of the battery’s tests and their implications.

“One way we know that Li-ion cells can – under certain conditions - survive a hard freeze is the result of a test chamber malfunction during low temperature testing. Instead of cycling as needed to maintain the desired test temperature, the chamber cooling system ran continuously and drove the temperature far below the intended test profile. Since it was obliviously an invalid test, it’s not clear that we retained test records, as would otherwise be the case. We do know that the result was definitely cell freezing, which means it was below -60 deg C (-76F) which is approximately the temp at which the electrolyte freezes solid. Yet after thawing, functionality was recovered.

“Another way we have learned about cell freezing is that there is a lab diagnostic technique that involves freezing cells in liquid Nitrogen, which would mean at least down at -196C / -321F. Since most of whatever physical degradation caused by freezing has already happened when the electrolyte changes phase from liquid to solid around -60C, going down further doesn’t have much more effect. We do know that cells frozen that cold can also recover – but only if the state of charge and load conditions that I mentioned last week are met. That would be highly unlikely for Phoenix (although, you have gotten me thinking of some ideas that might be interesting to brainstorm for the future…).

“A word of caution: When considering the phenomenon of CO2 freezing, remember that the environment is Mars, not Earth. If CO2 freezes here at -78.5C at 1 atm pressure, it would have to be colder than that to freeze at the much lower pressure on Mars. According to http://mars.jpl.nasa.gov/msp98/lander/science.html , that’s 148 Kelvin -125C / -193 °F).”

UPDATE: Bill Yalen has offered to answer any specific follow-up questions readers might have after reading this series. [Thanks, Bill!] You can reach him at wyalen[at]yardney.com.

Tomorrow -- One last Martian post: A downloadable Mars sunclock