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The more I think about it the more I'm convinced that the MER batteries are the unsung heroes of the missions. Here's the web page of the folks who made them, Lithion.
There's a reference to 2100 deep discharge cycles. I followed the earlier discussions on available watt hours with fascination, however we've now seen that insolation is sufficient as long as there are no major dust storms.
I am more interested in learning about the likely lifespan of the batteries based on their chemistry and present physical environment.
It is good to keep Li-Ion batteries at 0 Celsius when storing them (you should keep unused laptop batteries and iPods at 40% charge in the freezer according to several wags). So the cold environment on Mars should improve the longevity of the batteries (as long as they don't freeze and no doubt great care is taken to avoid that). Age (not cycles) is often described as the most important determinant of charging capacity of a Lithium-Ion battery. As the MER batteries age, are they holding less charge? Is there a chart showing this and can we project a lifespan based on current information? Has someone done this already? If so, sorry for trying your patience and repeating it. Please just kick me over to the relevant thread.