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Turns out Opportunity Mars rover is already working for nine years. It uses Li-Ion batteries to store energy acquired from the solar panels.

In my experience Li-Ion batteries when recharged frequently have about 40% of the original capacity after having been used for three years. I haven't tested, but I expect that in nine years they would barely charge.

Yet the rover batteries seem to have enough capacity left after nine years.

How is that possible?

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    \$\begingroup\$ I suspect a combination of overengineering (more battery capacity than required to allow for redundancy), luck (nine years is way beyond its design lifetime!), and meeting reduced capacity with reduced demand (eg no power used for failed robot arm). \$\endgroup\$
    – pjc50
    Jan 25, 2013 at 10:08
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    \$\begingroup\$ Could the Mars environment (atmosphere, temperature, etc.) possibly have a role in the longer lifespan of the batteries? Remember that on Mars, around freezing is scorching summer heat, and there is virtually no oxygen in the atmosphere, it being made up of almost exclusively carbon dioxide with some nitrogen and argon making up the lion's share of the remainder (those three add up to about 99.6%, roughly the same amount that nitrogen and oxygen add up to at 99.0% in Earth's atmosphere). \$\endgroup\$
    – user
    Jan 25, 2013 at 10:31
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    \$\begingroup\$ I'm not saying that it's an explanation, but it seems plausible that it would have an impact, even if small. (That is why I did not make it an answer; I just don't know. Just trying to reason through what factors might have an effect.) In any case, it's a good question. :) \$\endgroup\$
    – user
    Jan 25, 2013 at 10:36
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    \$\begingroup\$ Speculation : it's well known that lead acid batteries can be tuned for different applications; deep discharge cells, high discharge current, long term standby, low maintenance, low cost etc, each requirement is met by design changes to the batteries. Deep discharge batteries will cost more than regular car batteries for example. Now it's quite plausible that the same applies to Li-Ion batteries in ways that aren't so well known outside of battery makers - if your design goal is long life at extreme temperatures, I bet you get a different battery than if it's to be part of a $30 product! \$\endgroup\$
    – user16324
    Jan 25, 2013 at 11:10
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    \$\begingroup\$ It's a big scam. There is really a very long extension cord, but shhhh, don't tell anyone. \$\endgroup\$
    – Olin Lathrop
    Jan 25, 2013 at 12:11

2 Answers 2

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Batteries made by Yardley Lithion

Well discussed here:

Life verification of large capacity Yardney Li-Ion cells and batteries in support of NASA missions

One version 60% capacity after 3000 cycles deep discharge

Another - 25,000 cycles, 40% depth of discharge 35 min charge/discharge periods (LEO satellite simulation). Max Voltage = 3.8V/cell!

Next Generation and Domestically Produced Active Materials for Lithium-Ion Cells for Military and Aerospace Applications

Adding material from comments that increases usefulness of answer

PJC in Jan 2013 noted that majopr points from the paper cited above included

  • "the capacity fade at lower temperature is significantly decreased" (-20 degrees C).

and

  • " ... also to keep the rate and depth of discharge low ("battery could not support rate of discharge >C/2 at -20C when <75% charged after 1010 sols of operation"), and for long term storage to keep the batteries at 50-70% charge, not fully charged."

  • He also noted - The anode composition and surface coating may also be different from consumer batteries.

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    \$\begingroup\$ Key pullout sentence from that paper: "the capacity fade at lower temperature is signifigantly decreased" (-20 degrees C). The anode composition and surface coating may also be different from consumer batteries. \$\endgroup\$
    – pjc50
    Jan 25, 2013 at 14:31
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    \$\begingroup\$ .. also to keep the rate and depth of discharge low ("battery could not support rate of discharge >C/2 at -20C when <75% charged after 1010 sols of operation"), and for long term storage to keep the batteries at 50-70% charge, not fully charged. \$\endgroup\$
    – pjc50
    Jan 25, 2013 at 14:40
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NASA has a "Stirling Engine" for space use. They may have used it on the Mars mission.
They operate without air, zero gravity, no maintenance, no oil changes, and produce
about 110-140 watts, 24 /7 / 365/ => 14 years.
http://www.grc.nasa.gov/WWW/RT/2004/RP/RPT-shah.html
http://www.grc.nasa.gov/WWW/TECB/videos/animation_TDC_01_jeff.gif

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    \$\begingroup\$ Use an isotope energy source. Usually used on outer planet missions where solar energy is too low. Expensive overall, cause protests when satellites do gravity boosts after launch to slingshot on their way and fuel supply is finite (surprisingly). \$\endgroup\$
    – Russell McMahon ♦
    Jan 25, 2013 at 22:35

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